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471 PERSPECTIVE / PERSPECTIVE Lake Ontario: food web dynamics in a changing ecosystem (1970-2000)1 E.L. Mills, J.M. Casselman, R. Dermott, J.D. Fitzsimons, G. Gal, K.T. Holeck, J.A. Hoyle, O.E. Johannsson, B.F. Lantry, J.C. Makarewicz, E.S. Millard, I.F. Munawar, M. Munawar, R. OGorman, R.W. Owens, L.G. Rudstam, T. Schaner, and T.J. Stewart Abstract: We examined stressors that have led to profound ecological changes in the Lake Ontario ecosystem and its fish community since 1970. The most notable changes have been reductions in phosphorus loading, invasion by Dreissena spp., fisheries management through stocking of exotic salmonids and control of sea lamprey (Petromyzon marinus), and fish harvest by anglers and double-crested cormorants (Phalacrocorax auritus). The response to these stressors has led to (i) declines in both algal photosynthesis and epilimnetic zooplankton production, (ii) decreases in alewife (Alosa pseudoharengus) abundance, (iii) declines in native Diporeia and lake whitefish (Coregonus clupeaformis), (iv) behavioral shifts in alewife spatial distribution benefitting native lake trout (Salvelinus namaycush),

threespine stickleback (Gasterosteus aculeatus), and emerald shiner (Notropis atherinoides) populations, (v) dramatic increases in water clarity, (vi) predation impacts by cormorants on select fish species, and (vii) lake trout recruitment bottlenecks associated with alewife-induced thiamine deficiency. We expect stressor responses associated with anthropogenic forces like exotic species invasions and global climate warming to continue to impact the Lake Ontario ecosystem in the future and recommend continuous long-term ecological studies to enhance scientific understanding and management of this important resource.

Résumé : On trouvera ici un examen des facteurs 490 de stress qui ont modifié profondément lécosystme du lac Ontario et sa communauté de poissons depuis 1970. Les changements les plus importants ont été la réduction de lapport de phosphore, linvasion des Dreissena spp., la gestion de la pche, notamment lempoissonnement de salmonidés exoti-ques et le contrle de la grande lamproie marine (Petromyzon marinus), ainsi que la récolte des poissons par les pcheurs sportifs et les cormorans aigrette (Phalacrocorax auritus). La réaction ces facteurs a eu pour conséquen-ces: (i) le déclin de la photosynthse des algues et de la production du zooplancton épilimnétique, (ii) la diminution de labondance du gaspareau (Alosa pseudoharengus), (iii) la réduction des Diporeia indignes et des grands corégones Received 24 July 2002. Accepted 8 March 2003. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 30 May 2003.

J17013 E.L. Mills,2 K.T. Holeck, and L.G. Rudstam. Cornell University Biological Field Station, 900 Shackelton Point Road, Bridgeport, NY 13030, U.S.A.

R. Dermott, J.D. Fitzsimons, O.E. Johannsson, E.S. Millard, I.F. Munawar, and M. Munawar. Great Lakes Laboratory for Fisheries and Aquatic Sciences, Department of Fisheries and Oceans, Burlington, ON L7R 4A6, Canada.

R. OGorman and R.W. Owens. U.S. Geological Survey, Great Lakes Science Center, Lake Ontario Biological Station, 17 Lake Street, Oswego, NY 13126, U.S.A.

J.M. Casselman, J.A. Hoyle, T. Schaner, and T.J. Stewart. Ontario Ministry of Natural Resources, Glenora Fisheries Station, R.R. 4, Picton, ON K0K 2T0, Canada.

G. Gal. Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, U.S.A.

B.F. Lantry. New York Department of Environmental Conservation, Cape Vincent Fisheries Research Station, Cape Vincent, NY 13618, U.S.A.

J.C. Makarewicz. Department of Biological Sciences, State University of New York at Brockport, Brockport, NY 14420, U.S.A.

1 This paper forms part of the proceedings of a workshop convened at The University of Toronto at Mississauga, 18-20 May 2000.

The workshop was sponsored by the Great Lakes Fishery Commission to revisit Great Lakes ecosystem change, three decades since the SCOL Symposium, which was convened at Geneva Park, Ontario, in July 1971 and for which the proceedings were subsequently published as a special issue of the Journal of the Fisheries Research Board of Canada (Volume 29, Number 6, June 1972). The first paper in the SCOL II series (Madenjian et al. 2002) was previously published in this journal.

2 Corresponding author (e-mail: elm5@cornell.edu).

472 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 (Coregonus clupeaformis), (iv) les modifications comportementales de la répartition spatiale des gaspareaux, ce qui a favorisé les populations indignes de touladis (Salvelinus namaycush), dépinoches trois épines (Gasterosteus aculea-tus) et de menés émeraude (Notropis atherinoides), (v) les augmentations spectaculaires de la clarté de leau, (vi) les impacts de la prédation des cormorans sur certaines espces de poissons et (vii) les goulots détranglement dans le recrutement des touladis reliés une déficience de thiamine causée par les gaspareaux. Nous prévoyons que des réactions des facteurs anthropiques, tels que linvasion de poissons exotiques et le réchauffement climatique global, continueront affecter le systme du lac Ontario dans le futur et nous recommandons la poursuite détudes écologiques long terme pour favoriser la compréhension scientifique et la gestion de cette importante ressource.

[Traduit par la Rédaction] Mills et al.

Introduction Lake Ontario fish community, a better understanding of food web function was needed. During the 1980s and 1990s, our The Laurentian Great Lakes have been subjected to accel- understanding of the Lake Ontario food web increased, and erated ecological change since the arrival of European set- we realized that exotic fishes, particularly alewife, played a tlers 250 years ago. Lake Ontario and other Great Lakes much larger role in the destruction of the original fish com-ecosystems experienced numerous stresses including overfish- munity than was previously believed (OGorman and Stew-ing, colonization by exotic species, cultural eutrophication, art 1999; Ketola et al. 2000). In addition, we now have a and contaminant discharge leading to degradation in water greater appreciation for the significant role that humans play quality, loss and change of habitat, and the decline of native as a driving force in shaping the Lake Ontario ecosystem fish communities in the 1950s and 1960s. By the 1970s, and its food web. Attempts to restore the Lake Ontario eco-Lake Ontarios major native fish stocks had been pushed to system to historic conditions over the past 30 years through near extinction (Christie 1972). Atlantic salmon (Salmo active management to reduce phosphorus, restore native spe-salar), deepwater sculpins (Myoxocephalus thompsoni), lake cies, and control sea lamprey have been modified by anthro-trout (Salvelinus namaycush), burbot (Lota lota), and core- pogenic impacts associated with exotic species invasions, gonids (Coregonus spp.) had all disappeared or had seriously habitat modification, and climate change.

declined in abundance, whereas non-native fish like alewife In this paper, we build upon the scientific understanding (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), of the Lake Ontario fish community in SCOL I through as-and white perch (Morone americana) proliferated. Overfish- sessment of the dynamics of the Lake Ontario food web ing and sea lamprey (Petromyzon marinus) predation were from 1970 through 2000. We hypothesize that although oli-considered destabilizing factors in the Lake Ontario fish gotrophication (defined as the reverse of eutrophication) has community. Sea lamprey predation on salmonids and burbot driven the recovery process of the Lake Ontario ecosystem, likely increased as the number of dams in the Lake Ontario the lake will not return to historic conditions but will take a watershed decreased and acted in concert with commercial new path in response to unplanned exotic species introduc-fishing to virtually eliminate large piscivores (Christie 1972). tions. Our approach was to analyze available long-term data Cultural eutrophication, a major destabilizing force of the series on the Lake Ontario food web and to examine anthro-Lake Ontario ecosystem from the 1940s to the 1970s, led to pogenic stressors that were responsible for ecological and nuisance algal blooms and water quality deterioration fishery changes in the lake since 1970. We examined the im-(Schelske 1991). pact of stressors associated with nutrient abatement, exotic The sequence of events that led to a deterioration of habi- species introductions, and fish management on fish commu-tat and fishery stocks throughout the Great Lakes, including nities and species and later speculated on the future of the Lake Ontario, led to two important milestones. The first was Lake Ontario ecosystem. In our synthesis and integration of the 1972 Great Lakes Water Quality Agreement (GLWQA) links across trophic levels, we acknowledge the importance between the United States and Canada, which resulted in of concurrent scientific reviews in a book entitled State of controls and permissible phosphorus loadings to each of the Lake Ontario: Past, Present, and Future, edited by M.

Great Lakes and marked a new era of ecosystem manage- Munawar (2003), as a source of information. This book orig-ment and recovery. The second was the 1971 symposium on inated from a symposium jointly sponsored by the Interna-Salmonid Communities in Oligotrophic Lakes (SCOL I), tional Association of Great Lakes Research and the Aquatic which yielded new insights about anthropogenic stressors on Ecosystem Health and Management Society.

fish communities in Great Lakes and comparable ecosys-tems. SCOL I was an important milestone in the advance of Great Lakes science and was an important stimulus leading The Lake Ontario ecosystem and pivotal to a broader thrust in thinking about fish and fisheries within events the context of a lake ecosystem.

