ML070160216
| ML070160216 | |
| Person / Time | |
|---|---|
| Site: | FitzPatrick  |
| Issue date: | 12/31/2004 |
| From: | Adams J, O'Gorman R, Owens R, Prindle S, Schaner T Govt of Canada, Ontario Ministry of Natural Resources, State of NY, Dept of Environmental Conservation, US Dept of Interior, Geological Survey (USGS) |
| To: | Office of Nuclear Reactor Regulation |
| jmm7 | |
| References | |
| Download: ML070160216 (12) | |
Text
STATUS OF MAJOR PREY FISH STOCKS IN THE U.S. WATERS OF LAKE ONTARIO, 2004 R. O'Gorman and R. W. Owens U.S. Geological Survey, Great Lakes Science Center, Lake Ontario Biological Station Oswego, New York 13126 S. E. Prindle New York State Department of Environmental Conservation Cape Vincent, New York 13618 J. V. Adams U.S. Geological Survey, Great Lakes Science Center Ann Arbor, Michigan 48105 T. Schaner Ontario Ministry of Natural Resources Picton, Ontario K0K 2T0 Abstract We began a comprehensive re-analysis of our bottom trawl assessments, conducted annually since 1978, with a re-evaluation of the alewife Alosa pseudoharengus assessment. Although, the re-evaluation resulted in numerous changes to the calculation of alewife abundance, the new indices showed the same trends as the historical indices (Spearman rank correlation, P < 0.0001, r 0.95). Numerical and weight abundance indices for adult (age-2 and older) alewives in U.S. waters of Lake Ontario during spring 2004 were similar to those in spring 2002-2003 and were well below the long-term averages. The numerical abundance index for yearling alewives (2003 year class) was about 25% smaller in 2004 than in 2002-2003 and was below the long-term average for the fourth consecutive year. Wet weight condition of adult alewife in fall 2004 was higher than in any year since 1980 suggesting that the alewife population was more in balance with the productive capacity of the lake in 2004 than in any of the previous 23 years. Numerical and weight abundance indices for age-1 and older rainbow smelt Osmerus mordax in 2004 were markedly higher than the record lows recorded in 2003. The increase was due entirely to a strong 2003 year class and not to a decrease in mortality rates of adult rainbow smelt --
age-2 and older smelt remain scarce. We have lost the ability to track abundance of slimy sculpin Cottus cognatus along the south shore -- dreissenid numbers now preclude towing the trawl gear historically used to assess sculpins, and trawling with other gear produced inconsistent results. One deepwater sculpin Myoxocephalus thompsonii was collected in spring 2004.
Presented at: Great Lakes Fishery Commission Lake Ontario Committee Meeting Niagara Falls, Ontario March 29-30, 2005 Page 1 PROVISIONAL DATA NOT TO BE CITED
Figure 1. - Lake Ontario showing 12 areas sampled with bottom trawls.
Introduction surveys will not be biased if the fish are randomly distributed. We have always assumed that the fish The U.S. Geological Survey (USGS) and New are randomly distributed in the geographic area in York State Department of Environmental which a transect is located and, because we have Conservation (NYSDEC) have cooperatively numerous transects spaced at regular intervals assessed Lake Ontario prey fishes each year since around the shore, that our abundance indices are 1978. Bottom trawling has been conducted during unbiased. However, not until 2004 did we spring, summer, and fall to assess alewives Alosa initiate acoustic sampling to test the assumption of pseudoharengus, rainbow smelt Osmerus mordax, random distribution within geographic areas (see and slimy sculpins Cottus cognatus. Timing of below). If the fish are not randomly distributed the surveys was selected to correspond with the within geographic areas, mean abundance will be season when bottom trawl catches of the target biased, although if the non-random pattern of fish species peaked during May to October trawling distribution persists through time the differences conducted in 1972. Twelve transects were in mean abundance between years will be established at roughly 25-km intervals along the unbiased (Warren in ICES 1992). Although U.S. shoreline (Figure 1). Bottom trawling was random sampling is preferable for estimating generally conducted at all 12 transects in spring to precision, the systematic, fixed-station sampling assess alewives, at 11 transects in summer to that we employ in Lake Ontario will often be assess rainbow smelt, and at 6 transects in fall to optimal for getting the most precise estimate of assess slimy sculpins. At each transect, trawl relative abundance even though the variance of hauls were usually made at 10-m depth intervals the estimated relative abundance will be biased through the range of depths occupied by the target (ICES 2004).
