ML073240729

Habitat Utilization and Biological Characteristics of Adult Atlantic Sturgeon in Two South Carolina Rivers

ML073240729
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Site:	Vogtle
Issue date:	01/31/2000
From:	Collins M, Pashuk O, Post W, Tanya Smith State of SC, Dept of Natural Resources
To:	Office of Nuclear Reactor Regulation
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Transactions of the American Fisheries Society 129:982-988, 2000

© Copyright by the American Fisheries Society 2000 Habitat Utilization and Biological Characteristics of Adult Atlantic Sturgeon in Two South Carolina Rivers MARK R. COLLINS,* THEODORE 1. J. SMITH, WILLIAM C. POST, AND OLEO PASHUK Marine Resources Research Institute, South Carolina Department of Natural Resources, Post Office Box 12559, Charleston, South Carolina 29422, USA Abstract.-Thirty-nine adult Atlantic sturgeon Acipenser oxyrinchus oxyrinchus (136-234 em total length) were caught in gill nets fished at historical sturgeon-fishing locations in the Combahee and Edisto rivers (South Carolina) during spring and fall 1998. All fish were tagged (with passive integrated transponders and darts), and radio and acoustic transmitters were surgically implanted in 29 fish. When possible, gonad biopsies were taken for sex and maturity-stage determination.

Locations of telemetered fish were determined several times per week from airplanes (radio) and boats (radio and acoustic). Nominal ages, based on microscopic examination of pectoral spine cross-sections, ranged from 7 to 20 years. Of the 28 fish for which sex was definitively ascertained, 21 (aged 7-15) were male and 7 (aged 15-20) were female. All fish moved out of the rivers during the period extending from October to November. Twelve fish returned the following spring (most in March), and many took up residence at the same sites utilized the previous year. Fall and spring spawnings were documented based on histological examination of gonad biopsies and directed upriver movements of fish during both seasons. Habitats used during summer were diverse and included the lower and upper estuaries, tidal freshwater, river, and perhaps even the ocean, as some fish left the system entirely. One male was captured in two successive springs and was in spawning condition (running ripe) both years.

The Atlantic sturgeon Acipenser oxyrinchus ox-yrinchus is an anadromous species inhabiting the Atlantic coast of North America from Labrador to Florida. Recorded exploitation dates prior to 2198 BC (Ritchie 1969). By 1860, major commercial fisheries were established in a number of states from Georgia through New York (Smith 1990).

Because of the high value of these fish, the fish-eries were vigorously pursued, and within a de-cade, all fisheries suffered drastic declines or total collapse (Murawski and Pacheco 1977).

After the collapse of the more northerly fish-eries, South Carolina became a major producer of sturgeon. In 1976, South Carolina produced 55%

of the total U.S. landings of Atlantic sturgeon (Smith et ai. 1984). As a result of increased fishing pressure following the embargo on Iranian caviar, landings continued to increase in South Carolina, but catch per unit effort was steadily declining. By 1984, it became apparent that the fishery was being overexploited, and in 1985 the South Carolina sturgeon fishery closed indefinitely until systems for estimating abundance and monitoring the fish-ery could be implemented. Recently the Atlantic States Marine Fisheries Commission (ASMFC

• Corresponding author: collinsm@mrd.dnr.state.sc.us Received August 2, 1999; accepted January 31, 2000 1998) implemented a long-term moratorium on all U.S. Atlantic sturgeon fisheries.

Little is known about the ecology of adult At-lantic sturgeon in the southeastern United States.

The ASMFC conducted an information survey while developing a Fishery Management Plan and noted that research on habitat use and reproduc-tion/recruitment should be considered a high pri-ority (ASMFC 1990, 1998). To date, however, no spawning areas have been identified in any south-eastern river, nor have specific spawning condi-tions been defined. Further, habitat use by adult Atlantic sturgeon during nonspawning seasons is unknown. Age-distribution and sex-ratio infor-mation are also unavailable for most southern pop-ulations. Thus, a broad range of biological data is needed to facilitate restoration and management of this valuable species (Smith 1985; Smith and Clugston 1997). To address these issues, a study was conducted to provide information on habitat use, age distribution, reproductive status, and sea-sonal movements of adult Atlantic sturgeon in two South Carolina rivers.

