ATLANTIC STURGEON POPULATION MONITORING FOR THE HUDSON RIVER ESTUARY: SAMPLING DESIGN AND GEAR RECOMMENDATIONS

Transcription

1 ATLANTIC STURGEON POPULATION MONITORING FOR THE HUDSON RIVER ESTUARY: SAMPLING DESIGN AND GEAR RECOMMENDATIONS FINAL STUDY REPORT by Mark B. Bain, Douglas L. Peterson, Kristin K. Arend, and Nancy Haley New York Cooperative Fish and Wildlife Research Unit Department of Natural Resources - Cornell University Ithaca, NY for Andy Kahnle Hudson River Fisheries Unit Bureau of Marine Resources NY Department of Environmental Conservation 21 South Putt Corners Road New Paltz, NY and The Hudson River Foundation 40 West 20th Street, 9th Floor New York, NY November 1999

2 Bain M. B, D. L. Peterson, K. A. Arend, and N. Haley Atlantic sturgeon population monitoring for the Hudson River estuary: sampling design and gear recommendations. Final Report to the Hudson River Fisheries Unit, New York Department of Environmental Conservation, New Paltz, NY and The Hudson River Foundation, New York, NY. New York Cooperative Fish and Wildlife Research Unit, Cornell University, Ithaca, NY, 34 pages. Summary The goal of this report is to identify a protocol for assessing the status of Atlantic sturgeon in the Hudson River on an annual basis and at a practical level of effort and cost. Life history information is reviewed to determine what life stages provide an opportunity for population assessment. Large-scale sampling, telemetry, and targeted sampling results are reviewed for designing field assessment efforts. Finally, a gear evaluation experiment is reported to identify the best techniques for sampling Atlantic sturgeon in the Hudson River. Executing the recommended methods should provide the key information needed to judge the benefits of the current fishery moratorium, and assess progress toward population restoration goals specified in the current fishery management plan. The life cycle of Atlantic sturgeon greatly limits the opportunity to monitor population size through time even in relative terms (e.g., catch per unit effort). Monitoring of young sturgeon is the only opportunity available to assess the whole Hudson River population because adults display variable spawning periodicity, a protracted spawning season, and the use of multiple spawning areas. Habitats of the Hudson River that support juvenile Atlantic sturgeon are primarily concentrated in the lower half (south of about Poughkeepsie NY) of the tidal Hudson River. The distribution pattern of juvenile Atlantic sturgeon reflects a gradual downriver shift in distribution with increasing age and size with an affinity for brackish water habitats at larger sizes. The summer occurrence of mostly 1 year old Atlantic sturgeon in south Newburgh Bay offers the best opportunity for monitoring. This group of fish is the largest sized sturgeon with no evidence of emigration from the Hudson River, and they are found at a location distinct from that used by older fish that have begun emigrating. Ultrasonic telemetry of 30 juvenile Atlantic sturgeon resolved some key details of the biology of juvenile sturgeon. Telemetry and targeted gill netting in 1997 revealed a concentration of age 2 and older Atlantic sturgeon in north Haverstraw Bay. About half of these fish remained in the Hudson River well into the summer of The other half of the marked age 2 fish appear to have left the river in the fall of 1997 or the spring of These observations indicate that a significant number of juvenile Atlantic sturgeon emigrate from the Hudson River in the year they reach about 630 mm FL as Dovel and Berggren (1983) concluded. The distribution of age 2 and older sturgeon and their emigration behavior precludes confident use of sturgeon catch in Haverstraw Bay as an index of recruitment and population trend. Gill net catch rates were significantly greater than catch rates from trawling but gill nets varied much more than trawling in catch. Despite the statistical differences in catch, the two forms of sampling were roughly similar. Nevertheless, gill nets would be preferred over trawling because gill net sampling is less demanding in terms of boat capabilities, crew expertise, and equipment cost. Compared to trawls, gill nets are cheap, easily mastered by summer field staff, repairable, and usable from small boats. Our main finding then is that standardized gill netting in south Newburgh Bay during summer will provide data on the relative status of recruitment and population trend for the Hudson River Atlantic sturgeon population. Targeting the age 1 sturgeon holds the promise of assessing the whole Hudson population by annual tracking of a single critical year class. 2