species. Fixed station sampling designs, such as ours, are commonly used for assessing fish Two vessels participated in prey fish surveys populations in the Great Lakes and in northern during 1978-1982, the 19.8-m, steel hull R/V Europe (ICES 2004). The underlying assumption Kaho (USGS) and the 12.8-m, fiberglass hull R/V is that changes in relative abundance at the fixed Seth Green (NYSDEC). During 1983-1985, all stations are representative of changes in the whole assessment trawling was conducted by the Kaho population. Mean abundance from fixed station (the fiberglass Seth Green was permanently Page 2 PROVISIONAL DATA NOT TO BE CITED
retired in fall 1982). In 1985, the NYSDEC Re-analysis of the Alewife Assessment accepted delivery of a new R/V Seth Green and this 14-m, steel hull vessel participated with the An independent peer review of the USGS-Kaho in prey fish surveys during 1986-2002 and NYSDEC bottom trawling assessments of prey in 2004. Because of personnel shortages within fishes (primarily alewife) conducted in fall 2003 the NYSDEC, only the Kaho was used to assess found that the assessments provided reliable prey fish stocks in 2003. Intercalibration studies indices of trends in relative abundance and were conducted to determine if fishing power of suggested a number of strategies for improving the Kaho differed from that of either Seth Green assessment design and data analysis (New York (see below). Sea Grant 2004). In response to this review, we began a re-analysis of the alewife assessment and A 12-m (39-ft, headrope) bottom trawl and flat, plan on initiating similar re-analyses of other rectangular trawl doors were used for assessment assessments in the near future. The reviewers also fishing until 1997 when fouling by dreissenids suggested using acoustics to examine fish forced a change to a 3-in-1 trawl (18-m/59-ft distribution during the alewife assessment to test headrope) and slotted, cambered V-doors. We whether fish are concentrated near bottom and made a series of paired tows to determine homogenously distributed in the areas between the calibration factors for the two gears to allow transects. We initiated acoustic sampling during comparison of alewife and rainbow smelt catches the 2004 alewife assessment. We also extended made by the new gear with those made by our sampling to greater depths (170 m / 558 ft) but traditional trawling gear. However, up until 2004, have not as yet incorporated catches made there we continued to use the traditional trawling gear into the index calculations.
to assess slimy sculpins in those areas (mainly in deep water) where dreissenid density was We began our review of the alewife assessment by sufficiently low to allow us to trawl unimpeded. building, and verifying, an electronic file of all In 2004, the 3-in-1 trawl was used to assess slimy trawl catches made during the alewife assessments sculpins because increased dreissenid density in conducted during 1978-2004. A single electronic deeper water had greatly reduced not only the catch file for the survey was not previously number of depths where we could tow a trawl but available. Since the mid 1980s, yearly abundance also the amount of time we could tow at most indices were calculated by use of a spreadsheet depths. and prior to that by use of hand calculators. Next we revisited the validity of using fishing power In 2004, the number of trawl hauls made for correction factors (FPC) to account for changes in assessment of alewives, rainbow smelt, and slimy survey vessels and gear, redrew the sampling sculpins totaled 243 118 during April 20-May 8, frame and strata, and recast rules for adding zero 93 during June 1-June 10, and 32 during October catches (see below). Finally, we recalculated 8-25. The number of trawl tows made to assess alewife abundance indices and compared the new alewives was the largest since 1993 despite the indices to the old indices by use of the Spearman fact that the population can now be indexed more rank correlation.
reliably with fewer trawl hauls than in the past because the geographic and bathymetric Fishing Power Correction Factor distribution of alewives narrowed after dreissenids for Vessel and Gear colonized the lake in the early 1990s (OGorman et al. 2000). Alewives are no longer found off the A FPC has been applied to alewife catches made eastern shore in spring and off the south shore, by the steel Seth Green based on the results of they now concentrate in a narrow depth range. about 50 paired tows with the Kaho during 1985-Trawling effort during the rainbow smelt 1989 -- t-tests of the differences in log assessment was similar to that in recent years transformed catches indicated marginally whereas effort during the slimy sculpin significantly larger catches of yearling (P = 0.07) assessment was lower than that in most recent and adult alewives (P = 0.12) by the Seth Green.
years.