Methods With assistance from three former sturgeon an-glers who were familiar with the study areas, stur-geon gill nets were deployed in the Combahee and Edisto rivers of the Ashepoo, Combahee, and Edis-to rivers (ACE) basin (South Carolina) during 982

HABITAT UTILIZATION OF ADULT ATLANTIC STURGEON 983 TABLE I.-Histological criteria used to establish matu-rity stages of male and female Atlantic sturgeon from mi-croscopic examination of sectioned and stained gonad samples.

spring and fall 1998. Gill nets were 11-23 m long, with 29.2-45.7-cm stretch mesh. Nets were an-chored perpendicular to the bank at previously pro-ductive fishing locations and were checked during morning and evening. The same locations were sampled and the same gear was used during spring and fall. Fish were brought into the boat and placed upside down in a V-shaped trough with the anterior portion of the fish positioned in a tank that was receiving flow-through river water. Total length (TL) and fork length (FL) were measured, and a marginal pectoral spine was removed for age es-timation. Sturgeon were tagged at the base of the dorsal fin with a passive integrated transponder (PIT) tag and a dart tag. Individuals suitable for telemetry studies (i.e., those that were not injured or highly stressed) received a 5-cm incision just off the midline of the abdomen, about 10 em an-terior to the vent, for insertion of radio and acous-tic transmitters and for sex determination. Radio (Advanced Telemetry Systems, Inc., model 12AA) and acoustic (Sonotronics, Inc., model CHP-87-L) transmitters were sterilized with betadine and in-serted anteriorly through the incision. Sex was ini-tially determined by gross examination, but later an Eppendorfer biopsy punch was used to take a small sample of gonad. The incision was closed using Ethicon Ethibond Excel 2/0 braided poly-ester sutures, and the fish was released. In one instance the acoustic transmitter was attached ex-ternally at the base of the dorsal fin, and in another instance both transmitters were attached external-ly, one on each side of the dorsal fin.

Gonad biopsy specimens were preserved in 10%

formalin. After a 1-2-week fixation, they were transferred to 50% isopropanol, processed, vacu-um-infiltrated in a modular vacuum tissue proces-sor, and embedded in paraffin. Blocked samples were sectioned at 7 f,1m, stained with double-strength gill hematoxylin, and counterstained with eosin-yo Sections were examined microscopically, and sex and maturity stages were assigned using histological criteria modified from Wallace and Selman (1981; see also West 1990; Amiri et al.

1996a, 1996b; Van Eenennaam and Doroshov 1998; Vorobyova and Markov *1999) (Table 1).

Ripe and running ripe fish of both sexes and fe-males with ovaries containing postovulatory fol-licles were classified as being "in spawning con-dition."

Pectoral fin spines were sectioned with a low-speed diamond-blade saw. Three sections from the base of each spine were cut to a thickness of 0.5 mm and mounted on glass slides. Each section was Stage Immature Developing Early Mid Late Running ripe Spent Resting Immature Developing Early Mid Late Ripe Running ripe Recently spawned Spent Resting Characteristics Males Predominance of spermatogonia, some cysts of primary spermatocytes may be present, no evidence of previous spawning Nearly equal amounts of cysts with spermatogonia and cysts with prima-ry and secondary spermatocytes, some spermatids may be present Lobules contain approximately equal amounts of cysts with spermatogo-nia, spermatocytes and spermatids, some spermatozoa may be present in lobular lumina Ripe; predominance of cysts with sper-matids, and spermatozoa more abun-dant, some cysts with spermatogonia and spermatocytes Lobules filled with spermatozoa, little or no spermatogenesis; spermiation Only residual spermatozoa present in lobules that have contracted, prolifer-ation of interstitial tissue and phago-cytosis of germ cells Predominance of interstitial tissue, small empty lobules evident, sper-matogonial proliferation at the end of this stage Females No evidence of atresia, predominance of chromatin-nucleolar oocytes, abundant oogonia along periphery of lamellae, perinucleolar oocytes present at the end of this stage Cortical alveoli appear in the peripheral cytoplasm of the largest oocytes Predominance of primary (yolk-granular) and secondary (yolk-globular) vitellogenic oocytes The most advanced oocytes are in ter-tiary vitellogenic stage (yolk glob-ules fuse into yolk platelets), with two-layered zona radiata Nucleus at the animal pole in the most advanced oocytes Mature oocytes ovulated into the ab-dominal cavity and spawning takes place Postovulatory follicles (POF) present indicating that fish spawned during previous few hours to 2 d (dependent upon water temperatures)