Christie (1972) synthesized long-term fish community Lake Ontario (Fig. 1) ranks as the 17th largest lake in the changes in Lake Ontario and examined abiotic and biotic world with a surface area of 18 960 km2 (Beeton et al.

stressors from the 1800s to 1970 to assess shifts in fish 1999). The lakes watershed is dominated by forests (49%)

stocks. The lesson learned from Christies analysis was that and agriculture (39%), and 7% of the basin is urbanized although sea lamprey predation, overfishing, and water qual- (Stewart et al. 1999). Approximately six million people live ity were identified as primary destabilizing factors in the in the watershed with nearly 70% residing in the province of

Mills et al. 473 Fig. 1. Bathymetric map of Lake Ontario showing long-term sampling locations ().

Ontario. The maximum depth of Lake Ontarios main basin by nearly 50% from a peak of 15 036 tonnes (t)*year-1 in 1969 is 244 m, and the relatively shallow Kingston Basin, with its to 7410 t*year-1 by 1981, close to the loading target of 7000 numerous embayments, peninsulas, and islands, accounts for t*year-1 established in the GLWQA (International Joint Commis-more than 50% of the lakes shoreline. A total of 86% of sion 1988; Fig. 2a). Spring total phosphorus (TP) concentrations Lake Ontarios inflow comes from the upper Great Lakes declined by over 50% from 20-25 µg*L-1 in the early 1970s to via the Niagara River. just below the target concentration of 10 µg*L-1 by 1986 The Lake Ontario ecosystem has been subject to numer- (9.9 µg*L-1). From 1986 to 1993, TP concentrations fluctuated ous socio-political influences, management actions, and un- within +/-0.5 µg*L-1 of the target of 10 µg*L-1. This recovery pe-planned events since the late 1960s and early 1970s; these riod (12 years) was consistent with early phosphorus models events have been crucial to understanding ecosystem changes (Chapra and Sonzogni 1979) suggesting that the response time over the last three decades (Table 1). Pivotal events for Lake for Lake Ontario to achieve target concentrations resulting from Ontario were initially management actions taken to remediate phosphorus control measures would range from 8 to 22 years.

anthropogenic abuses to the aquatic ecosystem and to restore In contrast to phosphorus (P), silica exhibited no long-term a balanced fish community. The year 1968 saw the first of changes in the open waters of Lake Ontario (Johannsson et al.

what was to become annual releases of Pacific salmon to re- 1998; Millard et al. 2003), whereas nitrate concentrations as duce alewife and create a recreational fishery. Large-scale nitrate increased significantly, especially from 1968 to 1987 management activities accelerated through the 1970s with (Millard et al. 2003; Fig. 2b). Mean spring nitrate concentra-the signing and implementation of binational agreements to tions nearly doubled from 215 µg*L-1 in 1968 to about 400 reduce phosphorus loading to the lake, the initial treatment µg*L-1 by the late 1980s but leveled off during the 1990s of streams tributary to Lake Ontario to kill larval sea lam- (Stevens and Neilson 1987; Lean 1987; Neilson et al. 1994).

prey, and the start of annual releases of hatchery-reared lake Lean (1987) concluded that the increase in nitrate was asso-trout for population restoration. However, the pivotal event ciated with higher loading from the watershed and was not for the ecosystem in the 1990s was not a planned, science- associated with reduced algal demand because the nitrate in-based, management action but rather was the unintentional crease occurred before implementation of phosphorus con-establishment and proliferation of a suite of exotic species trol. Millard et al. (2003) showed that the rate of nitrate from Eurasia that gained entry to the Great Lakes in associa- increase paralleled nitrogen (N) fertilizer use in the Great tion with transoceanic shipping. Lakes basin and mirrored the observed Lake Ontario mid-lake increase up to the mid-1980s.

Nutrient dynamics and oligotrophication Ratios of N to P in the early 1970s (Stevens and Neilson 1987) were in the range (16:1) where N and P might have Mandated programs to control phosphorus in the Great Lakes co-limited phytoplankton growth (Forsberg et al. 1978; Vin-and Lake Ontario were an undeniable success (Stevens and cent 1981). However, the steady increase in nitrate and de-Neilson 1987; Millard et al. 2003) and initiated the process of crease in TP over the last two decades has elevated the N:P oligotrophication. Phosphorus loading to Lake Ontario declined in excess of 50:1 (Johannsson et al. 1998), which suggests

474 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 Table 1. Management actions, socio-political influences, and unplanned events that have been pivotal to understanding ecological changes in the Lake Ontario ecosystem since 1968.

Year Pivotal event Citation 1968 First annual release of Pacific salmon for alewife control and Owens et al. 2003 recreational fishing 1970 Canada limits phosphates in detergents Stevens and Neilson 1987 1971 First treatment of Canadian tributaries to Lake Ontario with Pearce et al. 1980 lampricide 1972 New York limits phosphates in detergents Stevens and Neilson 1987 1972 U.S.-Canada Great Lakes Water Quality Agreement sets in Stevens and Neilson 1987 motion programs to control P discharges 1972 First treatment of New York tributaries to Lake Ontario with Pearce et al. 1980 lampricide 1973 First annual release of hatchery lake trout for population Elrod et al. 1995 restoration 1974 Twenty-two cormorant nests on Little Galloo Island Weseloh and Ewins 1994 1982 First record of fry produced in lake by hatchery lake trout Marsden et al. 1988 1983 Last record of bloater Owens et al. 2003 1989 First record of zebra mussel T. Schnaer, Ontario Ministry of Natural Resources, Glenora Fisheries Station, Picton, ON K0K 2T0, personal communication 1991 First record of quagga mussel Mills et al. 1993 1992 Start of Diporeia collapse Lozano et al. 2001 1993 Start of annual, successful reproduction by hatchery lake trout OGorman et al. 2000 1995 First record of blueback herring Owens et al. 1998 1996 First record of Echinogammarus ischnus Dermott et al. 1998 1998 First record of Cercopagis pengoi MacIsaac et al. 1999 1998 First record of round goby (Neogobius melanostomus) Owens et al. 2003 that Lake Ontario is more P-limited (Forsberg et al. 1978; early 1980s when a decrease of about 2 µg*L-1 was observed Vincent 1981) for at least 3 months during the summer in between 1976 and 1985 (Stevens and Neilson 1987; Millard et the open deep waters of the lake (Lean et al. 1987; Millard al. 2003); Chl remained at these lower levels (2.5-3.5 µg*L-1) et al. 1996a). The shallow Kingston Basin is P-limited for a between 1985 and 1993 (Fig. 2b). Summer Chl concentra-much longer period than the offshore habitat because spring tions declined in the Kingston Basin from maxima of 5.0-water column light intensities can exceed growth-limiting 6.0 µg*L-1 in the early to mid-1980s to 3.5-4.5 µg*L-1 levels at full vertical mixing in this part of the lake, but not through the late 1980s and 1990s. The recent spring (pre-offshore (Millard et al. 1996a). stratified) Chl decline in the Kingston Basin was particularly evident after 1992, in spite of stable TP concentrations dur-Phytoplankton dynamics, primary production, ing this time (Millard et al. 2003). The significance of the Chl decline in the 1990s was that it occurred in the Kingston and the microbial loop Basin and not in offshore waters of the open lake.

Phytoplankton dynamics in Lake Ontario since 1970 re- Studies of primary productivity (PP) in freshwater lake flected shifts toward oligotrophy. Comparison of lakewide ecosystems have generally correlated seasonal or annual surveys conducted in 1970 (high phosphorus) and 1990 (low changes in PP with P loading (Millard and Johnson 1986).

phosphorus) showed an increase of oligotrophic over eutrophic In Lake Ontario, seasonal areal phytoplankton photosynthe-species (Vollenweider et al. 1974; Munawar and Munawar sis (1 May to 31 October) declined by 30% coincident with 1996; Munawar et al. 2003). Predominant eutrophic species the decline in P concentrations for the period 1972 to 1992 of diatoms and cyanobacteria have either been replaced by (Millard et al. 1996b). However, the observed decline in al-oligotrophic species or occur in very low numbers, and the gal photosynthesis was not proportional to the 50% decline relative abundance of oligotrophic species of diatoms and in Lake Ontario phosphorus concentrations because of the chrysophytes has increased. Other shifts in phytoplankton positive compensating effect that increased light penetration dynamics in response to the invasion by Dreissena spp. in has on depth of photosynthesis. Light penetration and deep-1989 might be anticipated, but lakewide responses remain ening of the euphotic zone increased further in the 1990s largely unknown. following establishment of Dreissena spp. In the Kingston Summer phytoplankton growth in Lake Ontario has proba- Basin, for example, the extinction coefficient of photosyn-bly always been P-limited, even at the higher concentrations thetically active radiation (PAR ) declined 25% from a sea-before P control (Lean et al. 1987; Millard et al. 1996a). sonal mean of 0.35 m-1 in 1991 to 0.26 m-1 in 1994-1995; However, algal biomass, as indicated by summer chlorophyll this equates to a deepening of the euphotic zone by about (Chl), did not respond immediately to reduced P loadings 5 m. In spite of this increase in light penetration, areal photo-and concentrations. Lakewide Chl did not decline until the synthetic rates have not likely been maintained at the same

Mills et al. 475 Fig. 2. (a) Long-term trends in phosphorus loading () and mean spring total phosphorus concentrations (bars) (spring surveillance cruises) in Lake Ontario (from Millard et al. 2003). (b) Long-term trends in mean summer chlorophyll a (bars) and mean spring nitrate

(). Values represent combined means from Bioindex stations 41 and 81 and from surveillance cruises. (c) Trends in total epilimnetic zooplankton production between 15 June and 31 October. Estimates based on samples collected from either the top 20 m or the epilimnion of Lake Ontario, whichever was shallower, at stations 41 () and 81 () of the Bioindex program.

levels as observed before dreissenid mussel invasion unless production during the eutrophic period of the early 1970s there has been a drastic increase in photosynthesis per unit (Vollenweider et al. 1974). Although there are no size-biomass of algae (Millard and Sager 1994). fractionated primary production estimates from Lake On-Lakewide studies of size-fractionated primary production tario in the early 1970s, estimates in nearby Lake Erie indi-in Lake Ontario from 1990 to 1997 reflect periods of post- cated that netplankton contributed to the bulk of primary phosphorus reduction and post-dreissenid establishment production at that time (Munawar and Burns 1976).

