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Aspects of the Wintering Biology of Striped Bass at a Power Plant Discharge
Kimberly Williams and John Waldman

Northeastern Naturalist, Volume 17, Issue 3 (2010): 373–386

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2010 NORTHEASTERN NATURALIST 17(3):373–386 Aspects of the Wintering Biology of Striped Bass at a Power Plant Discharge Kimberly Williams1,2 and John Waldman3,* Abstract - The warm water outflow at a power station in Northport, NY aggregates Morone saxatilis (Striped Bass) in winter, which creates unnatural angling opportunities. Substantially more Striped Bass were available to anglers in the winter of 1995–1996 than in the winter of 1996–1997. Although Striped Bass were observed within and around the heated effluent of the plume, tracking using ultrasonic transmitters indicated they were not “trapped” in the area by the colder water surrounding the discharge. However, there also was little evidence of a natural forage base. Tag returns from marked Striped Bass over subsequent years suggest that they displayed similar movement patterns to Striped Bass found in Long Island waters during other seasons. Differences in availability of Striped Bass among years may reflect seasonal changes in temperature, cohort abundance, and other factors. Introduction More than 20 electric generating stations in New England rely upon intake from nearby coastal waters to reduce heat formed as a byproduct of their operations. However, there has long been concern about the effects of power stations on the environment. Issues regarding aquatic life in the vicinity of power stations include direct physical harm to fishes and other organisms from entrainment and impingement (Boreman et al. 1981, Kerr 1953, Marcy 1973), alterations to near-field water quality (Kerr 1953), and thermal pollution from heated discharges (Kerr 1953, Setzler et al. 1980). During autumn in North Temperate regions, elevated temperatures of power plant effluents appear to attract some organisms that would normally migrate south as waters cool. As waters surrounding a power plant plume continue to cool with the coming of winter, fish and other animals appear to become “trapped” within the thermal influence of the plume. Organisms that remain at these sites become subject to starvation or stress during the coldest months if they deplete their forage base or if water quality deteriorates (Pawson and Eaton 1999, Wagenheim 1974). Another hazard is the potential for rapid temperature changes due to plant shutdowns. Disruptions of thermal discharge have been observed to cause excessive mortality to fish and other organisms (Englert et al. 1988, Silverman 1971). 1Marine Sciences Research Center, SUNY Stony Brook, Stony Brook, NY 11794. 2Smithtown High School Science Department, 100 Central Road Smithtown, NY 11787. 3Biology Department, Queens College, 65-30 Kissena Boulevard, Flushing, NY 11367. *Corresponding author - 374 Northeastern Naturalist Vol. 17, No. 3 In New England and some Mid-Atlantic states, the fish that aggregate in heated effluents from power plants represent an important winter recreational angling opportunity (Marcy and Galvin 1973, Moore and Frisbie 1972). Even as far south as Maryland, anglers fish for Striped Bass in heated effluent (Hart 1997). Moore and Frisbie (1972) found that substantially more fish were captured in heated power plant waters than were captured when these waters were not heated. However, most of the recent information available regarding the attraction of fish to power plants in New England is anecdotal, existing primarily in the sportfishing media (e.g., Sampson 1996). The anadromous Morone saxatilis (Walbaum) (Striped Bass) is one of the most popular sport fish and economically important commercial fish along the mid-Atlantic and New England coasts of the US (Richards and Rago 1999). The prey items of these carnivores include a variety of invertebrates and fishes, depending on age, season, and availability of prey (Dunning et al. 1997, Manooch 1973). Major migratory populations of Striped Bass occur in Chesapeake Bay and the Delaware and Hudson Rivers (Waldman and Fabrizio 1994). Subadult and adult Striped Bass from these populations occur routinely in Long Island Sound during spring, summer, and autumn (Hickey 1981, Schaefer 1968). Some enter the Long Island Sound via the East and Harlem Rivers (Waldman et al. 1990); others enter through the “Race” between Long Island and Connecticut (Austin and Custer 1977). Most Striped Bass are believed to exit the Sound by late autumn, to overwinter in their natal estuaries or in coastal waters between the New York Bight and Cape Hatteras (Waldman et al. 1990). Due to their ecological and economic importance, movements of Striped Bass