3 Introduction Atlantic sturgeon (Acipenser oxyrhynchus ) of the Hudson River historically supported major fisheries in New York and New Jersey coastal waters and in the river. These fisheries had one period of very high harvest (pre-1900s), a long period ( ) of minimal harvest and slow population recovery, a period ( ) of renewed abundance and high harvest, and finally another decline ( ). The greatly diminished abundance of Hudson Atlantic sturgeon in recent years is now widely recognized, and the fisheries were closed in 1996 by joint actions of the Atlantic States Marine Fisheries Commission (1996) and the New York Department of Environmental Conservation (1996). Annual monitoring of sturgeon populations, including the Hudson River stock, are not now available, and there are no standard methods to monitor population responses to the current fishery moratorium. The management plan for Atlantic sturgeon (Atlantic States Marine Fisheries Commission 1990, 1998) specifies that methods need to be developed for monitoring Atlantic sturgeon populations. Some method is needed to at least assess population trends so that the effect of fishery regulations can be evaluated through time. The current low abundance of Atlantic sturgeon in the Hudson River has been reported by Peterson et al. (2000). Using gill net captures of marked and unmarked juveniles, they calculated that there were 4,313 wild age-1 Atlantic sturgeon in the Hudson River in 1995, a decline of about 80% from a similar population estimate conducted in 1977 by Dovel and Berggren (1983). Also, catch per unit effort for Atlantic sturgeon in the Hudson River Estuary Monitoring Program (Electric Utilities of the Hudson River) over the past 11 years indicates that recent abundance has been at its lowest recorded levels. Peterson et al. (2000) concluded from an assessment of these different results that Atlantic sturgeon are at a very low abundance in the Hudson River, and that recruitment over the last 6 years has been very poor. 3

4 Previous research (e. g., Kieffer and Kynard 1993; Van Eenennaam et al. 1996; Waldman et al. 1996a, 1996b; Boreman 1997; Johnson et al. 1997; Stevenson 1997; Van Eenennaam and Doroshov 1998) on Atlantic sturgeon addressed topics like habitat, distribution, stock structure, age and growth, and reproductive behavior. These studies have provided valuable insights, understanding, and data on ecology and behavior of the species; information vital for assessments of population abundance. Past Atlantic sturgeon research on the Hudson River has also relied heavily on gill nets as the main sampling method. Gill nets are inexpensive, relatively easy to deploy, and effective for capturing sturgeon in the Hudson River, but they can be used only during brief periods of slack tide (typically one set per day for less than 1 hour). The feasibility of obtaining abundance data by trawl sampling was demonstrated by the estimate of Atlantic sturgeon abundance reported in Dovel and Berggren (1983). Trawling in the Hudson River Electric Utilities monitoring program also records Atlantic sturgeon, and trawling is used in other waters with sturgeon. Trawling has the advantage of being usable at all times, but the gear requires expensive, specialized equipment and an accomplished field crew. The goal of this report is to identify a protocol for assessing the status of Atlantic sturgeon in the Hudson River on an annual basis and at a practical level of effort and cost. Life history information is reviewed to determine what life stages provide an opportunity for population assessment. Largescale sampling, telemetry, and targeted sampling results are reviewed for designing field assessment efforts. Finally, a gear evaluation experiment is reported to identify the best techniques for sampling Atlantic sturgeon in the Hudson River. A protocol is recommended that is practical to implement and feasible for obtaining annual population status information. Executing the recommended methods should provide the key information needed to judge the benefits of the current fishery moratorium, and assess progress toward population restoration goals specified in the current fishery management plan (Atlantic States Marine Fisheries Commission 1998). 4