(Munawar and Munawar 2003). Except in the spring of Our understanding of the Lake Ontario food web in 1970 1990, the bulk of primary production has been associated did not include knowledge of the microbial food web (MFW),

with smaller-sized nanoplankton (2-20 µm) and which includes bacteria, heterotrophic nanoflagellates, cili-picoplankton (<2 µm). The importance of smaller-sized frac- ates, nanoplankton, and picoplankton (Munawar et al. 2003).

tions contributing to Lake Ontarios pelagic primary produc- The MFW is now considered an essential pathway of energy tion in the 1990s has likely been a major change from the transfer to zooplankton (Munawar and Munawar 1999). Hete-presumed higher netplankton contribution to overall algal rotrophic flagellates and bacteria are active in biodegradation

476 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 of organic material that regenerates mineral substances, which al. 1991, 1998; Johannsson 2003; Fig. 2c). In the mid-lake, are consequently used by unicellular autotrophs. In turn, flag- epilimnetic zooplankton production declined from 17-42 g ellates are consumed by microzooplankton such as ciliates dry weight (dw)*m-2*season-1 (season = 15 June - 31 October)

(10-80 µm). With the shift toward a more oligotrophic state, between 1981 and 1985 to 8-19 g dw*m-2*season-1 from we can only speculate that the plankton community has shifted 1986 to 1995. In the Kingston Basin, zooplankton produc-toward autotrophic nanoplankton and picoplankton with a tion declined from 28-52 g dw*m-2*season-1 between 1981 tight coupling between heterotrophic nanoflagellates and bac- and 1984 to 7-13 g dw*m-2*season-1 between 1993 and 1995.

teria (Munawar et al. 2003). The decline in zooplankton production in association with declines in phosphorus led Johannsson (2003) to rule in fa-vor of nutrient decline instead of fish zooplanktivory as the Zooplankton dynamics and production driving force leading to lower zooplankton production in Lake Ontario in the 1980s and early 1990s.

The shift toward oligotrophy of the Lake Ontario ecosystem The zooplankton web has become more complex since was expected to impact the zooplankton community composi- 1970 with the addition of two exotic spiny-tailed cladocerans tion through a reduction in eutrophic species. Whole-lake, and veliger larvae of Dreissena spp. The cladoceran Bytho-large-scale studies in the late 1960s and early 1970s (Patalas trephes longimanus (formerly known as B. cederstroemi) was 1969, 1972) revealed that the zooplankton community was first seen in 1982, and Cercopagis pengoi was first observed dominated by small cladocerans and cyclopoid copepods, in 1998 (MacIsaac et al. 1999). Both B. longimanus and primarily Diacyclops thomasi (formerly Cyclops bicuspidatus C. pengoi are predators on other zooplankton; recent evi-thomasi), Tropocyclops extensus (formerly T. prasinus mexi- dence by Benoit et al. (2002) indicates that C. pengoi feeds canus), Bosmina spp. (B. leideri and B. freyi formerly known on small copepods and Bosmina and likely decreases juve-as B. longirostris), and Daphnia retrocurva. Mesotrophic and nile copepod production through direct predation and a shift eutrophic zooplankton species such as Eubosmina coregoni of copepod vertical distribution to deeper, colder waters. In-and Ceriodaphnia lacustris were present regularly, whereas terestingly, B. longimanus has only been observed in Lake Chydorus sphaericus, Eurytemora affinis, Acanthocyclops Ontario in 1987 and 1994 (primarily in the fall), whereas vernalis, Leptodora kindtii, and Mesocyclops edax were seen C. pengoi has been observed lakewide since 1998 (highest only occasionally (Makarewicz 1993; Johannsson et al. 1998). densities in August and September) (Makarewicz et al. 2001).

This community remained intact through the 1980s At present, B. longimanus has little impact on food resources (Johannsson et al. 1991). The response of the zooplankton of fish. Cercopagis, on the other hand, is very abundant in the community to phosphorus reductions, however, was subtle. summer and its impact on fish is currently unknown.

Chydorus sphaericus disappeared in the early 1990s, and Mysis relicta, an abundant omnivore with important links Ceriodaphnia lacustris declined in abundance and was not in Lake Ontarios benthic and pelagic food web, is a domi-observed in the mid-lake in 1995 (Johannsson 2003). nant prey of rainbow smelt and a common prey of alewife The impact of alewife on the zooplankton species compo- and slimy sculpin (Cottus cognatus) (Owens and Weber sition since the early 1970s in Lake Ontario has been signifi- 1995). Interestingly, mysid population densities have re-cant, and intense planktivory by these fish has structured the mained relatively stable since the mid-1980s despite shifts zooplankton community toward small species (e.g., Bosmina). toward oligotrophy and reductions in zooplankton produc-Zooplankton are the principal food of juvenile and adult ale- tion (Johannsson et al. 2003). However, mysid production in wife (Mills et al. 1992; Urban and Brandt 1993), and alewife the offshore (1984-1995) increased with declines in alewife were responsible for >96% of the predation on zooplankton predation in the 1990s (Johannsson et al. 2003).

by Lake Ontario fish as late as 1990 (Rand et al. 1995). Ale-wife abundance declined 42% from the early 1980s to the early 1990s (OGorman et al. 2000), and subtle changes The benthic macroinvertebrate community were observed in the zooplankton community coincident with this decline. Observed changes included an increase in abun- With mandated binational programs to improve water dance of larger zooplankton species, and August mean quality conditions in Lake Ontario in the 1980s and 1990s, cladoceran length was larger in 1990-1991 than in the late expectations were high that the benthic community would 1980s (Johannsson 2003). Relative production of Daphnia benefit greatly from these efforts, but unfortunately, toxic increased from <15% (1981-1985) to 30-50% (1986-1995) contaminants in Lake Ontario sediments have dampened of total zooplankton production (Johannsson 2003); over this such expectations leading to a decline in certain pollution-period, the abundance of summer cyclopoid copepods (1987- sensitive species. For example, studies by Nalepa and 1995 in the mid-lake) and of total zooplankton (in the spring Thomas (1976) and Barton and Anholt (1997) both found over the 1981-1995 period) also increased (Johannsson et al. that pollution-sensitive Diporeia were rare at sites near the 1998). Decreases in planktivory should have led to detect- mouth of the Niagara River. Such scarcity has been attrib-able increases in zooplankton productivity; however, no sig- uted to high loadings of chlorinated hydrocarbons from in-nificant increase in zooplankton production was observed dustries (Durham and Oliver 1983; Nalepa 1991).

between 1987 and 1995 (Johannsson et al. 1998; Johannsson One of the most significant changes in the benthic macro-2003). To the contrary, zooplankton production declined sig- fauna of Lake Ontario has been the establishment of two spe-nificantly between 1981 and the early 1990s in the main cies of Dreissena. The zebra mussel, Dreissena polymorpha, lake, along the south shore, and in the Kingston Basin and was first detected in the lake in 1989, and by 1991, Dreissena paralleled the decline in TP concentrations (Johannsson et bugensis (also known as the quagga mussel) was observed co-

Mills et al. 477 existing with the zebra mussel (Mills et al. 1993). South-shore predators to control exotic prey fish like alewife (Christie et studies between 1992 and 1995 showed that total Dreissena al. 1987a). Further control measures included efforts to min-biomass had increased and that areas of lake bottom domi- imize loadings of mercury, chlorinated hydrocarbons of nated by zebra mussels in 1992 were dominated by quagga DDT (dichlorodiphenyltrichloroethane) and PCBs (polychlo-mussels in 1995 (Mills et al. 1999). Associated with the dra- rinated biphenyls), and municipally produced phosphorus matic increase in Dreissena spp. was a collapse of the larger (Minns et al. 1986). The full effects of these initiatives on fingernail clams (Sphaerium spp., mainly S. corneum and Lake Ontario fish were not expected to be known until the S. nitidum; Fig. 3a); this collapse was likely due to competi- 1980s and 1990s.