have been well studied. Tagging studies indicate that many Striped Bass undergo coastal migrations which are generally northward in the spring and summer, and southward in the autumn and winter (Merriman 1941, Waldman et al. 1990). Not all, however, undertake “typical” migration patterns; some have been reported to overwinter in embayments along the coast rather than following their migrating conspecifics. Striped Bass have been documented overwintering in northern Atlantic coastal sites including bays of the Long Island Sound (Neville 1940, Raney 1952), the Thames River, CT (Sampson 1996), and the upper Bay of Fundy, Canada (Rulifson and Dadswell 1995). For almost a half century, the sportfishing media have focused attention on the fishery associated with a power plant located on the north shore of Long Island in Northport, NY. The heated effluent from the Long Island Power Authority (LIPA; formerly known as LILCO) station in Northport supports the only major recreational Striped Bass fishery of its kind on Long Island (Fig. 1). However, little research has been performed on the nature of this fishery or of the biology of the Striped Bass that support it. 2010 K. Williams and J. Waldman 375 In one of the first studies of the Northport power plant fishery, Austin et al. (1973) described the presence in winter 1971–1972 of Pseudopleuronectes americanus (Walbaum) (Winter Flounder), Menidia menidia (L.) (Atlantic Silversides), Morone americana (Gmelin) (White Perch) and two hakes, Urophycis chuss (Walbaum) (Red Hake) and Merluccius bilinearis (Mitchill) (Silver Hake). That winter, the recreational fishery was characterized as small. A creel study conducted at the site showed most anglers capturing no fish of any species during February, with only one Striped Bass landed. In another study of the Northport power station, Young (1979) described the winter fishery as being substantial enough to support both sport and commercial fisheries, with commercial rod and reel landings of 950 pounds that January. It was unknown how many pounds were taken by sport anglers. During winters when Striped Bass are abundant at the Northport plant, recreational angling is popular. In 1984, harvest of Striped Bass in winter was prohibited, so the winter Striped Bass fishery at Northport became strictly catch and release. This study characterizes the winter Striped Bass fishery associated with the heated effluent around the power station in Northport. Biological data regarding length, weight, age, and condition of Striped Bass are presented and compared with similar data for specimens captured during winter across Long Island Sound off the coast of Connecticut. Additional information on the sex, diet, local and long-distance movements, and stock origins of Striped Bass captured in the study area also is provided. Figure 1. Map of power plant region and location in Long Island Sound. 376 Northeastern Naturalist Vol. 17, No. 3 Methods Field-site and sampling season description Our primary site was the outflow of the power station on the north shore of Long Island at Northport, NY (Fig. 1, site A). Water from Long Island Sound is drawn into the station through pumps fed from an intake lagoon. After being used, the water is released into a cooling lagoon before spilling as a thermal plume into Long Island Sound at Crab Meadow Beach. Fish collection was conducted from 14 January 1996 through 6 April 1996 (Field Season I), and from 22 December 1996 through 18 February 1997 (Field Season II). Unless otherwise noted, for the purpose of this study, “winter” will refer to the months of December through April. Results from trawl sampling in open waters across Long Island Sound were also used in the study as a result of notification from the Connecticut Department of Environmental Protection that Striped Bass were bycaught in both mobile and fixed fishing gear at substantially higher rates than usual during the winter of 1995–1996. Data collection All fish sampling was conducted during daylight hours. During each sampling day, salinity, air temperature, and water temperature at 30 cm of depth were measured at the heated effluent waterfall (Fig. 1, site A) and at the shoreline approximately 100 m east of the outfall (Fig. 1, site B). With the assistance of volunteers, Striped Bass were captured at site A across both field seasons using a variety of gears. During Field Season I, 776 specimens were caught via angling. During Field Season II, all 42 Striped Bass caught were captured via angling. Fishing was conducted at all tidal stages. However, most tagging occurred during the first two hours of the outgoing tide, when captures appeared to be highest. In Field Season I, three gill nets also were used to capture specimens from the thermal plume. Each net measured 46 x 