Unshed advanced oocytes atretic, POF no longer present Predominance of perinucleolar oocytes, traces of atresia may be present

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examined microscopically, and the fish were as-signed nominal ages based on the number of annuli observed (see Cuerrier 1951; Brennan and Cailliet 1989).

Overflights were used to survey the freshwater segments of the rivers for telemetered fish, and fish locations were pinpointed by boat. Egg col-lectors, which consisted of anchored plastic matrix pads (floor buffing pads; see Sulak and Clugston 1998) and which provided an artificial substrate to which sturgeon eggs would adhere, were deployed at possible spawning locations. Varying numbers of these collectors were placed in the vicinities of telemetered fish, and collectors were checked ev-ery 2 d. Habitats utilized regularly were charac-terized in terms of depth, substrate, water tem-perature, dissolved oxygen, and salinity. When fish used brackish water habitats, acoustic tracking was utilized because of attenuation of radio signals.

Results Thirty-nine adult Atlantic sturgeon, ranging in size from 136 to 234 em TL, were captured in the Edisto and Combahee rivers (Figure 1). Most fish were caught at night and removed when the nets were checked in the morning. Catch per unit effort (CPUE) of three cooperating former sturgeon an-glers in the Combahee and Edisto rivers was 0.0-0.456 fish/net-day (1 net-day = 91 m of gill net fished for 1 d) during spring 1998. The CPUE was greater for two of the three anglers in fall 1998, with a range of 0.288-0.408 fish/net-day.

Nominal ages were determined for 38 fish, as one spine was considered unreadable. Nominal ages were 7-20, with a modal age of 9 and a mean age of 10.8 (Figure 2). The oldest, and also the largest, fish captured was a female taken in spring 1998. Females were aged 15-20, and males were aged 7-15.

Sex information obtained by gross examination was not used, except for fish that were running ripe. Of the 28 sturgeon for which sex was deter-mined, 21 were male (139-195 em TL), and 7 were female (180-234 ern TL), providing a 3: 1 sex ra-tio. In the spring, running ripe males were captured as early as 2 March, and a running ripe female (at Edisto River river kilometer [RKM] 56) was cap-tured on 7 March, when the water temperature was 13.6°C. One male that was captured in March 1998 and recaptured in March 1999 was in spawning condition (running ripe) both years. Spent males were captured as early as late March, and spent females were captured as late as mid-May.

Running ripe males began reappearing at the end of August. During September, ripe and running ripe males were present in approximately equal numbers, and during October, only one of seven males captured was not running ripe. A late-developing female was captured on 2 June, and histological examination indicated that this female had not spawned in the spring. This fish moved upriver to RKM 105 in late September. Spent fe-males, including one that had spawned very re-cently (postovulatory follicles still present), were captured in late September and October, when the water temperature was 17-18°C.

Adult Atlantic sturgeon were present in the Edisto River from March through October 1998.

In early 1999, 12 telemetered fish returned to the

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river, primarily during the month of March. Only one fish, a male that was ripe in spring 1998, moved upriver. The rest remained in the lower riv-er and estuary, many of them using the same lo-cations that they had utilized the previous summer.