tion with Dreissena for food and space. Coincident with the In the early 1970s, efforts to control nuisance levels of ale-ascent of Dreissena spp., numbers of the shallow water am- wife, to establish a sport fishery, and to restore lake trout led to phipod Gammarus fasciatus increased, perhaps because they an acceleration of stocking of fish predators including lake benefitted from the structural complexity associated with trout, brown trout (Salmo trutta), rainbow trout (Oncorhynchus mussel colonies and the energy transfer to the benthos through mykiss), Atlantic salmon, chinook salmon (O. tshawytscha),

pseudofecal deposition (Stewart and Haynes 1994; Haynes et and coho salmon (O. kisutch). Limited stocking of kokanee al. 1999). However, Gammarus may be replaced in the future salmon (O. nerka) during 1965-1972 was not successful and by the newly established amphipod Echinogammarus ischnus; was discontinued in 1973 (Pearce et al. 1980). These intro-by 1996, E. ischnus was present at the mouth of the Niagara ductions initially failed to produce significant fisheries be-River, where it displaced Gammarus fasciatus (Dermott et al. cause of high parasitic sea lamprey-induced mortality 1998). Colonization of Lake Ontario by the filter-feeding (Pearce et al. 1980). Sea lamprey control was initiated in Dreissena spp. has likely decreased crustacean zooplankton 1971, and by the mid-1980s, stocks of lake trout, brown production, particularly in nearshore (defined as <30 m depth) trout, chinook salmon, and coho salmon all responded posi-regions, if the ecological response is similar to that of Lake tively to reduced numbers of parasitic lamprey. These suc-Erie, where dreissenid mussels depressed zooplankton pro- cesses led to a new set of management issues what is the duction through their impact on pelagic primary production most suitable fish species mixture for the lake and what level (Johannsson et al. 2000). Finally, the nearshore macrobenthos of stocking is necessary to maintain a balance between pred-community has undergone further change with the replace- ator demand and prey fish supply?

ment of the gastropod snails Amnicola spp. and the Valvata Enhanced survival of trout and salmon led to an expansion spp. with the exotic New Zealand mud snail (Potamopyrgus of hatchery stocking programs in both New York and On-antipodarum; Zaranko et al. 1997). tario and the dawn of a massive recreational fishing industry Historically, the burrowing amphipod Diporeia represented (OGorman and Stewart 1999). Pacific salmonines played a 60-80% of benthic biomass in Lake Ontario (Johannsson et pivotal role in the transformation of the recreational fishery.

al. 1985) and was critically important as a food source for As the salmonid fishery expanded with the onset of acceler-lake whitefish (Hoyle et al. 2003). In the Kingston Basin, ated stocking efforts (beginning in 1980) and the potential density of Diporeia increased between 1983 and 1989 and for record fishery yields was eminent, management concerns reached a seasonal average just over 13 000*m-2 in 1988 emerged about the sustainability of the salmonid fishery (Fig. 3b). Dry biomass ranged from 2.3 to 3.0 g*m-2 before with a waning alewife population (OGorman and Stewart 1985 (Johannsson et al. 1985; Dermott and Corning 1988) 1999). By the mid-1980s, the state of New York and the and peaked at 5.0 g*m-2 in 1988. A rapid population increase province of Ontario agreed to limit stocking to 8 million of Diporeia between 1987 and 1989 in offshore waters was salmonids (including chinook, coho, and Atlantic salmon in synchrony with similar events in the Kingston Basin (site and lake, rainbow, and brown trout) annually (Fig. 4; Kerr

81) and in the Bay of Quinte (Dermott 2001). After 1990, and Le Tendre 1991). The outcome of these efforts to reha-Diporeia density in the Kingston Basin (at depths <35 m) bilitate the Lake Ontario fishery resulted in a multimillion plummeted to <4*m-2 by October 1995 and to zero in April dollar recreational fishing industry (OGorman and Stewart 1996 (Fig. 3b; Dermott 2001). Lozano et al. (2001) also ob- 1999). Total annual expenditures by anglers participating in served a significant decline in Diporeia density between 1972 Lake Ontarios recreational fisheries were impressive; expen-and 1997 at depths of 12-88 m. A zone of low Diporeia ditures were $53 million (Canadian) for the province of On-density (<4 individuals*m-2) encompassing a significant por- tario in 1995 (Department of Fisheries and Oceans 1997) tion of the soft sediment habitat in Lake Ontario currently and $71 million (U.S.) for New York in 1996 (Connelly et extends to 26 km offshore and as deep as 160 m (Lozano et al. 1997). Concerns about salmonid predator demand and al. 2001). This reduction of Diporeia is expected to have a prey supply were rekindled in the early 1990s (Jones et al.

significant impact on the fish of Lake Ontario that are de-1993); by 1993, salmonid stocking levels were reduced to pendent on these organisms for their growth and survival.

4.5 million and since have been maintained at between 4 and 5.5 million annually (Fig. 4).

Fish community dynamics Development of a world-class salmonid fishery produced tremendous fishing harvest and effort by anglers. Harvest Fish managers recognized the numerous stresses on the also reflected angler preference chinook salmon were Lake Ontario fish community before the 1970s and instituted most sought after by anglers, followed by rainbow trout, new measures to set the stage for the recovery process. Most lake trout, brown trout, and coho salmon (Stewart et al.

notable were sea lamprey control (Pearce et al. 1980), quotas 2003). Harvest rate declined from 1985 to 1995 and the re-to regulate commercial fishing effort, and stocking of salmonid duction in salmonid stocking was followed by declines in

478 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 Fig. 3. (a) Shell-free dry weight biomass in October of Sphaerium (bars) and Dreissena () at a sandy-silt inshore site (station 93) in Lake Ontario, 1991-1995. (b) Yearly mean density of the amphipod Diporeia at Bioindex stations 41 () and 81 () in Lake Ontario, 1981-1996, and catch-per-gillnet (shaded area; sum of catch adjusted to 100 m of each mesh size, 3.8- to 15.2-cm stretch measure) of age-1 and older lake whitefish in the Kingston Basin of Lake Ontario, 1981 to 2000.

Fig. 4. Numbers of coho salmon (Oncorhynchus kisutch; solid), brown trout (Salmo trutta; light shading), rainbow trout (Oncorhynchus mykiss; horizontal hatching), lake trout (Salvelinus namaycush; open), chinook salmon (Oncorhynchus tshawytscha; diagonal hatching),

and Atlantic salmon (Salmo salar; dark shading), stocked in Lake Ontario, 1968-2000 (excludes fish stocked at a weight <1 g).

fishing effort. By 1995, fishing effort declined to about half community was dominated by non-native planktivores of the 1990 peak and harvest declined by two- to four-fold (alewife and rainbow smelt) and a native benthivore, slimy for all species from 1985 to 1995. sculpin (Owens et al. 2003). By the 1990s, exotic In the 1970s and early 1980s, Lake Ontarios offshore fish planktivores declined as did slimy sculpins, but native fishes

Mills et al. 479 like threespine stickleback (Gasterosteus aculeatus) and em- macrozooplankton (e.g., Bythotrephes), macroinvertebrates erald shiner (Notropis atherinoides) became more common. (e.g., Mysis and Diporeia), and larval and juvenile fish (OGorman 1974). Numbers and biomass of rainbow smelt Alewife fluctuated widely and without trend in U.S. waters of Lake The Lake Ontario fish community had massive numbers of Ontario during 1978-1998 (Fig. 6). The most dramatic alewife in the decades before 1970, and fishery biologists rec- change in the rainbow smelt population was the marked de-ognized the need to develop indices of alewife abundance cline in large rainbow smelt that was evident by the mid-(OGorman and Stewart 1999; Owens et al. 2003; Casselman 1980s. Christie et al. (1987a), Casselman and Scott (1992),

and Scott 2003). A mass mortality of alewives during the and Owens et al. (2003) all considered size-selective preda-winter of 1976-1977, precipitated by unusually cold temper- tion by salmonids as the most plausible explanation for the atures, depressed alewife numbers during the late 1970s reduction of large rainbow smelt.

(OGorman and Schneider 1986; Bergstedt and OGorman 1989). Following the 1976-1977 mass mortality, bottom Slimy sculpins trawl catches showed alewife numbers increasing in both the Slimy sculpins are native benthic fish and are important to Kingston Basin and U.S. waters (Fig. 5). In the 1980s, the the diet of lake trout (Elrod and OGorman 1991). Numbers of number of alewives in the Kingston Basin during summer slimy sculpins fell sharply in southern Lake Ontario between gradually rose, whereas the number of alewives in U.S. wa- fall 1982 and fall 1984 because of predation by stocked juve-ters of the lake in spring slowly declined. In the early 1990s, nile lake trout (Owens and Bergstedt 1994). Numbers slowly alewife numbers spiked sharply in the Kingston Basin but rose from 1984 to 1991, declined abruptly in 1992, and re-remained stable in U.S. waters. By the late 1990s, alewife mained low during 1993-1998 in both U.S. and Canadian numbers were at very low levels in both regions of the lake. waters (Casselman and Scott 2003; Owens et al. 2003). The Heavy fish predation was one likely factor causing reduction 1992 decline in overall numbers was due entirely to the col-in alewife numbers in the lake during the late 1990s, but per- lapse of the dense population of poor conditioned, slow-haps more important was the absence of a strong year-class growing fish in the southeast corner of the lake at depths during 1992-1997. Year-class strength was judged from >70 m (Owens and Weber 1995; Owens and Noguchi 1998).

catches of age-1 fish in spring bottom trawls in U.S. waters. Owens et al. (2003) hypothesized that the decline of slimy Of the six year-classes produced during 1992-1997, two sculpins was due to reductions in productivity brought on by were the smallest produced during 1977-1997, one was the nutrient abatement and to reductions in Diporeia, an impor-sixth smallest produced, and two were close to the long-term tant food of slimy sculpin, brought on by Dreissena coloniza-average. The waning population of alewife posed a new tion.