3 m, with mesh sizes of either 10 cm, 18 cm, or a combination of 8 cm and 13 cm. These nets were set by boat within 15 m of shore in depths from 1 to 9 m and retrieved by hand. Set times of the gill nets ranged from 15 min to 1 hr. One-hundred-fifty-one Striped Bass were captured in the 10-cm net, 5 were captured in the 18-cm net, and 153 were captured in the 8-cm and 13-cm net. Additionally, 149 Striped Bass were captured with a bottom trawl operated by the US National Marine Fisheries Service (USNMFS) near Milford, CT, over several dates during Field Season I. The 27-m mouth-size otter trawl had 1-m x 1-m doors and was towed at about 3 knots for 30 min at depths ranging from 9 to 15 m. Data also were obtained from nine Striped Bass that had been victims of a fish kill and had been found in the impingement grating of the power plant on 16 January 1996. Biological data were recorded from 1281 Striped Bass. Total length was measured to the nearest mm, and weight was measured with a spring scale 2010 K. Williams and J. Waldman 377 to the nearest 45 g. Specimens were examined for external signs of illnesses such as lymphocystis, with its white to gray tumor-like patches on the body and fins, and for gross injuries and abnormalities. All specimens were scanned with a Northwest Technologies handheld coded-wire-tag detector. The machine detects the presence of a wire (1.5 mm long x 0.25 mm in diameter) implanted in the cheek prior to their release from hatcheries. The wire tags are scored with a code for each specimen and include state, hatchery, and ancillary information. Most specimens with coded wire tags were sacrificed for later tag recovery. Once recovered, tags were sent to the United States Fish and Wildlife Service (USFWS) for code interpretation. Ten to fifteen scales were removed from below the dorsal fins on the left side of each individual. Acetate impressions of scales were made to facilitate reading of the annuli in estimating the age of individuals. Striped bass diets were characterized by examining stomachs from 80 sacrificed specimens: 32 from the coast of Connecticut and 48 from the power station. Additionally, 33 live individuals were examined for diet by the use of gastric lavage (Light et al. 1983): 2 from Connecticut and 31 from the power station. Gonads from 79 Striped Bass were examined for sex determination. Prior to releasing them in their area of capture, 1177 Striped Bass were tagged with externally visible tags. This effort was coordinated with an ongoing mark-recapture study of Striped Bass along the east coast of the US. Using a scalpel, a 4-mm long subcutaneous incision was made in an area posterior and ventral to the pectoral fin on the fish’s left side, into which a Floy FM-84 tag was inserted. Instructions for anglers who caught tagged Striped Bass were posted at the sampling site as well as in local tackle shops. Similar postings appear in tackle shops and fishing media along the Atlantic coastal range of the Striped Bass. During Field Season II, Striped Bass distribution and movements around the thermal plume were studied by tracking the positions of seven specimens tagged with Sonotronics Sonic Temperature Sensing Tags (Model CTT-83- 2-E), Manual Tracking Receiver (Model USR-5W), and Hydrophone (Model DH-2) with headphones. Tags were 62 mm by 16 mm, weighed 8 g in water, and had an expected 14-month battery life. Sonic tags were distinguished from one another by a unique sequence of “pings” that emitted from each tag and were audible through the hydrophone. The seven specimens tagged with temperature tags (9–18 February 1997) ranged in total length from 441 mm to 635 mm. A hollow needle was used to create two holes in the dorsal musculature of the fish through which a stainless steel wire was fed to attach the tag. The site was visited 12 times during different tidal stages between 9 February and 4 April 1997, using the hydrophone to determine the presence of tagged fish. 378 Northeastern Naturalist Vol. 17, No. 3 Results Angler use Observations and questioning of anglers using the site revealed that the most popular angling times were afternoons, weekends, and holidays. Although a few anglers indicated that they fished at night, no surveying was performed from dusk to dawn. Anglers fishing in and around the heated effluent in boats were less common than anglers fishing from shore. Even though the average air temperature was colder during the first sampling season, anglers did not have difficulty capturing Striped Bass, many of which were made available for use in this study. Angler use of the sampling site was greatly reduced during Field Season II when anglers reported having to invest more effort to capture few, if any, Striped Bass. The sampling crew and the recreational anglers frequently