Movements of three males suggest that Atlantic sturgeon may not differentiate among ACE basin rivers. All were initially captured at RKM 53 in the Combahee River, one in the spring and two in the fall of 1998. The fish caught in the spring was recaptured after 13 d at large at RKM 56 on the Edisto River. The recapture allowed for exami-nation of the incision and sutures. The incision had entirely closed, and the sutures were gone. The two fish captured in the fall left the Combahee River in October 1998 and were located in the lower Edisto River in early June 1999.

Movements of two fish were strongly indicative of a fall spawning migration. Both fish were tele-metered in late May 1998. They spent the summer in the lower Edisto River, then in October moved upriver to RKM 190. No sturgeon eggs were col-lected with the artificial substrates, so spawning sites were not confirmed. However, a running ripe female that may have been in the process of spawn-ing was captured in the spring at Edisto RKM 56, and a very recently spawned female was captured at the same location in fall. The substrate in that area is limestone. Fish were located during spawn-ing seasons in three other areas that had hard sub-strates: RKM 55-60 in the Combahee River and the areas of RKM 105 and 190 in the Edisto River.

In the Combahee River, no movements upriver of RKM 60 were noted during spring or fall.

A wide variety of habitats was utilized by adult Atlantic sturgeon during summer. In the Edisto River, habitats included locations in the vicinities of RKM 90-105, RKM 64-72, RKM 40 in the upper fresh/brackish interface zone, RKM 18-29 in the lower interface zone, and RKM 8-10 in the high-salinity section of the estuary. Salinities at locations inhabited by fish varied from 0.0 to 28.6 ppt, dissolved oxygen was 3.4-8.3 mg/L, and tem-peratures were as high as 33.1°C. Substrates in-cluded fine mud, sand, pebbles, and shell hash.

Depths utilized varied from 1.5 to 13.0 m, but in nearly all cases, fish were in the greatest depth available in the immediate area. No telemetered fish remained in the Combahee River during sum-mer.

Discussion The CPUE of the three former sturgeon anglers in fall 1998 and the CPUE of two of the anglers (one caught no sturgeon) in spring 1998 were ap-parently greater than that recorded from the com-mercial fishery in the same rivers in 1981-1982.

Smith et al. (1984) reported CPUE in terms of weight (15.8 and 13.2 kg/net-day for 1981 and 1982, respectively), but they also gave the mean weight of fish landed in 1982 (73.3 kg). Thus, CPUE in numbers of individuals was approxi-mately 0.216 and 0.180 fish/net-day for 1981 and 1982, in comparison with 0.0-0.456 fish/net-day during spring 1998 and 0.288-0.408 fish/net-day in fall 1998. This suggests that the abundance of adult Atlantic sturgeon may have increased and, therefore, that the population(s) may be recovering because of the closure of the fishery. However, it is possible that the anglers who participated in this

986 COLLINS ET AL.

study chose the most productive of all historical fishing sites and that additional nets in other lo-cations would produce fewer fish per net-day, thus reducing the overall CPUE.

The youngest mature male caught in the ACE basin was age 7, although Smith (1985) reported that males mature as early as age 5 in South Car-olina. Van Eenennaam and Doroshov (1998) re-ported that the youngest mature male they en-countered in the Hudson River (New York) was age 12, supporting Smith's (1985) assertion that Atlantic sturgeon mature at younger ages in the south. Atlantic sturgeon from northern populations reportedly achieve much greater ages than those observed in this study (e.g, age 60; Magnin 1964).

Smith (1985) reported a maximum nominal age of 30 for fish sampled from the Winyah Bay, South Carolina commercial fishery, and the oldest fish in the present study was age 20. Similarly, Sulak and Clugston (1999) found a maximum age of 25 among 5,000 Gulf sturgeon Acipenser oxyrinchus desotoi in the Suwannee River (Florida).