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Figure 2. - Stratified mean catch of adult alewives (age-2 and older) with bottom trawls in U.S.
waters of Lake Ontario shoreward of the 160-m (525 ft) bottom contour in late April-early May, 1978-2004. Mean catch in 2001 was estimated from bottom trawl catches in June 2001. For weight indices, 1kg =2.2 lb.
Figure 3. - Relative standard error (RSE) for yearling and adult alewife abundance indices in U.S. waters of Lake Ontario, 1978-2004. The RSE (RSE = 100%*{standard error of the index/the index})is a measure of variability in abundance indices.
Page 4 PROVISIONAL DATA NOT TO BE CITED
Munro (1998) developed a decision rule for FPC model as simple as possible, we searched for applying FPCs to trawl survey data. The a single depth cut off to be used for both life objective was to balance the trade-off between the stages of alewife based on all possible pair-wise reduction in bias and the increase in variance that ratios of log transformed catches. To reduce the comes with applying correction factors. Recently, effects of extreme ratios (caused by occasional a new decision rule (Adams and OGorman, in zero or very high catches), we minimized the prep.) was developed based on improvements median absolute deviation, medianlxi -
made to the one proposed by Munro (1998). The median(xi)l, of the ratio (rather than the sums of new decision rule is based on the root mean squares). The depth cut off that minimized the square error (RMSE) of a change in catch rate median absolute deviation was 91.5 m (300 ft).
(e.g., a change in abundance from one year to the next), which can be fixed in a simulation. The Fishing power corrections for each gear were RMSEs of the estimated change are estimated estimated for each alewife life stage and depth separately for each of three options: (1) no zone separately (<91.5 m and >91.5 m). Vessel correction factor applied to either vessel, (2) a effects were ignored in this analysis because correction factor applied to vessel A, and (3) a previous analyses of the two vessels pulling the correction factor applied to the vessel B. If the same 12-m trawl indicated that fishing power RMSE of the estimated change in catch rate is differed little between the two vessels. We used smaller when a correction factor is applied the Adams and OGorman (in prep.) modification regardless of vessel (2 and 3) than when no of the Munro (1998) decision rule to determine correction factor is applied (1), then, and only whether the estimated FPCs for gear should be then, is a FPC recommended. The RMSEs are applied. Measurement variability of both yearling calculated from simulated data, based on observed and adult alewives was high at depths <91.5 m distributions of catch in paired trawl hauls, over a (300 ft), contributing to the decision not to apply wide range of fishing power differences. Using the FPC to alewife catches made at those depths.
this method, we found that the FPC for alewife Measurement variability was much lower at calculated from the side-by-side trawling fell depths >91.5 m (300 ft) and the decision rule within the range that would not reduce error in indicated that a FPC should be applied to alewife tracking catch rates over time and concluded that catches at these depths.
a FPC was not needed for the steel Seth Green.
Recalculation of Alewife Abundance Indices A FPC was never applied to catches made by the fiberglass Seth Green, indeed, there was never a Indices of alewife abundance are simply stratified, rigorous analysis of 17 paired tows conducted weighted mean catch per tow.
with the Kaho in 1980. We again used the Adams and OGorman (in prep.) modification of the Yh N h Munro (1998) decision rule to evaluate if it would N be appropriate to apply a FPC to alewife catches and found that a FPC was not needed for the fiberglass Seth Green. Where Yh is the mean catch in either numbers or kg per 10-min tow in the hth stratum, N h is the To maintain continuity of our trawling data sets, we had applied a FPC to catches made with the 3- number of hectares in the hth stratum, and N is the in-1 trawl during 1997-2003. Because there total number of hectares in the sampling frame.
appeared to be a relation between the fishing Historically, the strata were depth intervals, six power of the two gears and depth, the correction 20-m (66 ft) strata from shore to the 120-m (394 factor was based on a linear regression of the ft) contour (47% of U.S. waters) and one stratum difference in log transformed catches with depth. encompassing the area in U.S. waters beyond the Catch data were from paired tows conducted 120-m (394 ft) contour (53% of U.S. waters).
during 1995-1998 using two vessels, the Kaho and About 33% of U.S. waters are >160 m (525 ft) steel Seth Green. In the reanalysis, to keep the deep and yet trawling was rarely conducted at depths >160 m. Therefore, when recalculating Page 5 PROVISIONAL DATA NOT TO BE CITED
alewife abundance indices, we defined the outer m (525 ft) bottom contour, off Rochester, NY on limit of the sampling frame as the 160-m (525 ft) April 21, 2004 while the Kaho was bottom contour and divided the area between shore and trawling. The area sampled with hydroacoustics that contour into eight, 20-m (66 ft) depth strata. corresponded to the area sampled with bottom Survey precision is reported as the relative trawls. Scheduled hydroacoustic evaluation of standard error (RSE); 100%* standard error of the fish distribution between bottom trawling index / index. transects was thwarted by equipment malfunction.