described the fish they captured during Field Season I as having a relatively “healthy” appearance. This was noted in sharp contrast to the “sick” or “ragged” appearance of the specimens that were landed during Field Season II. Environmental conditions During Field Season I, salinity at the site ranged from 25 to 32 ppt. Water temperatures measured ranged from 6 °C to 13 °C inside the plume (Fig. 1, site A) and -3 °C to 9 °C outside the plume (Fig. 1, site B). During Field Season II, temperatures ranged from 9 °C to 13 °C inside the plume and 2 °C to 4 °C outside the plume. To gauge differences in regional seasonal temperatures, monthly mean air temperatures during the two autumns preceding the field seasons were obtained from the National Climatic Data Center for their Setauket Station, approximately 16 km east of Northport. Autumn 1995 was cooler. For the months of September through December of 1995, monthly deviations from long-term means were -0.9, +0.8, -2.9, and -3.0 °C; whereas for the same months in 1996 the deviations were +0.7, -0.2, -1.5, and +3.0 °C. Length, age, and sex of specimens Striped Bass captured in this study ranged in total length and weight from 167 mm to 995 mm, and 0.2 kg to 10.4 kg, respectively; the mean size of Striped Bass examined was 450 mm and 1 kg. Total lengths differed significantly among the gear types used (ANOVA: P < 0.001), and each gear differed significantly from all other gears (post-hoc test of all combinations: P < 0.007). Captures made by hook and line produced the broadest range in lengths, from 275 mm to 770 mm. Among the gill nets, the 10-cm net captured a smaller range of lengths than the 8-cm and 13-cm net. Striped Bass caught in the power plant impingement were the largest of all examined (799 mm ± 118 std). The shortest specimens (383 mm ± 54 std) were those captured by trawl in Long Island Sound off Connecticut. Ages at time of tagging of Striped Bass that had been tagged at the study site and later recaptured ranged from 2 to 9 years. 2010 K. Williams and J. Waldman 379 Most of the Striped Bass sacrificed and examined were female (ratio = 68:12). Ratios of females to males were highest among fish caught in the Connecticut trawl survey in February (23:4) and were lowest among hookand- line catches at the power plant in March (1:1). Striped Bass that were caught in the impingement grating at the power plant were the only collection in which the sex ratio favored males (5:4). Incidence of lymphocystis and other abnormalities During Field Season I, physical abnormalities were recorded in 76 (7%) of 1094 Striped Bass examined. Some abnormalities involved non-fatal wounds that had been incurred recently or in the past; others involved illness or deformity. The most common non-fatal wound—observed in 56 (5%) individuals— was missing part or all of a caudal fin or caudal peduncle. Other abnormalities included physical deformities, such as lordosis (curvature of the spine) and abnormally shaped opercula. Wounds such as gashes or cuts were most often documented in the body, head, or fins. Illnesses included the presence of parasitic copepods and lymphocystis. During Field Season I, lymphocystis was present in 3% (34 of 1094) of Striped Bass examined at the Northport sampling location and in 2% (3 of 149) of specimens captured by trawl and examined at the other (Long Island Sound) sampling location. These fractions were not significantly different (2 x 2 test of independence: P < 0.480). Forty-six percent of Striped Bass captured that had lymphocystis lesions during Field Season I had them on 25% or less of their body. Lymphocystis was found in 29% (12 of 42) of Striped Bass examined and was significantly more prevalent during Field Season II (2 x 2 test of independence: P < 0.001). During Field Season II, the majority (83%) of Striped Bass with lymphocystis lesions were afflicted on 75% or greater of their body surfaces. Hatchery origin In total, nine Striped Bass tested positive for coded wire tags (0.7% of specimens examined). All were captured during Field Season I and all were from the 1992 year class. Seven of these were sacrificed. All were female and were determined to have been reared in hatcheries around the Chesapeake Bay in 1992. Three of these were reared by the State of Maryland in the Pepco hatchery and were released into the Patuxent River in Maryland in 1992. These three individuals were captured together in the same gill net set at the Northport study four years later, in March 1996. Diet Of Striped Bass examined for stomach contents, 77% of specimens sacrificed for the study had empty stomachs. Even though 70% of the individuals upon which gastric lavage had been performed had some food in their stomachs, the fullest stomach examined contained only six Gammarus spp. (amphipods). Amphipods were the most common food item present. 