problem for fishery managers. Because the alewife fueled an economically important recreational fishery, managers rec- Threespine stickleback, emerald shiner, and deepwater ognized that they must conserve the diminished alewife pop- sculpin ulation to preserve the recreational fishery. The recent emergence of native fishes like threespine stickle-Alewives are both primary prey of salmonines and impor- back and emerald shiner reflects a significant change in the tant predators on zooplankton (Urban and Brandt 1993), lar- Lake Ontario fish community. Owens et al. (2003) suggested val fish (Krueger et al. 1995; Mason and Brandt 1996), and that the seminal event that allowed these native fishes to re-macrozooplankton (Bythotrephes and Mysis; Mills et al. produce successfully was a relaxation of predation on their 1992). Zooplankton stocks, juvenile alewife abundance and larvae resulting from the shift of alewife to deeper water.

growth, and growth of young salmonines were tightly Deepwater sculpin, once abundant in the deeper waters of linked. OGorman et al. (1987) found that first-year growth the main basin (Dymond et al. 1929), were not reported in of coho salmon did not depend on size at release but was re- southern Lake Ontario during 1943-1971, and Christie lated to the biomass of age-0 alewives, whereas second-year (1973) reported that the last specimen identified from north-growth of coho salmon depended on biomass of yearling ale- ern Lake Ontario was taken in 1953. Deepwater sculpin wife (OGorman et al. 1987). In Lake Ontario, growth of were listed by Crossman and Van Meter (1979) as being age-1 alewives during 1984 to 1991 was slower than that in present in 1972-1975, although they noted that the fish were Lake Michigan during the 1960s and Lake Huron during the extremely rare and considered endangered. After 1972-1970s, when alewives were an important forage species in 1975, deepwater sculpin were not reported until 1996-1998, the fish communities of those two lakes. Growth of age-1 when one was caught in the Ontario Ministry of Natural Re-alewives in Lake Ontario was dependent on epilimnetic zoo- sources index trawling program in 1996. Thereafter, a few plankton density and the number of competing age-0 ale- were taken near mid-lake in the northeast (Casselman and wives (OGorman et al. 1997). Consequently, the link of the Scott 2003), and in 1998, one was caught off the southwest youngest cohorts of alewife to zooplankton is important, as shore, the first sighting of this formerly abundant fish in these fish represent the bulk of alewife production in Lake U.S. waters since 1942. The reappearance of deepwater scul-Ontario and are critical to supporting a substantial piscivore pin in Lake Ontario was the last in a series of changes in the sport fishery. open-water fish community that followed the shift of rain-bow smelt and alewives to deeper water in the early 1990s Rainbow smelt (OGorman et al. 2000). The distribution shift coincided with, Rainbow smelt are the second most abundant open-water and was probably a result of, the colonization of Lake On-fish in Lake Ontario (Casselman and Scott 2003) and feed on tario by dreissenids and the concomitant increase in water a variety of food resources including zooplankton, non-native clarity.

480 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 Fig. 5. Indices of relative abundance of alewives yearling and older for a 21-year period, 1978 to 1998, caught in trawl tows of 10-min duration in late April - early May in U.S. waters of Lake Ontario (). Index is a 3-year running mean standardized to the maximum stratified CPUE (catch-per-unit-effort) indicated. Indices of abundance of alewife yearlings and older for a 27-year period, 1972 to 1998, caught in deep-water 1/2 nautical mile trawl hauls in the summer months, May-August, in the Canadian Kingston Basin of Lake Ontario (6 sites) (). Index is a 3-year running geometric mean standardized to the maximum CPUE indicated (from Casselman and Scott 2003).

Fig. 6. Indices of relative abundance () and biomass (bars) of age-1 and older rainbow smelt for a 21-year period, 1978 to 1998, caught in trawl tows of 10-min duration in late May - early June in U.S. waters of Lake Ontario. Indices are 3-year running means standardized to the maximum stratified CPUEs (catch-per-unit-effort) indicated.

Coregonids, blueback herring, and round goby along the south shore, but he noted that deepwater Remnant populations of lake herring (Coregonus artedii) coregonids were extremely scarce. Only one deepwater core-continue to persist in Lake Ontario (Casselman and Scott gonid (bloater) was caught in northwestern Lake Ontario in 2003; Owens et al. 2003), whereas four species of deepwater 1972 (Owens et al. 2003), the next documented catch was in coregonids that were present in Lake Ontario in the 1960s 1983, and thereafter none were caught. Although some na-(Todd and Smith 1992) have disappeared. In 1964, Wells tive fishes have disappeared in Lake Ontario, two new ex-(1969) reported catching bloater (C. hoyi), shortnose cisco otic fishes were reported in the late 1990s, the blueback (C. reighardi) and kiyi (C. kiyi) in experimental gill nets herring (Alosa aestivalis; Owens et al. 1998) and the round

Mills et al. 481 goby (Neogobius melanostomus; Charles ONeill, New York to 1.7 million fish annually from 1994 to 1996. Because of Sea Grant, Brockport, NY 14420, personal communication). stakeholder demand and a second management review (Stew-Historically, lake whitefish (Coregonus clupeaformis) were art et al. 1999), stocking was increased slightly in 1997 and an important component of the Lake Ontario commercial ranged from 2.0 to 2.2 million fish annually from 1997 to fishery and an abundant species in the cold-water fish com- 1999.

munity of eastern Lake Ontario (Christie 1973; Hoyle et al.

2000). Most of the commercial harvest was taken from the Lake trout Kingston Basin, where two major spawning stocks exist, a Native lake trout declined to extinction in Lake Ontario by stock that spawns in the Bay of Quinte and a stock that the 1950s (Christie 1973). Hatchery-reared lake trout stocked spawns along the south shore of Prince Edward County. By in the Kingston Basin during 1953-1964 survived well (Pearce the mid-1960s, these two stocks and the fishery that they et al. 1980), but few survived to sexual maturity because of supported had collapsed; only a remnant population per- harvest by commercial gill nets and predation by sea lam-sisted through the late 1960s and 1970s. Bottom trawling preys (Christie 1973). Control of sea lamprey began in 1971 surveys indicated that age-0 lake whitefish production began (Elrod et al. 1995), and lake trout stocking was renewed in to increase in the late 1970s (lake stock) and early 1980s 1973 with the goal of restoring a self-sustaining population (bay stock). After 1986, significant age-0 production was (Schneider et al. 1983). Nine genetic strains were used in the more consistent, as evidenced by large year-classes in 1987, restoration effort, six non-Great Lake strains, two Lake Su-1991, 1992, 1994 and 1995. Cold fall and winter conditions, perior strains, and one mixture of genetic strains of hatchery followed by more ideal warm summers, combined with the fish that survived to maturity in Lake Ontario (Elrod et al.

reduction of large rainbow smelt, resulted in a substantial 1995). Lake trout stocking increased from 66 000 fish in lake whitefish resurgence in the late 1970s and early 1980s 1973 to 1.9 million fish in 1985 and was maintained above (Casselman et al. 1996). By the early 1990s, the stocks had 2.0 million fish annually until 1992. Changes in stocking recovered to historically high levels of abundance, and had policy to reduce predation on alewife (OGorman and Stew-accumulated a large spawning-stock biomass composed of art 1999; Stewart et al. 1999) resulted in reductions in lake several strong year-classes (Casselman et al. 1996). Lake trout stocking in 1993. Management efforts to minimize whitefish abundance peaked in the early 1990s but declined mortality from sea lampreys, anglers, and commercial fish-rapidly after 1992. The deepwater amphipod Diporeia has ers and the shift in stocking to mostly Seneca strain (a ge-been an important food item of Lake Ontario lake whitefish netic strain with a higher survival rate than other strains)

(Ihssen et al. 1981), and with the collapse of this important contributed positively to rebuilding the lake trout population food resource in the Kingston Basin by the mid-1990s, lake (Marsden et al. 1989; Elrod et al. 1995).

whitefish shifted to other prey items, notably dreissenid Despite seemingly adequate numbers of mature lake trout mussels. Associated with the shift in diet to Dreissena came through much of the 1980s and into the early 1990s (Selgeby a dramatic decline in lake whitefish body condition and their et al. 1995), there were few reports of naturally produced population decline (Hoyle et al. 2003). fish except for fry captured on spawning shoals (Casselman 1995; Elrod et al. 1995; Krueger et al. 1995). Failure of the Chinook salmon hatchery-origin fish to reproduce was puzzling because The combination of angler preference for large, fast- many of the factors thought to contribute to reproductive growing salmon, the desire of fishery managers for a predator failure of lake trout in the Great Lakes were relaxed, and that would control large numbers of alewife, and the compa- ecosystem quality seemed to be improving (Fitzsimons et al.

rably lower hatchery production costs of chinook salmon led 2003). Contaminant levels, which were of great concern for to chinook salmon becoming the key player in the species adult lake trout (Huestis et al. 1996) and lake trout fry mix of Lake Ontarios salmonid community. By 1982, chi- (Fitzsimons 1995), declined in the 1970s and 1980s. Nutri-nook salmon was the principal salmonid predator in Lake ent levels (Nichols and Hopkins 1993) and excessive peri-Ontario and represented between 32 and 54% of annual phyton growth, which might cause low oxygen levels in stocking from 1982 to 1999. Predation by chinook salmon interstitial spaces on spawning reefs during egg incubation on alewife was so effective that managers became concerned (Sly 1988) had declined and were now less likely to impact that predator demand might outweigh prey supply. Because lake trout recruitment. Alewives, however, were suspected to of their high abundance and fast growth, chinook salmon ac- impede resurgence of lake trout because of their ability to counted for an estimated two-thirds of the lakewide predator exert heavy predation pressure on lake trout fry (Jones et al.