5 Opportunities for Population Monitoring Hudson River Atlantic sturgeon spawn at multiple sites in the freshwater portion of the Hudson River, with the most well known spawning site near Hyde Park, NY (Bain 1997). After spawning (late May through mid-august), males remain in the river and females may go to the sea or remain in the lower Hudson River until early fall (Figure 1). Stevenson (1979) estimated that females spawn once every four years from optical and chemical analyses of fin rays and otiliths, whereas males are thought to spawn every year (Vladykov and Greeley 1963; Bain 1997). Larvae appear a month after spawning starts, and they are believed to remain in the freshwater portion of the river and gradually move downstream during the summer and fall (Dovel and Berggren 1983). This movement may be governed by the development of salinity tolerance in young Atlantic sturgeon, as suggested by studies of salinity tolerance in other sturgeon species (McEnroe and Cech 1987). Juvenile Atlantic sturgeon grow rapidly in the Hudson River exceeding 70 cm total length [TL] (63 cm fork length [FL]) by three years of age (Stevenson 1997; Stevenson and Secor 2000). Juvenile sturgeon of this size and age begin migrating to marine waters (Dovel and Berggren 1983; Bain 1997), and Dovel and Berggren (1983) concluded that all or most age 6 or younger sturgeon are marine migrants. These marine migrant juveniles have been observed to move back into coastal river systems during summer (Smith 1985; Keiffer and Kynard 1993), and one location (river kilometer [km: distance above the downstream tip of Manhattan Island, New York, NY] 78) near Garrison, New York supports a concentration of marine migrant juveniles (>630 mm FL or >700 mm TL), adults, and fish that had spawned earlier in the same year (Bain et al. 2000, Figure 1). Male and female Hudson River Atlantic sturgeon reach maturity at 117 and 173 cm FL (133 and 197 cm TL) and ages 12 and 14 respectively (Van Eenennaam et al. 1996, Van Eenennaam and Doroshov 1998). 5

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7 Aside from the challenge posed complexity, the life cycle of Atlantic sturgeon greatly limits the opportunity to monitor population size through time even in relative terms (e.g., catch per unit effort [CPUE]). Typically, fishery monitoring is done by measuring relative adult numbers as CPUE at spawning sites or during spawning migrations. This monitoring strategy is not viable for Hudson Atlantic sturgeon for three reasons: variable spawning periodicity (Van Eenennaam et al. 1996; Stevenson 1997), a protracted spawning season (Van Eenennaam et al. 1996; Bain 1997), and the use of multiple spawning areas in the Hudson River (Bain 1997). Large migrating juveniles offer no opportunities for monitoring since these fish occupy multiple rivers, estuaries, and coastal waters over a large part of the US Atlantic coast. Eggs and larvae are difficult to sample due to their small size and location in deep, river channels. Also, these earliest life stages may not reflect adult population levels due to the typically high mortality in the early life of most fish species. The only remaining option is the monitoring of early juveniles fish because this is the most advanced life stage where all members of the population remain in one waterbody - the Hudson River estuary. The precise size or age of juvenile emigration from the Hudson River to the Atlantic Ocean has not been established. Dovel and Berggren s (1983) conclusion that emigration begins as early as age two is based on the decline in abundance of juvenile sturgeon by size and age group when they studied the species in the 1970s. Dovel and Berggren reported that juvenile Atlantic sturgeon grow quickly in the first three years of life: reaching 70 cm TL at age 3. Available life history information and the conclusions of Dovel and Berggren (1983) were used by Bain (1997) to propose a period when all juvenile Atlantic sturgeon of the Hudson River population remain in the river. This key, whole-population resident period, is supported by all available life history information and is termed by Bain (1997) as the early juvenile stage. 7

8 The early juvenile stage begins after the transition from the larval stage (1 month age, about 30 mm TL) and extends to age 2 and about 440 mm FL or 490 mm TL. An intermediate or transitional juvenile stage follows and this period includes sturgeon may outmigrate from the river. The intermediate juvenile stage extends from age 2 and about 440 mm FL (490 mm TL) to age 6 and 630 mm FL (700 mm TL). These size and life stage criteria are largely from Dovel and Berggren (1983) who were not able to directly estimate emigration ages and size. However, they fit the general growth pattern reported by Stevenson and Secor (2000), and the pattern of declining in the abundance of juvenile Atlantic sturgeon in the Delaware River (Lazzari et al. 1986). Peterson et al. (2000) constructed seasonal length-frequency histograms for all small juvenile Atlantic sturgeon captured in the Hudson River in 1995 and identified wild age-1 fish as those attaining 383 to 473 mm FL ( mm TL) from March through July, mm FL ( mm TL) during the month of August, 473 to 540 mm FL ( mm TL) during the month of September, and 518 to 563 mm FL ( mm TL) from October to mid-december. These sizes are larger than reported from other sources listed above for age 1 sturgeon but still within a reasonably consistent range. The young Atlantic sturgeon grow very fast through the summer of age 1, and they then begin slowing in growth especially during the second year of life or third season of growth. If the 700 mm FL criteria for the initiation of emigration is assumed, then age 2 sturgeon will be of a size to start outmigration and that will most likely begin in the fall of the second year of life. If true, the age 2 fish in their third growing season could be considered as a safe group for assessing the total Hudson River population of Atlantic sturgeon. Otherwise, the only period for confident and practical population assessment will be the first year of life or second growing season. 8