380 Northeastern Naturalist Vol. 17, No. 3 Other food items observed included polychaetes, crabs, mantis shrimp, and Menidia sp. (Atlantic silverside). Large-scale movements As of 2006, about 21% of Striped Bass tagged in the Northport study had been recaptured and reported to USFWS, 228 from Field Season I and 8 from Field Season II (Table 1). Four individuals, all of which were tagged and released in 1996, were reported with insufficient recapture data to be used in recapture assessments. Tagged individuals were recaptured from North Carolina to Maine, with most recaptures reported from New York (32%), Massachusetts (22%), and Rhode Island (14%). The majority of recaptures were made during the spring and summer immediately after tagging, with many of these occurring in Long Island Sound, Rhode Island, and Massachusetts. During the tagging process, cooperating anglers reported recapturing at the power station 26 individuals that had been marked for this study. In winters subsequent to tagging, none of the Striped Bass marked for the study were recaptured at the Northport site. Local movements During the time that the seven Striped Bass with sonic tags were monitored, ambient Long Island Sound water temperatures outside the effluent varied between 2 °C and 4 °C. However, Striped Bass marked with sonic tags occupied temperatures between 2 °C and 14 °C. The tag signal pattern indicated that some individuals repeatedly occupied the heated water of the effluent, in water temperatures at least 1 °C above ambient Long Island Sound temperature, while others were observed both in and out of the plume. Six of the seven Striped Bass fitted with sonic tags were heard again after Table 1. Recaptures by year from 1146 Striped Bass tagged and released in winter 1995–1996 and 31 tagged and released in winter 1996–1997 that were recaptured from 1996 to 2006. Recaptures from winter 1996–1997 releases appear in parentheses. Ns = no state reported Total Year recaptured per State 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 State ME 2 3 - - - - - - - - - 5 NH 1 - 1 1 - - - - - - - 3 MA 27 12(1) 7 3 1 1 - - - - - 52 RI 25 2 2 2 - 1 1 - - - - 33 CT 15 4 3 1 - 1 - - - - - 24 NY 53 6(5) 2(1) 4(1) 2 1 - - - - - 75 NJ 1 8 3 4 1 1 - - 1 - - 19 DE 1 2 - 1 - - - - - - - 4 MD 1 4 1 1 1 - - 2 - 1 - 11 VA 1 2 1 - - - - - - 1 - 5 NC - - - - - - 1 - - - - 1 Ns - 2 1 - 1 - - - - - - 4 Total 127 45(6) 21(1) 17(1) 6 5 2 2 1 2 - 236 2010 K. Williams and J. Waldman 381 their initial capture; however, as time progressed, sonically tagged fish were located less often. The longest time between tagging and observation of an individual in the plant vicinity was 33 days. This individual had been initially fitted with a sonic tag on 16 February 1997, and was last detected on 21 March 1997. By the end of Field Season II on 4 April 1997, none of the individuals fitted with sonic tags were detectable. Discussion Thermal discharges of electric-generating stations can aggregate Striped Bass in temperate waters and, consequently, provide winter angling opportunities that might otherwise not exist. The station at Northport, NY is well known for this phenomenon and it sometimes supports substantial catchand- release Striped Bass fishing. The popularity of this site is indicated by the fact that shore-based anglers must hike 1 km of sand beach, often in frigid conditions, and that it draws anglers from across Long Island and even New Jersey. However, our two-year study revealed surprisingly high interannual differences in the availability of Striped Bass. Levels of angling activity appeared correlated with these differences, probably through direct feedback from individual angler success and from word of mouth and printed and internet reports of catch levels. We don’t know if the winter abundance of Striped Bass in western Long Island Sound during either of the two field seasons was anomalous compared with long-term averages; however, catch data from Field Season I are far more robust and may allow a more meaningful examination of the characteristics of the Striped Bass that seasonally occur there. The length and age range of Striped Bass captured from all sources in Field Season I indicate they were composed of both immature and mature individuals (Hoff et al. 1988). The significantly different lengths of specimens captured by the different sampling gears used at the Northport site may reflect gear biases, with hook and line being the least selective. The trawl-caught individuals showed the shortest average lengths. However, it is unclear if the trawl was selecting for smaller fish or just encountered an aggregation of smaller fish at the time of sampling. The overall female-to-male sex ratio of 5.7:1 during Field