demand for alewives (Jones et al. 1993). Consequently, man- 1995; Krueger et al. 1995) and because of their ability to in-agement of predator demand required modification of chi- duce mortality of fry through thiamine deficiency brought nook salmon stocking levels, but because of the popularity about by a maternal lake trout diet of alewife (also known as of this recreational fishery, changes in stocking levels were early mortality syndrome (EMS); Fitzsimons et al. 1999).

controversial. As a result, chinook salmon stocking numbers Although thiamine levels and the resulting EMS were not received considerable attention and public scrutiny in the sufficient to completely block recruitment, the sublethal ef-United States and Canada (OGorman and Stewart 1999; fects resulting from the thiamine deficiency pose a significant Stewart et al. 1999). Stocking numbers peaked in 1984 at 4.2 bottleneck to recruitment. Nevertheless, naturally produced million fish and ranged from 3.2 to 3.6 million fish from age-1 and older lake trout of the 1993-1998 year-classes 1985 to 1992. Chinook salmon stocking was reduced sub- were found in low numbers throughout U.S. waters stantially during 1993-1994, based on a management review (OGorman et al. 1998; Owens et al. 2003). Reasons for the in 1992 (OGorman and Stewart 1999), and ranged from 1.5 abrupt shift from consecutive years of total reproductive fail-

482 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 ure to consecutive years of limited reproductive success, as (Casselman 2002). Even though recruitment conditions were evidenced by nearly 150 young native lake trout caught in especially good in the late 1990s, particularly in the ex-U.S. and Canadian waters from 1993 to 1999, are not clear. tremely warm summers of 1995 and 1998, smallmouth bass However, the shift coincided with a peak in potential egg abundance has not shown any significant resurgence.

deposition (B.F. Lantry and R. OGorman, unpublished data) and a change in the springtime distribution of alewives Double-crested cormorants away from nearshore lake trout nursery areas to deeper wa- Double-crested cormorants were first observed in Canadian ter (OGorman et al. 2000). waters of Lake Ontario in 1938 and in New York waters of Lake Ontario in 1945 (Weseloh and Ewins 1994). Breeding Walleye, yellow perch, and smallmouth bass numbers remained low until the late 1970s mainly because of Walleye (Stitzostedion vitreum) is an important keystone contamination of Lake Ontario by organochlorine compounds, predator of the inshore fish community of eastern Lake On- particularly DDE (dichlorodiphenyldichloroethylene), which tario and the associated embayments. Walleye reached record- resulted in thinning and breakage of eggshells and reproduc-setting high levels in the Bay of Quinte in the early 1990s. tive failure (Weseloh 1987). The combination of reduced lev-This increase occurred as the result of a major resurgence in els of DDE, augmented by an abundant food supply (primarily the late 1970s after the population had been at record-setting alewife), and their protected status in both the U.S. and Can-low levels in the early 1970s. The resurgence began as a ada led to a dramatic resurgence of double-crested cormorants result of an extremely large year-class in 1978 after the in the 1980s and 1990s. The number of nests occupied by winterkill of its larval predators, alewife and white perch, cormorants on Little Galloo Island (LGI) in eastern Lake which occurred after the severe winters of 1976-1977 and Ontario and numbers of the Lake Ontario population increased 1977-1978 (Casselman and Scott 2003). In the late 1980s dramatically after 1974 (Fig. 7). LGI nest numbers continued and early 1990s, the walleye population of the Bay of Quinte to increase through 1996, after which they declined in re-moved down the bay as spawning runs of alewife, an impor- sponse to intense human activity associated with pellet collec-tant prey species for walleye, diminished. Although large tions and eventually egg oiling. Reproductive output and walleye have seasonal migrations between the Bay of Quinte survival of cormorants appears to be tightly linked with ale-and eastern Lake Ontario, this shift, along with the increased wife abundance; years with abundant alewife populations have abundance of walleye, initiated their dispersion out of the also been years of high postfledging survival of these birds lower Bay of Quinte into eastern Lake Ontario. This was ac- (Wesloh and Ewins 1994). Although cormorants seemingly celerated in the early 1990s by the progressively increasing have had little overall effect on alewife stocks, subsequent transparency caused by dreissenid colonization (Casselman studies in U.S. waters of the eastern basin of Lake Ontario in-and Scott 2003). In the mid-1990s, walleye abundance in- dicated adverse impacts on smallmouth bass (Lantry et al.

creased in New York waters of Lake Ontarios eastern basin. 2002) and yellow perch (Burnett et al. 2002).

This increase, which was also seen in the upper St. Lawrence River, no doubt reflected the dispersion of the Bay of Quinte The Lake Ontario food web stock. Coincident with this decrease, yellow perch (Perca flavescens) abundance increased substantially throughout the Ecological changes in the Lake Ontario food web have Bay of Quinte at a time when the species was generally de- been dramatic since SCOL I (Fig. 8). In the decades of the creasing in the eastern basin of Lake Ontario in both New 1950s and 1960s, pessimism prevailed among Great Lakes York and Ontario waters. scientists and managers as depreciation of the fish communi-Yellow perch were at record-setting high levels in north- ties and degradation of water quality proceeded unabated.

eastern Lake Ontario in the late 1970s and early 1980s but By 1970, lake trout populations were gone; the few remain-declined precipitously in the mid-1980s. Among the many ing salmonids were riddled with sea lamprey wounds; cul-factors associated with these dynamics was a massive winter- tural eutrophication resulted in excessive algal growth and kill of alewives, significant predators of yellow perch larvae low water clarity; proliferation of alewife led to intense zoo-(Mason and Brandt 1996), in the late 1970s followed by a planktivory and a predominance of small cladocerans and strong rebound in the 1980s. A shift in alewife distribution cyclopoid copepods; and only a remnant population of lake in the early 1990s boosted yellow perch reproductive success, whitefish persisted (Figs. 8a and 8b). In ensuing years, oli-but it was followed by increased double-crested cormorant gotrophication would drive the recovery process and the food (Phalacrocorax auritus) predation that appears responsible for web would be greatly altered (Figs. 8c and 8d). In 2000, sea decreasing yellow perch abundance in eastern Lake Ontario lamprey predation no longer plagued salmonids and burbot, in recent years (Burnett et al. 2002). In unison, smallmouth exotic alewife supported a Pacific salmon recreational fish-bass (Micropterus dolomieu), which were at record-setting ery, and invasion of Dreissena spp. fostered new trophic high levels in the late 1970s, 1980s, and early 1990s interactions. The microbial food web, including bacteria, cil-throughout the eastern basin of Lake Ontario, also reached iates, heterotrophic nanoflagellates, and picoplankton, was record-setting low levels in the late 1990s, with some weak identified as an essential pathway of energy transfer to zoo-recovery. This decrease has been associated with cormorant plankton in 2000, but less so in 1970. Food web changes predation in both U.S. and Canadian waters (Lantry et al. were most evident in the nearshore: Diporeia disappeared, 2002; Casselman et al. 2002). Some of the poorest year- seven new invasive species were established (Cercopagis, classes seen in decades occurred in the early 1990s as a re- zebra and quagga mussels, round goby, New Zealand mud sult of several extremely cold summers, especially in 1992 snail, blueback herring, and Echinogammarus), fingernail as a result of the Mount Pinatubo eruption in 1991 clams disappeared, double-crested cormorant populations

Mills et al. 483 Fig. 7. Numbers of double-crested cormorant nests on Little Galloo Island (solid) and elsewhere (shaded) in Lake Ontario, 1974-2001 (after Weseloh and Ewins 1994; D.V. Weseloh, Canadian Wildlife Service, 4905 Dufferin Street, Downsview, ON M3H 5T4, and I.M. Mazzocchi, New York State Department of Environmental Conservation, 317 Washington Street, Watertown, NY 13601, personal communications).

greatly expanded, and water clarity increased dramatically. time of the GLWQA, however, was the connection between In offshore waters, quagga mussels dominated over zebra phosphorus supply and higher trophic level production, es-mussels, and Cercopagis proliferated in late summer and pecially to fish. Subsequently, scientists realized the need early fall. Despite these changes, omnivorous Mysis popula- for long- and short-term studies that focused on food web tions remained relatively stable, coupling the benthic and pe- linkages and managers recognized their value in decision-lagic food webs. making. Scientists and managers would take longer to ac-knowledge that the issues of water quality and fisheries pro-Discussion duction were linked.