Smith et aL (1984) reported a mean nominal age of 15 for fish landed in South Carolina during 1978-1980; this represents an age that is substan-tially greater than the mean of 10.8 years that we observed in the present study. It is not known whether the age distribution observed is "normal" for this region or whether it is a result of intense fishing pressure prior to the fishery's closure. As-suming that the assigned ages are accurate, only eight fish were from year-classes spawned prior to the closure of South Carolina's commercial fishery in 1985. It is possible that only a few fish in this population avoided harvest and that the population is now slowly rebuilding. It should be noted that full age-growth studies have not been conducted for southern populations of Atlantic sturgeon, and the annual nature of marks on hard parts of adult Atlantic sturgeon has not been validated.

There have been reports of fall runs or migra-tions of Atlantic sturgeon, but they are generally considered to represent postspawning downriver migrations (Smith 1985). Dovel and Berggren (1983) found no convincing evidence for fall mi-grations in the Hudson River. Smith et aL (1984) reported indications of both spring and fall runs of Atlantic sturgeon in the ACE basin rivers, but they did not confirm that the runs were related to spawning. That spawning takes place in both sea-sons, at least in this system, has now been histo-logically verified. Spring and fall spawning are reported to occur in a number of Eurasian stur-geons. Berg (1959) reported that most of these species consist of separate races, which he termed "vernal" and "hiemaL" The former commercial sturgeon anglers who participated in the present study stated their belief that fall spawners and spring spawners are separate stocks or races. They based this belief primarily on an assertion that fish of both sexes caught in spring were larger than those caught in fall, as pointed out by Smith et aL (1984). We compared the lengths of males (too few females) captured in the two seasons during

]998. The number of spring spawners was insuf-ficient for statistical comparison, but the length ranges (spring: 189-194 cm TL; fall: 139-]52 cm TL) do not overlap.

Group-synchronous development of oocytes in ovaries of migrating female Atlantic sturgeon (only three modes of oocytes present: tertiary vi-tellogenic, cortical alveoli, and perinucleolar),

along with the fish size differences between sea-sons, suggest that the same females do not spawn twice in the same year. Moreover, the large sep-aration between clutches of mature and previtel-logenic oocytes within the same ovary of females in spawning condition (no primary or secondary vitellogenic oocytes) indicates that after mature eggs are released, a long period is required for another clutch to mature. In South Carolina, ex-amination of presumed spawning marks on pec-toral rays indicated that intervals between spawn-ings is 3-5 years for females and 1-5 years for males (Smith 1985). However, it was not verified that the marks were actually the result of spawn-ing. The capture of a running ripe male during consecutive springs (1998 and] 999) verifies that some males may spawn in consecutive years, but there was no indication that the same individuals participate in both spring and fall spawns.

Spawning locations were not verified by collec-tion of eggs. Achieving this goal will likely require substantial additional effort, especially if spawn-ing activity is restricted to a very small area at each site, as appears to be the case for Gulf stur-geon (Sulak and Clugston 1999). However, infer-ences concerning spawning sites can be drawn from telemetry, histology, and substrate data. It is likely that spawning occurs at several locations in the Comabahee and Edisto rivers, as is the case for Gulf sturgeon in the Suwanee River (Sulak and Clugston 1998, 1999) and the Choctawhatchee River (Florida-Alabama) (Fox and Hightower 2000). The running ripe female captured in the vicinity of limestone outcrops at RKM 56 in the Edisto River may have been in the process of spawning. Similar substrate has been identified as

HABITAT UTILIZATION OF ADULT ATLANTIC STURGEON 987 being typical of spawning sites for a number of sturgeon species (Foltz and Meyers 1985; Parsley et al. 1993; Kieffer and Kynard 1996; Sulak and Clugston 1999).