Season I may reflect stock origins. Striped Bass of Hudson River origin are believed to migrate seaward at approximately 1:1 sex ratios (McLaren et al. 1981). However, Striped Bass from Chesapeake Bay have shown pronounced female biases, perhaps as much as 10:1 (Merriman 1941, Schaefer 1968). The observed sex ratio is between the values estimated for the two primary stocks and is suggestive of a mixed stock, as would be expected in northeastern coastal waters (Waldman and Fabrizio 1994). Long-term recaptures of Striped Bass tagged in this study at the power station also show a wide geographic range for these fish that is suggestive of 382 Northeastern Naturalist Vol. 17, No. 3 Chesapeake, Hudson, and possibly Delaware River origins. This is not true, however, for recaptures of tagged hatchery-released Striped Bass. Individuals of hatchery origin composed 0.7% of all specimens examined. Despite the proximity of the Northport site to the Hudson River and the large releases that had occurred from the hatchery on the Hudson River, all seven specimens sacrificed for retrieval of their coded wire tags were of Chesapeake Bay origin. This corresponds with results from the ocean side of Long Island’s south fork where the ratio of hatchery-origin Chesapeake to Hudson River fish was 62:1 in autumn 1991 and 37:2 in autumn 1992, despite approximately only twice as many individuals being stocked in the Chesapeake vs. the Hudson between 1983 and 1989 (Waldman and Vecchio 1996). Waldman and Vecchio believed this imbalance between the ratio of releases in each system and subsequent recaptures was due to differences in survival that may have stemmed from a larger average size at release of Chesapeake Bay stockings, in addition to differences in condition. The recapture together, in one gill net set at the power station in March 1996, of three Striped Bass released together in 1992 from the Pepco Hatchery on the Patuxent River suggests that some long-term schooling occurs in this species. Although food consumption rates for Striped Bass typically decline sharply in winter (Dunning et al. 1997, Hurst and Conover 2001), the individuals in the thermal plume were actively feeding, as indicated by their willingness to strike artificial lures and by the presence of food in some stomachs. However, food items in Striped Bass stomachs were not numerous and were mainly crustaceans, with piscine prey limited to Menidia. Evidence of Striped Bass feeding was not limited to the thermal plume. Striped Bass captured in trawls in open Long Island Sound waters also contained prey items. Capture rates at the site were substantially higher in Field Season I than in Field Season II, probably reflecting a larger number of Striped Bass in the region that winter. This difference was true not only for the Northport study, but also was reported by USNMFS scientists performing a trawl survey for winter flounder in Long Island Sound (R. Alix, USNMFS, Milford, CT, pers. comm.). This trawl survey, initiated during winter 1980–1981, captured few Striped Bass until 1995–1996, when voluminous catches of Striped Bass of approximately 400–450 mm in length were made. The first large catch occurred on 25 January with more than 600 in one tow, followed by a tow 6 days later that included an estimated 2000–3000 Striped Bass, with smaller but substantial catches also made in early February. However, large Striped Bass catches were not repeated the following winter. There was a pronounced difference in the incidence of lymphocystis between Field Seasons I and II, but in an unexpected direction. Lymphocystis virus replicates in the cytoplasm of hypertrophic connective tissue cells that form the external lesions (Lorenzen et al. 1991). The waterborne virus then enters the new host via its epithelium, most often on its gill surfaces. 2010 K. Williams and J. Waldman 383 This transmission mode favors crowded conditions, like those that may occur in thermal plumes (Krantz 1970). However, we saw a manifold greater incidence and a higher degree of external coverage of individuals in Field Season II, when there appeared to be far fewer Striped Bass present at the Northport site. Indeed, the incidence at the Northport site in Field Season I was not significantly different from that seen in aggregations of Striped Bass trawled in unheated waters in Long Island Sound, suggesting that their higher densities in the thermal plume did not favor transmission of lymphocystis. Weather associated with Field Season I was colder than in Field Season II. However, it was during Field Season I that Striped Bass showed fewer physical abnormalities and lymphocystis. This is counterintuitive to the idea that a harsher winter should be more stressful to fish. It would seem that a winter in which the temperature is closer