The acoustic data were collected with a Biosonics In the early 1980s, as the alewife population DT-X 120 kHz split-beam echosounder, and rebounded from the 1976-1977 winter die-off, the analyzed with SonarData Echoview software.
time required to handle large trawl catches made Estimates of targets were stratified vertically fishing all possible depths at a transect along the path of the acoustic track into three problematic. In general, we adopted the practice layers - within 3 m (10 ft) of bottom, 3 to 10 m of always fishing at mid-depths, where catches (10 to 33 ft) above bottom, and >10 m (33 ft) were consistently high, and at the shoreward end above bottom and <5 m (16 ft) from surface.
of the distribution, terminating fishing after a Maximum vertical opening of our bottom trawl catch of 50 or fewer alewives, assuming a catch of was 3.25 to 3.75 m (10 to 12 ft) at those depths zero at shallower depths that would have been where alewives were abundant.
fished if time allowed. When shallower depths occasionally were sampled after a catch of less Within 3 m (10 ft) of bottom at depths <80 m (262 than 50 to check this assumption, catches were ft), acoustic sampling showed a concentration of -
small and contributed little to the total abundance 55 to -45 dB targets, too small to be alewives, and estimate (OGorman and Schneider 1986). After few targets of alewife size. Bottom trawl catches the dreissenid mediated shift of alewives to deeper at <80 m (262 ft) were dominated by threespine water in 1994 (OGorman et al. 2000), we reduced stickleback Gasterosteus aculeatus; rainbow smelt sampling effort at depths <40 m (131 ft) and and alewives were rare. At depths >80 m (262 ft),
abandoned the use of assuming a catch of zero at acoustics detected a concentration of -50 to -35 standard sampling depths that were not fished dB targets some of which could be alewives.
after a catch of 50 or fewer alewives. Bottom trawl catches were dominated by rainbow smelt at 85 m (279 ft) and alewives at 95 to 150 m In summary, when recalculating alewife (312 to 492 ft).
abundance indices, we increased the number of strata from seven to eight, reduced the total area From 3 to 10 m (10 to 33 ft) above bottom, there within the sampling frame by 33%, abandoned the were relatively few acoustic targets and no targets zero catch assumption from 1994 onward, with a strength corresponding to that of alewife.
dropped the FPC for the steel Seth Green, did not In the large volume extending from 10 m (33 ft) include a FPC for the fiberglass Seth Green, and above bottom to within 5 m (16 ft) of the surface, used a FPC to account for the 1997 gear change targets smaller than alewife were prominent above only for catches at depths >91.5 m (300 ft). The bottom depths of 20 to 40 m (66 to 131 ft) and recalculated alewife abundance indices for decreased markedly beyond the 80-m (262 ft) numbers and weight mirrored the historical bottom contour. In sum, acoustic sampling failed indices for yearlings (Spearman rank correlations; to detect large numbers of fish with target Ps < 0.0001, rs = 0.98) and adults (Spearman strengths corresponding to that of alewife above rank correlations, Ps < 0.0001, rs > 0.95). the zone sampled with the bottom trawl and where acoustics detected near bottom concentrations of Acoustic Evaluation of Fish Distribution in Spring fish with signal strengths similar to alewife, bottom trawl catches were dominated by rainbow To evaluate the distribution of fish during the smelt or alewives.
alewife assessment, hydroacoustic data were collected from the Seth Green along four parallel tracks running perpendicular to shore, to the 160-Page 6 PROVISIONAL DATA NOT TO BE CITED
Figure 4. - Stratified mean catch of yearling alewives with bottom trawls in U.S. waters of Lake Ontario shoreward of the 160-m (525 ft) bottom contour in late April-early May, 1978-2004. Mean catch in 2001 (*) was estimated from bottom trawl catches in June 2001.
Figure 5. - Wet weight of a 165-mm (6.5 in) alewife (predicted from annual length-weight regressions) in spring and fall, Lake Ontario, 1976-2004. 1 gram = 0.035 ounce.