9 Methods During the summers of 1995 and 1996, we sampled the suspected range of juvenile Atlantic sturgeon in the Hudson River: km 43 (Tappan Zee Bridge, Nyack, NY) to km 245 (I-90 Bridge, Albany, NY). We used a stratified random sampling design, based on seven strata identified by Coch and Bokuniewicz (1986; Figure 2), to apportion effort equally throughout the 173-kilometer study area. Individual sampling stations (located at discrete river kilometers) were selected using a random numbers table. To insure equivalent temporal effort throughout the study period, an equal number of samples were taken in each stratum per month (i.e., June, July, August/September). Whenever possible (i.e., when conditions were suitable for deploying gill nets), an equal number of stations was sampled along each shore of the river within each stratum. At each station, we deployed three gill nets (3 m high x 94 m long) with stretch mesh sizes measuring 5-, 10-, and 15-centimeters. Previous research efforts on Atlantic sturgeon in the Hudson River showed that these net sizes are effective for capturing the range of juvenile-size sturgeon ( mm FL; Bain, unpublished data; Dovel and Berggren 1983; Haley et al. 1996). Gill nets were deployed as follows: parallel to and approximately 30-m apart from one another; only in daylight and during slack tide for optimal fishing and to minimize gear damage; anchored on the bottom and set perpendicular to shore for an average of 40 minutes; and positioned between mid-channel and the shoreline, extending from the shoal region (3-6 m water depth) to the main channel. Captured fish were immediately removed from the nets and placed into floating cages along side the research vessel. All sturgeon were released within 1 hr of capture and no mortalities occurred during capture, handling, or release. We analyzed the frequency distribution of juvenile Atlantic sturgeon collected in 1995 and 1996 to determine which zone of the river were used most and which zone were avoided by juvenile Atlantic sturgeon. 9

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11 In October 1994, 4,929 hatchery-reared Atlantic sturgeon from the U.S. Fish and Wildlife Service (USFWS) Lamar Fish Hatchery (Lamar, Pennsylvania) were stocked into the Hudson River by the New York Department of Environmental Conservation just downstream of Storm King Mountain (km 89-90, New York Department of Environmental Conservation 1994). The USFWS clipped the left-pelvic fins and implanted coded wire tags in all stocked fish before they left the hatchery. Our random gill-net sampling program began six months after the release of the hatchery fish (i.e., 1995). The hatchery produced Atlantic sturgeon remained identifiable and an important part of the pool of study fish through the end of our work in In 1997, we modified our gill netting protocol to target regions of the river where the highest numbers of juveniles had been captured through random netting conducted from April- December in From mid-april through May in 1997, we captured 31 juvenile sturgeon (14 hatchery-raised and 17 wild) that were telemetry tagged with external ultrasonic tags made by Sonotronics. Tagged fish ranged in size from mm FL (mean 594, standard deviation 56) and in weight from g (mean 1526, standard deviation 468). Thus, according to the length criteria of Bain (1997), the telemetry marked fish included 1 river-resident juveniles (< 440 mm FL), many (19) freshwater-marine transitional juveniles ( mm FL), and some (11) marine-migratory juveniles (>630 mm FL). The tag of one fish recaptured in a gill net approximately 2 weeks after tagging had partially ripped out of the dorsal fin and consequently was removed and attached to another fish captured at a later date. Therefore, 30 tagged fish were being monitored at any one time. Tagged sturgeon locations and movements were monitored by boat throughout an 138-km stretch of the Hudson River, between the Tappan Zee Bridge near Nyack, NY (km 43) and the Rip Van Winkle Bridge near Catskill, NY (km 181). Monitoring began approximately 2 weeks 11