to that preferred by the fish should create an environment where fish are less stressed and thus, appear to be in better physical condition. One possible explanation for this finding is that during Field Season II, when the weather was milder, healthier individuals did not need to use the site for refuge. Perhaps healthier individuals were able to continue on a southern migration pattern that is typical of Striped Bass in winter. In this case, perhaps the fish that used the heated effluent of the power plant plume for refuge were those in poorer condition. Although our study provided insights into the biology of Striped Bass wintering at power plants and the Northport station in particular, many unknowns remain. A fundamental question is what controls the numbers of Striped Bass that utilize the site in a given year? We revealed a large difference in the apparent availability of Striped Bass between the two years of our study. One common theory mentioned by anglers is that the schools become attracted to and then “trapped” in the warm water as ambient temperatures of Long Island Sound decline. A corollary is that more individuals become trapped when temperatures decline unusually rapidly towards the end of the stock’s southerly migration than if temperatures remain higher longer, allowing a greater percentage of the stock to pass to the south. However, in Field Season II, we demonstrated movement of individuals in and out of the thermal plume. Moreover, in Field Season I, trawl surveys in the open waters of Long Island Sound, ≈35 km from Northport and far from the influence of power station discharges, held large numbers of Striped Bass. Thus, we reject the notion that Striped Bass are thermally trapped at this location. An alternative hypothesis is that the numbers of Striped Bass wintering at the latitude of Long Island Sound varies interannually and that the availability of individuals at the Northport site is correlated with the number of individuals wintering in open waters. We see several possible and not mutually exclusive explanations for this variability. One is that the autumn migration southward is influenced by the rate of declining water temperatures 384 Northeastern Naturalist Vol. 17, No. 3 and that a temperature threshold at which migration ceases may be crossed in different years with varying percentages of the migratory stock having moved past Long Island. There was some support for this on a gross level from the autumn temperature data available across the two field seasons. Another possible influence is the proportion of southerly migrating individuals that pass through Long Island Sound vs. the alternative of traveling along the south shore of Long Island; Merriman (1941) believed that storms influenced Striped Bass to move farther towards the Sound before crossing to Long Island. Austin and Custer (1977) also discerned intra-Sound autumn movement patterns in which Striped Bass from both sides of the Connecticut shore moved towards its center before crossing the Sound to Long Island and then exiting in both directions. Finally, there may be density-dependent factors in the size of the wintering range of Striped Bass. The range of marine fishes often is positively correlated with population size (MacCall 1990). Large abundances of Striped Bass may have more northerly migrating individuals (Merriman 1941) and thus, more northerly wintering individuals than lesser abundances. The 1993 Maryland Striped Bass juvenile index, at nearly 40, was (at that time) the highest recorded since the inception of the index in 1957, was far higher than the 1992 (9.0) and 1994 (16.1) values, and would be expected to generate large numbers of first-time migrants from the bay as two-year olds in 1995. Acknowledgments For many kinds of assistance, we thank Robert Alix, Mark Bain, Jeff Buckel, Tony Calabrese, Pam Carlsen, Roy Cash, Nancy Craig, Felix Edwards, Guy Hilbert, Tom Hurst, Bill Kobel, Terry Marburger, Andy Matthews, Conky Nostrand, Kim Roberts, Teresa Rotunno, Art Schweitheim, Nancy Steinberg, Byron Young, and, especially, Carl Lobue. We also thank for financial support the Long Island Beach Buggy Association and the Sounds Conservancy Grants Program. Literature Cited Austin, H., and O. Custer 1977. Seasonal migration of Striped Bass in Long Island Sound. New York Fish and Game Journal 24(1):53–68. Austin, H., J. Dickinson, and C. Hickey. 1973. An ecological study of the ichthyofauna at the Northport Power Station Long Island, New York. LILCO Contracted Report Number SR 72-23F. New York Ocean Science Laboratory, Montauk, NY. Boreman, J., C. Goodyear, and S. Christensen. 1981. An empirical methodology for estimating entrainment losses at power plants sited on estuaries. 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