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Alewife days nearshore water is <4° C (39° F) during the first winter after hatch (an index of winter The numerical index of abundance for adult duration) (OGorman et al. 2004). Year class size alewives (age 2 and older) in U.S. waters of Lake is also influenced by the abundance of spawners Ontario in April-May 2004 was similar to that in in a curvilinear manner - weak year classes are 2003 whereas the weight index of abundance was produced by large and small spawning stocks 16% higher than that in 2003 (Figure 2). The whereas strong year classes are produced by 2004 weight index was about double the record intermediate spawning stocks. In spring 2004, the low of 1999, about one quarter of the record high size of the spawning stock was intermediate.
of 1981, and about 40% less than the long term May-July water temperatures, however, were mean. Relative standard error of the 2004 adult much colder than average (8th coldest out of 29 abundance indices was 29%, which was not only years) indicating unfavorable conditions for above average but also the third highest RSE on reproduction. Conversely, the duration of winter record (Figure 3). Age-3 fish made up 38% of the is apparently going to be shorter than average adult catch, age-5 fish made up 22%, and age-2 indicating favorable conditions for survival of and age-6 fish (from the record 1998 year class) juveniles. Nonetheless, unfavorable conditions each made up 18% of the catch. for reproduction in May-July 2004 will likely outweigh favorable conditions for juvenile The numerical abundance index for age-1 survival in winter 2004-2005 leading us to alewives (2003 year class) in U.S. waters in spring anticipate a weak 2004 year class.
2004 was about 25% lower than numerical indices for the two previous years and about 35% below The prognosis is poor for the Lake Ontario the long term average (Figure 4). Although alewife population returning to the early 1990s yearling alewives are not fully recruited to our intermediate levels of abundance. In recent years, sampling gear, we consider the yearling the population was able to rebound to abundance index a rough indicator of year class intermediate abundance levels in 2000-2001 only strength because the indices are correlated with because of the unusually large 1998 year class.
the catch rates of the same year class at age 2 But with the population at an intermediate level, (Spearman rank correlation, n = 26, r = 0.60, P = adult condition declined and the population 0.001). Relative standard error of the 2004 quickly returned to a lower level. The process of yearling abundance index was 28%, similar to the food web disruption, mediated by exotic species, long term average (26%) (Figure 3). The may well have eroded lower trophic level support moderately weak 2003 year class, 12th smallest for the Lake Ontario alewife population to below out of 27 at age 1, apparently will not provide that of the early 1990s. With reproductive success sufficient age-2 recruits to propel adult abundance average or below during 2000-2002, the above spring 2004 levels in 2005. Our index of population has been stable at a low level and adult adult alewife condition is the wet weight of a 165- condition has improved. With the carrying mm (6.5-in) alewife predicted from annual length- capacity of the lake reduced, the alewife weight regressions. The predicted weight in fall population at a low level and made up of a high 2004 was higher than the predicted weight in fall proportion of fish age 5 (44%), and 2003 suggesting that the alewife population was environmental conditions unfavorable for not expanding and depressing food resources production of age-1 alewives, we expect the (Figure 5). Indeed, condition of adults in fall indices of adult alewife abundance to be at, or 2004 was better than in any year since 1980 below, 2004 levels through 2006.
indicating that the alewife population was more in balance with Lake Ontarios productive capacity Rainbow Smelt in 2004 than in any of the previous 23 years.
Indices of rainbow smelt abundance are, like Size of alewife year classes at age 1 is positively indices for alewife, simply stratified, weighted linked to nearshore water temperatures during mean catch per tow. Whereas the sampling frame May-July and negatively linked to the number of for alewife extends from shore to the 160-m (525 Page 8 PROVISIONAL DATA NOT TO BE CITED
ft) bottom contour in U.S. waters, the sampling visible and thus more vulnerable to predation by frame for rainbow smelt extends from shore to the all trout and salmon. The shift to deeper water 140-m (459 ft) bottom contour in U.S. waters probably resulted in the distribution of rainbow because historically few smelt were found at smelt overlapping more completely with that of depths >140-m (459 ft). The rainbow smelt lake trout Salvelinus namaycush.