Control of the sea lamprey was effective in improving sur-Fisheries exploitation, exotic species introductions, and eutro- vival of salmonid piscivores in Lake Ontario, and advances phication were highlighted in SCOL I as factors contributing in fish culture technology made it possible to establish large to the degradation of the Great Lakes fish communities. Sea populations of salmonid predators that were effective in con-lamprey and the alewife were recognized as particularly im- trolling alewife. With an expanding salmonid predator popu-portant predators impacting the Lake Ontario fish commu- lation, fish managers were forced to address ecological issues nity (Christie 1972; Christie et al. 1987b). Since 1970, related to food availability, habitat, genetics, disease patho-negative effects of exploitation on native fishes, eutrophi- gens, and exotics, as well as the concerns of the stakeholder cation, sea lamprey, and alewife have been largely amelio- public.

rated, thereby contributing greatly to restoration efforts in Managers hoped that the path to recovery of the Lake Lake Ontario. However, establishment of new non-native spe- Ontario ecosystem following the 1950s and 1960s would be cies continues to hamper goals to restore Lake Ontarios his- a return to historic conditions. Early signals were consis-toric fish communities, and the resultant ecological effects tent with this expectation. For example, nuisance algal may be irreversible. blooms dramatically declined, and native species like lake Managers of the Lake Ontario ecosystem in the late 1960s trout responded positively to nutrient abatement and sea and early 1970s recognized that water quality degradation lamprey control (Elrod et al. 1995). Further, structural was so extreme that the lake environment had to be changed, changes in the size and species composition of phyto-but that there were no guarantees of success. Results of eco- plankton indicated a shift from a eutrophic to an oligotrophic logical studies indicated that eutrophication could be reversed community, and associated with this change, smaller-sized or-through mandated controls on phosphorus loading to the ganisms contributed more to primary productivity lake (Neilson and Stevens 1987). The GLWQA signed by (Munawar and Munawar 2003; Munawar et al. 2003). How-the U.S. and Canada in 1972 provided the backdrop to re- ever, the recovery of Lake Ontario would take a new eco-duce total phosphorus levels in offshore waters of Lake On- logical path one that would be markedly influenced by tario to a target level of 10 µg*L-1 and set the stage for unplanned exotic species introductions. Restoration of lake oligotrophication. Skeptics predicted that environmental trout, for example, would be hampered by a thiamine defi-change was either irreversible or would take decades, but ciency of the fry brought about by adult lake trout feeding Lake Ontario (and Lake Erie) responded soon after remedial on non-native alewife (Fitzsimons et al. 1999). Lake measures were in place. What was not appreciated at the whitefish populations would no longer thrive because of

Fig. 8. The food web of Lake Ontario in (a and c) nearshore and (b and d) offshore habitats in (a and b) 1970 and (c and d) 2000. Thin solid arrows indicate the direction of 484 energy flow, thin dotted arrows represent energy flow to and from exotic species introduced since 1970, wide bidirectional arrows represent migration through the water col-umn, and wide unidirectional arrows represent relative depth of light penetration. All exotic species are stippled, and those that arrived post-1970 are dark stippled. Predator-prey interactions involving larval fish are not depicted.

Mills et al. 485 the loss of their preferred food, the native burrowing am- clarity may have caused alewife, rainbow smelt, and age-2 phipod Diporeia. The lesson of the last three decades has lake trout to shift to deeper water (OGorman et al. 2000).

been clear: the trajectory of the recovery process of the The outcome of this shift to deeper waters is currently un-Lake Ontario food web has taken new and unpredicted eco- clear; we can only speculate that this behavior modification logical paths. will increase predation on Mysis.

Restoration of Great Lakes native fish communities has Zebra mussel-induced water clarity changes likely redi-often been considered a signal toward the return to a healthy rected nearshore energy production to the benthic habitat ecosystem. Re-establishment of the bloater into Lake On- and perhaps made this habitat more vulnerable to invasion tario would satisfy those who desire an ecosystem that sup- by other non-native species. So far, Echinogammarus, the ports a self-reproducing diverse fish community. Historically, New Zealand mud snail, and the round goby have become bloater populations developed migration patterns in close as- established in nearshore benthic habitat during the 1990s.

sociation with their migrating prey, Mysis. Mysids are abun- Round gobies will likely play a special role in the coming dant in Lake Ontarios abyss; offshore biomass of Mysis can decades by functioning as a benthic prey and an energy vec-exceed that of alewife and rainbow smelt combined tor between Dreissena spp. and other fish species, especially (Johannsson et al. 2003). The lack of a significant deepwater with the current declines in Diporeia and slimy sculpins.

predator on Mysis is a missing link in the Lake Ontario food Gobies, however, are unlikely to replace slimy sculpins as web, a link that limits movement of energy in the offshore the preferred prey of juvenile lake trout during thermal strat-pelagia. Evidence of improved environmental conditions, re- ification because bathymetric distributions of the two species duced alewife and rainbow smelt densities, and an abundant differ, gobies on bottom above the thermocline and juvenile offshore mysid population in Lake Ontario would seemingly lake trout on bottom well below the thermocline. However, provide a window of opportunity to re-establish bloaters. if gobies migrate to greater depths in fall as thermal stratifi-However, collapse of Diporeia has eliminated the likelihood cation weakens, they will likely provide food for juvenile of fully restoring bloater because Diporeia was the most and adult lake trout and, as such, would partly fill the func-important food in bloater diets, followed closely by Mysis tional role of slimy and deepwater sculpins. Interestingly, fu-(Wells and Beeton 1963). Although competition for food ture food web scenarios for Lake Ontario will no doubt with dreissenids is suspected as the cause for collapse of ponder organism functionality: does it matter whether lake Diporeia, other factors like disease from pathogens are also trout feed on gobies or sculpins if these benthic fishes serve possible. If mysid populations collapse, restoration of bloater similar functional roles in the food web?

to any level of abundance would be impossible. In spite of dramatic ecological changes in the Lake Ontario Concerns about the role of persistent organic contaminants ecosystem since 1970, Mysis populations have exhibited little on the demise of lake trout in Lake Ontario and continued response to oligotrophication (not surprising considering its prevention of successful rehabilitation when reintroduced to origins in the Great Lakes as a glacial relict), increased wa-the lake in the 1970s have persisted for decades (Zint et al. ter clarity, and top-down effects of fisheries management. In 1995; Cook et al. 1997). The suspected historic causality of contrast to zooplankton, Diporeia, and alewife, Mysis have contaminants on lake trout mortality can never be defini- shown no trend in abundance over time. However, Mysis tively determined because we have only recently seen the production has increased, due possibly to declines in larger development of the appropriate analytical methods to detect alewife, which feed on Mysis (Johannsson et al. 2003). This and quantify such contaminants. Nevertheless, new method- low level of response of the mysid population could be due ologies and assessment techniques (Cook et al. 1997) now to their migration patterns and their use of a habitat that by permit much more definitive statements of the current poten- volume is the largest in the lake. Combined, these factors ef-tial of contaminants to block fish reproduction (Fitzsimons fectively minimize predation on Mysis from visually feeding 1995). The effects of contaminants on lake trout and other fish. In addition, omnivory allows Mysis to switch to algae fish species will continue to evolve in the coming decades. or detritus if their primary zooplankton prey declines. The Although larval lake trout are highly sensitive to dioxin and main importance of Mysis for alewife, the primary forage dioxin-like chemicals, residue levels are now below those as- fish in the lake, may be one of energy storage (Johannsson et sociated with acute toxicity. The synergistic effects of these al. 2003), as metalimnetic zooplankton production can be other organochemicals on newly discovered phenomena like stored as mysid biomass and fed on by alewife from fall diet-induced thiamine deficiency (Fisher et al. 1996; through spring when distribution of the two species overlap.

Fitzsimons et al. 1999) on fish, for example, is unknown but The consequence of the availability of Mysis as food for ale-needs to be explored. wife in the fall could have significant implications for their One of the most dramatic changes in the Lake Ontario eco- growth, gonad development, and overwinter survival.

system since SCOL I was improved water clarity resulting from both oligotrophication and the invasion of dreissenids. Moving into the future Increased water clarity has resulted in far-reaching trophic interactions that could have profound effects on predator- The post-1970 era provided assurance to skeptics that en-prey interactions. For example, food availability of visual vironmental change in the Lake Ontario ecosystem associ-predators may be influenced more by changes in light re- ated with chemical pollution was reversible. What was not gime than by changes in food abundance. In addition, water so surprising from the lessons learned over the past 30 years clarity changes may also modify the behavior of organisms in Lake Ontario was that environmental change associated and their impact on the food web. Following the establish- with biological pollutants like invasive species is likely irre-ment of dreissenid mussels in Lake Ontario, increased water versible. Once established, exotics rarely disappear although

486 Can. J. Fish. Aquat. Sci. Vol. 60, 2003 their role in the food web may change significantly. For ex- creasing water temperatures in late fall and early winter may ample, the role of the alewife has shifted over the last six negatively affect fish survival and fry emergence, notably of decades from a nuisance species to a pivotal species that lake trout and lake whitefish (Casselman 1995). At the same supports a multimillion dollar salmonid sport fishery time, increasing water temperature could result in stronger (OGorman and Stewart 1999). Ironically, although environ- year-classes of warmwater species such as smallmouth bass mental conditions have greatly improved in Lake Ontario (Casselman et al. 2002) and alewife.

and the other Great Lakes since 1970, this period has coin- The initial introduction of salmonids into the Great Lakes cided with an acceleration of newly established exotics (Mills was an attempt to control nuisance levels of alewife but et al. 1993). Global transport of organisms associated with quickly focused on developing a multimillion dollar recre-shipping and the establishment of organisms from distant ational fishing industry (OGorman and Stewart 1999). The places like the Black, Caspian, and Baltic seas were unheard strategy for rehabilitation of lake trout, and later Atlantic of in the decades prior to 1970. Although the intent of the salmon, in Lake Ontario had strong scientific and ecological U.S. - Canada Water Quality Agreement was to restore and underpinnings (Schneider et al. 1983; Elrod et al. 1995).