Other areas that the data suggest might have been spawning sites were RKM 105 and 190 in the Edisto River and RKM 55 in the Combahee River. To reach the two upriver sites in the Edisto River, fish had to cross several very shallow (~0.5 m) stretches of river as a result of the low-flow conditions that prevailed during fall 1998. In spring, movements upriver of RKM 56 on the Edisto River were not observed, and no movement above RKM 60 was detected in the Combahee Riv-er. Dovel and Berggren (1983) suggested that At-lantic sturgeon in the Hudson River spawn be-tween RKM 55 and 136, although the salt wedge extends as far upriver as RKM 98. Based on his-tological examination of gonads, Van Eenennaam et al. (1996) concluded that spawning did not take place below RKM 196 in the Hudson River. They also pointed out that based on the sensitivity of sturgeon embryos and larvae to even low salini-ties, it is doubtful that successful spawning takes place below, or even immediately above, the salt wedge. The proposed spawning sites in the Edisto and Combahee rivers were all more than 12 km upriver of the maximum intrusion of the salt wedg-es, and they are substantially farther upriver when the salt wedges are displaced downriver during typical flow conditions. Shortnose sturgeon Aci-penser brevirostrum in general (Kynard 1997),

Gulf sturgeon in the Suwannee River (Sulak and Clugston 1999), and Atlantic sturgeon in the Hud-son River migrate more than 200 km upriver to spawn. However, this may not always be the case for Atlantic sturgeon in southern rivers, as some spawning in South Carolina may take place rela-tively close to the coast (but probably well above the salt wedge). This hypothesis is further sup-ported by the recent capture (April 1998) of two very early larval Acipenser sp., tentatively iden-tified as Atlantic sturgeon, at RKM 42 in the Sa-vannah River (T. Reinert, Georgia Cooperative Fish and Wildlife Research Unit, personal com-munication). Sulak and Clugston (1999) hypoth-esized that Gulf sturgeon spawning sites are de-termined primarily by a specific topographical, hy-drological, and chemical milieu, and such condi-tions may occur lower in the Edisto, Combahee, and Savannah rivers than in other rivers in which Atlantic sturgeon have been studied.

Migration patterns and general behavior of adult Atlantic sturgeon in the ACE basin closely parallel those of Gulf sturgeon (Sulak and Clugston 1998, 1999), perhaps to a greater extent than those of Atlantic sturgeon in the northern portion of their range. In general, both overwinter in the ocean, migrate into the river in early spring and move upriver to spawn, inhabit the river throughout the summer, and migrate out of the river in the fall.

Maximum observed ages for Gulf and southern Atlantic sturgeons are much younger than those reported for northern Atlantic sturgeon. There is also some evidence of fall spawning for Gulf stur-geon (Sulak and Clugston 1998; Ken Sulak, U.S.

Geological Survey, Biological Resources Divi-sion, personal communication) as well as for ACE basin Atlantic sturgeon. In contrast to Gulf stur-geon, however, Atlantic sturgeon in the ACE basin utilized a broad range of estuarine and riverine habitats during the summer.

This study provides new information on the movements and habitats of adult Atlantic sturgeon in the southeastern United States as well as evi-dence to substantiate a fall spawning period. How-ever, significant data gaps still remain. For ex-ample, a number of sites were identified as prob-able spawning areas, but collection of eggs for final verification was not achieved. Nominal age structure and catch data suggest that the population in the ACE basin is rebuilding following the clo-sure of the fishery, but population size estimates were not possible. It is not known whether the surprisingly diverse habitats used form a pattern that occurs in other rivers in the region. Atlantic sturgeon spend a substantial portion of their lives in the ocean and have been captured in depths of up to 40 m off the coast of South Carolina (Collins and Smith 1997), but little is known of their move-ments or specific habitats. Future efforts should focus on addressing these issues as well as on de-termining the status of Atlantic sturgeon popula-tions in other rivers.

Acknowledgments This study was funded by the National Marine Fisheries Service (S-K grant NA77FD0063) and the South Carolina Department of Natural Re-sources. We thank the ACE basin anglers Ivy Per-ry, Stanley Moore, and Ad Mixon, who assisted in this study. Chris Walling, Dan Russ, Sterling Bryson, Billy McCord, Doug Oakley, and Paulette Powers provided invaluable assistance in teleme-try efforts. We also thank Ken Sulak (U.S. Geo-logical Survey/BRD, Gainesville, Florida) for his comments, suggestions, and information. Refer-ence to trade names does not imply endorsement.

988 COLLINS ET AL.

This is South Carolina Marine Resources Center contribution 439.

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