sampling frame was divided into six strata by depth and geographic area where catches were Rainbow smelt year classes generally alternate homogenous. Beginning in 2000, we modified between strong and weak in Lake Ontario our stratification scheme for calculating rainbow apparently due to cannibalism, primarily by smelt abundance indices to account for the shift in yearling smelt on young-of-year (Figure 7). The distribution of smelt to deeper water (OGorman alternating pattern was interrupted by two et al. 2000). During 1978-1999, because catches successive weak year classes in 1982-1983 and made at 70 m (230 ft) were uniformly low, the again in 2001-2002. The catch of yearling area between the 70-m and 140-m bottom rainbow smelt in 2004 (2003 year class), however, contours was considered one stratum and few was the 5th largest during 1978-2004 and perhaps trawl tows were made there. After the distribution signals a resumption of the alternating pattern in shift, however, catches at 70 m (230 ft) were year class strength that had been intact during neither low nor homogenous. Therefore, 1984-2000. We had expected that abundance of sampling effort at depths 70 m (230 ft) was yearling rainbow smelt would increase in 2004 increased, the single 70 m (230 ft) strata was because abundance of yearling smelt was very divided into three strata in which catches were low in 2003. However, because the number of homogenous - 60 to 79 m (197 to 259 ft), 80 to 99 mature rainbow smelt in the population was at a m (262 to 325 ft), and 100 to 139 m (328 to 456 record low in 2003, the magnitude of the 2003 ft). Characterization of survey precision awaits a cohort was unexpectedly large.
reanalysis of the rainbow smelt database, similar to the ongoing reanalysis of the alewife database, The mean weight of rainbow smelt caught during which is scheduled to be completed in 2005. the June 2004 survey decreased to 2.4 g (0.08 oz) from 3.9 g (0.14oz) in June 2003, because Number and weight indices for yearling and older yearling rainbow smelt (the youngest age group in rainbow smelt in 2004 were the highest since the catch) dominated the catch in 2004 (Figure 7).
1997 and 1998 (Figure 6). Compared to the The heaviest mean weight for rainbow smelt was record low of 2003, indices in 2004 were higher 13.8 g (0.49 oz) in 1979.
by 17 fold (numerical) and 10 fold (weight). The spike in abundance was driven by a strong 2003 The relative and absolute abundances of large year class and not by increased survival of fish in rainbow smelt (150 mm or 5.9 in) remained earlier year classes -- abundance of age-2 and low in 2004. Large rainbow smelt made up less older rainbow smelt remained extremely low. than 3% of the population during 1989-2003 (range: 0.1 to 2.8%) and in 2004 they made up In the 1980s, rainbow smelt mortality rates about 1% of the population. The stratified mean declined when alewives produced large year catch per tow of large rainbow smelt ranged from classes, presumably because the young alewives 1 to 14 during 1989-2003 and was only 1 in 2004.
buffered smelt from predation. However, despite In contrast, during 1978-1983, large rainbow strong alewife year classes in 1998 and 1999, smelt were 10 to 26% of the population and mean smelt survival from age 2 to age 3 in recent catch per tow ranged from 55 to 205. The paucity years has remained uniformly low, averaging 11% of large rainbow smelt during 1989-2004 was during 1997-2003 compared with about 49% most likely due to heavy predation and, more during 1979-1996. Chronically high rainbow recently, several consecutive weak year classes.
smelt mortality rates followed an increase in water clarity and a shift of smelt to deeper water that We are forecasting that 2005 rainbow smelt began in the early 1990s (OGorman et al. 2000). abundance indices will be slightly higher for all Increased water clarity makes rainbow smelt more age groups combined and much lower for Page 9 PROVISIONAL DATA NOT TO BE CITED
yearlings. In all likelihood, any rise in rainbow could be a useful tool for assessing sculpins and smelt abundance will be short lived without a other small, demersal fishes on the dreissenid-relaxation of predation pressure. Rainbow smelt infested bottom. Due to the problematic have demonstrated considerable resiliency by performance of the net, the low catches of slimy rebounding from an extremely low level of sculpins in 2004 are quite likely not an indication spawner abundance which suggests that a of a severe population decline. In 2003, slimy prolonged population collapse is unlikely. sculpin numbers were at intermediate levels along the south shore at depths >70 m (230 ft).