maintain the chemical, physical, and biological integrity of However, the overwhelming desire of stakeholders to have a Great Lakes waters, release of untreated waters from the recreational fishery made up of naturalized and exotic ballast tanks of foreign vessels into the Great Lakes was not salmonids has been the driving force of fish management to considered within the mandate. We anticipate that future date. Consequently, the current Lake Ontario fish commu-policy efforts will institute measures that will reduce the risk nity is largely composed of a mix of exotic species that have of introducing new biological pollutants to Lake Ontario and no evolutionary sympatry. In addition, control of these other Great Lakes. salmonids and their associated top-down influence on fish Over the last two decades, Lake Ontario has experienced communities (Christie et al. 1987a; McQueen et al. 1989) is significant reductions in phosphorus with a concomitant shift largely regulated through stocking. As a result, conventional toward oligotrophy and a dramatic increase in water clarity ecological paradigms are difficult to apply, and descriptions resulting from both nutrient reduction and proliferation of of historical fish community structures are not useful for filter-feeding Dreissena spp. Macroinvertebrate activity has understanding or predicting species interrelationships or increased since invasion by Dreissena spp. (Stewart and equilibrium states (Christie et al. 1987b; Eshenroder and Haynes 1994), light penetration has increased, benthic- Burnham-Curtis 1999). Managers of the Lake Ontario fish-feeding round gobies have become established, and benthic ery resource will be challenged in the coming decades as the algae like Cladophora have reportedly reached nuisance lev- ecosystem changes and will need to rely on the use of such els again (Charles ONeill, New York Sea Grant, Brockport, tools as ecological modeling and risk assessment to gain in-NY 14420, personal communication). We suggest that these sights into outcomes and consequences of management deci-events are in response to greater light penetration and reflect sions.

a redirection of energy production from the pelagic to the Multiple biological and physical stressors have caused pro-benthic habitat. We contend that the combined effects of found changes in the Lake Ontario ecosystem and its fish oligotrophication and dreissenid-induced modifications will community during the last three decades. Major stressors favor benthic over pelagic energy pathways, particularly in over this period included oligtrophication, invasion by dreis-nearshore and embayment habitats of Lake Ontario. We ex- senids that led to increased water clarity and benthification, pect that this shift in the direction of energy flow will have fisheries management through stocking of exotic salmonids dramatic ecological consequences for Lake Ontario in the and lamprey control, climate change, establishment of other future by favoring colonization of bottom-dwelling organ- exotics, and harvest by anglers and cormorants. Responses isms, promoting fish communities that make efficient use of to these stressors have led to significant changes in the fish the benthic habitat, and enhancing growth rate cycles of ben- community, including declines in alewife, declines in native thic algae and submersed aquatic vegetation. Thus, we ex- sculpins and lake whitefish populations, increases in some pect this shift in importance of benthic processes (termed native fishes associated with lamprey control, shifts in ale-benthification) to be coupled with the oligotrophication of wife spatial distribution, and declines of fish species result-Lake Ontario. ing from double-crested cormorant predation. In addition, The role of climate and global climate warming are new fish recruitment bottlenecks have resulted from alewife-induced stressors that were not identified in SCOL I. Summer and thiamine deficiency. We expect stressor impacts to continue early winter inshore water temperatures have increased signif- to shape the Lake Ontario ecosystem in the future and rec-icantly in Lake Ontario over the past several decades, parallel- ommend continuous long-term ecological studies to enhance ing global warming and temperature extremes, particularly scientific understanding and management of this important those associated with El Nino and La Nina (Casselman 2002). resource.

We expect that future global warming will lead to increasing wa- As we move into the future, we will continue to wrestle ter temperatures in Lake Ontario and thereby affect fish commu- with the more global issue of sustainability of fishery re-nity dynamics and their habitat. Global warmings impact on fish sources. Some will argue for maximum utilization of the species may be either positive or negative depending on species- fishery resources, and others will be more concerned with specific thermal requirements and changes in thermal habitat. further restoration of native, natural species assemblages.

Rising temperatures associated with seasonal climate events Regardless of which fish community type is preferred, un-could positively affect salmonids by increasing the habitat vol- derstanding how fish integrate into the Lake Ontario food ume for cold-water species in well-oxygenated lakes like web and respond to environmental change has become em-Lake Ontario (Magnuson et al. 1990). On the other hand, in- bedded in the thinking of managers and scientists alike. The

Mills et al. 487 Lake Ontario ecosystem and its stakeholders have and will Burnett, J.A.D., Ringler, N.A., Lantry, B.F., and Johnson, J.H. 2002.

continue to profit from this philosophy in the coming de- Double-crested cormorant predation on yellow perch in the east-cades. The Lake Ontario food web and energy partitioning ern basin of Lake Ontario. J. Gt. Lakes Res. 28: 202-211.

within the food web has become more complex, particularly Casselman, J.M. 1995. Survival and development of lake trout eggs in the nearshore waters. Understanding ecological processes and fry in eastern Lake Ontario in situ incubation, Yorkshire involving benthic and pelagic food web pathways and their Bar, 1989-1993. J. Gt. Lakes Res. 21(Suppl. 1): 384-399.

linkages to fish will continue to challenge both our scientific Casselman, J.M. 2002. Effects of temperature, global extremes, and understanding and desire to manage large-lake ecosystems. climate change on year-class production of warmwater, coolwater, and coldwater fishes in the Great Lakes Basin. In Proceedings of The challenges for scientists, managers, and stakeholders in American Fisheries Society Symposium 32, Fisheries in a the coming decades will be monumental, as expectations Changing Climate, 20-21 August 2001, Phoenix, Ariz. Edited by will be hampered by ecological surprises resulting from an- N.A. McGinn. American Fisheries Society, Bethesda, Md. pp. 39-thropogenic forces like climate warming and exotic species 59.

invasions. We hope that society will invest in the future of Casselman, J.M., and Scott, K.A. 1992. Fish community dynamics Lake Ontario and other Great Lakes as they are some of the of the outlet basin of Lake Ontario. In Lake Ontario Fisheries greatest natural resources on earth. Unit, 1991 Annual Report, Section 18. Ontario Ministry of Nat-ural Resources, Picton, Ont.

Acknowledgments Casselman, J.M., and Scott, K.A. 2003. Fish-community dynamics of Lake Ontario long-term trends in the fish populations of We are grateful to the Operations staff at the Glenora eastern Lake Ontario and the Bay of Quinte. In State of Lake Fisheries Station and the legions of summer students, all of Ontario: Past, Present and Future. Edited by M. Munawar.

whom, for over more than four decades, have carefully and Ecovision World Monograph Series, Aquatic Ecosystem Health diligently conducted the Ontario Ministry of Natural Re- and Management Society, Burlington, Ont. In press.

sources (OMNR) sampling programs reported here. This Casselman, J.M., Hoyle, J.A., and Brown, D.M. 1996. Resurgence study is Contribution No. 01-01 of the Applied Research and of lake whitefish, Coregonus clupeaformis, in Lake Ontario in Development Branch, Aquatic Research and Development the 1980s. Gt. Lakes Res. Rev. 2: 20-28.

Section, Ontario Ministry of Natural Resources. We also Casselman, J.M., Brown, D.M., Hoyle, J.A., and Eckert, T.H. 2002.

gratefully acknowledge C.P. Schneider and J.H. Elrod for Effects of climate and global warming on year-class strength guiding the fishery assessment programs in U.S. waters for and relative abundance of smallmouth bass in eastern Lake On-two decades. We thank the crews of the research vessels Seth tario. In Black bass: ecology, conservation, and management.

Green, Kaho, Bayfield, Lauzier, and Limnos and the Techni- Edited by D.P. Philipp and M.S. Ridgway. American Fisheries Society, Symposium 31, Bethesda, Md.

cal Operations personnel, biologists, and technicians who Chapra, S.C., and Sonzogni, W.C. 1979. Great Lakes total phos-staffed these vessels during the often-arduous fish and phorus budget for the mid-1970s. J. Water Pollut. Control Fed.

biomonitoring surveys. This article is Contribution No. 206 51: 2524-2533.

of the Cornell Biological Field Station and No. 1228 of the Christie, W.J. 1972. Lake Ontario: effects of exploitation, introduc-U.S. Geological Survey Great Lakes Science Center. We tions, and eutrophication on the salmonid community. J. Fish.

thank H. Niblock, S. Carou, K. Ralph, and D. Graham (Fish- Res. Board Can. 29: 913-929.

eries and Oceans Canada), D. Lynn (University of Guelph), Christie, W.J. 1973. A review of the changes in the fish species and G. Sprules and M. Legner (University of Toronto) for all composition of Lake Ontario. Gt. Lakes Fish. Comm. Tech. Rep.

their contributions and R. Klumb, D. Fitzgerald, N. Lester, No. 23.

and two anonymous reviewers for providing helpful com- Christie, W.J., Scott, K.A., Sly, P.G., and Strus, R.H. 1987a. Re-ments on the manuscript. We are indebted to M. Hansen, S. cent changes in the aquatic food web of eastern Lake Ontario.

Kerr, and the Great Lakes Fishery Commission for facilitat- Can. J. Fish. Aquat. Sci. 44(Suppl. 2): 37-52.

ing this important endeavor. Christie, W.J., Spanlger, G.R., Loftus, K.H., Hartman, W.L., Colby, P.J., Ross, M.A., and Talhelm, D.R. 1987b. A perspective on Great Lakes fish community rehabilitation. Can. J. Fish. Aquat.

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