Sculpins In summer 2005, we intend to explore various Slimy sculpin modifications to the 3-in-1 net that might make it a more consistent sampler of benthic fishes. If a In 1996, we lost our ability to index the sculpin suitable modification is found, we intend to population at depths <70 m (230 ft) along the develop a FPC factor (and use a decision rule to south shore because dreissenid density had risen test whether it is worth applying) over the next to a level that made sampling with our 12-m trawl few field seasons from paired tows with the 12-m problematic - quantities of dreissenids in the net net in fall. Calibration factors can differ among were so large that they had the potential to alter seasons (ICES 2004), so an appropriate the fishing power of the net, hindered catch calibration factor for slimy sculpins can only be sorting, and, sometimes, even precluded winching developed from data collected in fall. This the cod end of the net onto the deck. We investigation of relative fishing power will be continued to use the 12-m trawl to assess sculpins needed to allow comparison of slimy sculpin at depths >70 m (230 ft) during 1997-2003 catches made in past years with the 12-m trawl to although tow times at depths <100 m (328 ft) catches made in future years with the modified 3-were continually reduced as the dreissenid in-1 trawl.
population expanded into ever deeper water. By 2003, in southwestern Lake Ontario, we were Deepwater sculpin unable to trawl at depths <80 m (262 ft) and the standard 10-min tow time had to be reduced to 5 During the alewife assessment in April 2004, we min or less at depths of 85 (279 ft) and 95 m (312 caught (and released) one deepwater sculpin ft). Moreover, in southeastern Lake Ontario, we Myoxocephalus thompsonii, albeit at a depth were, for the first time, forced to reduce tow time deeper than those usually fished during the at two depths. Therefore, we attempted to use the alewife assessment (170 m, 558 ft). This was the 3-in-1bottom trawl to assess slimy sculpins in first deepwater sculpin we have caught since 2004. 2000. During 1998-2000, we caught five deepwater sculpins at depths of 110-150 m (361-In southwestern Lake Ontario, few slimy sculpins 492 ft), two while conducting long-term were captured in 2004, so few in fact that we assessment trawling, and three while conducting suspect the net was in poor contact with the short-term assessment trawling that targeted bottom, perhaps due to unusually strong currents deepwater prey fishes in mid lake, along the in the aftermath of fall storms. In central Lake international boundary. After 2000, we did not Ontario, catches were about 1% of previous years conduct targeted trawling for deepwater sculpins whereas in the southeast catches were about 50% in mid lake. Prior to 1998, the last documented of previous years. Overall, our general record of a deepwater sculpin being captured in impression was that the net performed U.S. waters of Lake Ontario was over 50 years inconsistently but that with some modification it ago.
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Figure 6. - Stratified mean catch (+/-1 SE) of rainbow smelt (age 1 and older) with bottom trawls in U.S. waters of Lake Ontario shoreward of the 140-m (459 ft) bottom contour in June, 1978-2004.
For weight estimates, 1kg =2.2 lb.
Figure 7. - Stratified mean catch of age-1 rainbow smelt with bottom trawls in U.S. waters of Lake Ontario shoreward of the 140-m (459 ft) bottom contour in June, 1978-2004. Asterisks denote length-frequency based estimates.
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Acknowledgements power differences in trawl survey data. Fishery Bulletin 96: 538-546.
We thank M. Sanderson and J. H. Hasse for volunteering aboard the R/V Seth Green during New York Sea Grant. 2004. Coastlines. Volume the alewife assessment. 33/3:4-5.
References O'Gorman R., and C. P. Schneider. 1986.
Dynamics of alewives in Lake Ontario following Adams, J. V., and R. OGorman. in prep. A a mass mortality. Transactions of the American decision rule for applying fishing power Fisheries Society 115:1-14.
corrections.
OGorman, R., J. H. Elrod, R. W. Owens, C. P.
ICES. 1992. Report of the workshop on the Schneider, T. H. Eckert, and B. F. Lantry. 2000.
analysis of trawl survey data. ICES CM Shifts in depth distributions of alewives, rainbow 1992/D:6. 96 p. smelt, and age-2 lake trout in southern Lake Ontario following establishment of dreissenids.
ICES. 2004. Report of the workshop on survey Transactions of the American Fisheries Society design and data analysis (WKSAD). Fisheries 129:1096-1106.
Technology Committee. ICES CM 2004/B:07, Ref. D, G. 65 p. O'Gorman, R., B. F. Lantry, and C. P. Schneider.
2004. Effect of stock size, climate, predation, and Munro, P. T. (1998). A decision rule based on the trophic status on recruitment of alewives in Lake mean square error for correcting relative fishing Ontario, 1978-2000. Transactions of the American Fisheries Society 133: 855-867.
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