12 after the first sturgeon were tagged in 1997 and continued throughout and beyond the tagging period. Three to four days were typically required to complete a survey of the entire study area. Surveys occurred approximately once a week between 16 June and 31 October 1997, and on 22 December 1997 (km ). In 1998, surveys were conducted three times between March and April and every other week between 18 May and 3 July. Fish were located using directional hydrophones (Sonotronics; sensitivity -84 dbv ref 1 µbar) mounted on PVC pipe, and connected to a digital receiver (Sonotronics; KHz nominal frequency; LCD display of frequency or pulse interval; sensitivity 1 µvolt for 30 db [S+N]/N ratio). Using data provided by the telemetry marked fish, we further narrowed our 1997 targeted gill net sampling area in an effort to maximize our CPUE of juvenile Atlantic sturgeon. A trawling versus gill netting catch rate experiment was conducted in South Newburgh Bay (km ) in July and August The same gill nets used in previous years (3 m high x 94 m long, stretch mesh sizes measuring 5 and 10 cm) were deployed as described above. On each sampling day, 4 nets (2 of each mesh size) were set for approximately 40 minutes during the slack (before ebb) tide. This 4-net deployment constituted one unit of gill net effort because it was the level of sampling effort with gill nets that could be routinely deployed by one boat in a day of fieldwork. Trawling was conducted with a 6-m, 4-panel high-rise otter trawl with a 6.4-m headrope and a 7.3-m footrope. The trawl net was made of nylon netting of the following size mesh and thread: body of 51-mm stretch mesh #18 thread, codend of 38-mm stretch mesh # 18 thread, and a headrope of 12-mm poly dac net rope with three 20-cm plastic floats, spaced evenly apart. One unit of trawling effort was set to be 4 trawl tows of ten minutes each. Both gear types were used in the same location during the same day; gill netting was completed prior to trawling. This design was intended to results in an equal distribution of "field days" devoted to each sampling gear. 12

13 Results Large-Scale Random Sampling Fifty-three Atlantic sturgeon were captured in the randomly deployed gill nets (61 stations, 183 net sets) in 1995 and 36 were captured in 1996 (105 stations, 315 net sets). Fork lengths of the Atlantic sturgeon captured in 1995 ranged from 236 mm to 1,295 mm (mean = 712 mm FL; Figure 3). At least ten of these fish had fin clips that identified them in the field as hatchery-reared sturgeon, and these fish averaged 361 mm FL (range: mm FL). In 1996, captured Atlantic sturgeon ranged in size from 394 mm to 1,225 mm FL (mean = 653 mm FL; Figure 3). Fourteen of the juvenile Atlantic sturgeon captured in 1996 were stocked fish; ranging in fork length from 394 mm to 560 mm (mean = mm FL). Pooling captures across years, the Atlantic sturgeon were distributed non-uniformly among the sampling strata (X 2 (6 df) = 33.37; p <.001; Table 1). The Atlantic sturgeon occurred at more sites than expected in the Highlands Gorge and Wide River strata, and at fewer sites than expected in the two uppermost strata (Bifurcating Channel-Shoal and Confined and Dredged Channel; Figure 4). Stocked Atlantic sturgeon were captured between river kilometers 100 and 154 in 1995 and were centered in distribution around km 120. Wild sturgeon within the size range of the stocked fish ( < 600 mm FL; n = 10) were found between km 65 and 110 in 1995 and they were centered in distribution (km 95) farther downriver than the stocked fish (Table 2). The cohort of small Atlantic sturgeon was, on average, slightly larger than the stocked group (431 mm FL compared to 361 mm FL). In 1996, stocked Atlantic sturgeon were distributed more like their wild counterparts (Table 2). Stocked sturgeon were centered in distribution at km 94 (ranged, 57 to 140) with the similar-sized wild sturgeon centered at km 92 (range 68 to 131, Table 2). The sample of stocked Atlantic sturgeon collected in 1996 was larger on average (495 mm FL) than the stocked fish captured in When 1995 and 1996 distribution data are pooled, the mean distribution for stocked Atlantic sturgeon was not significantly different than the mean distribution of small Atlantic sturgeon. 13

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15 Table 1. Observed and expected capture counts (presence) and total chi-square for a test of the distribution of juvenile Atlantic sturgeon within the Hudson River. The distribution of fish was significantly (X2 (df = 6) = 33.37; p < 0.001) different from a uniform distribution. River strata Observed Expected Wide estuary Highlands gorge Wide river Narrow river Meandering channel-shoal Bifurcating channel-shoal Confined and dredged channel

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17 Table 2. Summary statistics on the river distribution of small (< 600 mm FL) wild and stocked Atlantic sturgeon by sampling period. Sampling Period Sturgeon Group N 1 Mean ± S.D. (rkm) Median (rkm) Range (rkm) 1995 Wild 6 95 ± Stocked ± Wild 5 92 ± Stocked ± & Wild ± Stocked ± N = number of unique capture stations 17

18 Telemetry and Targeted Sampling The 30 sonic telemetry marked juvenile Atlantic sturgeon tracked in 1997 provided some detailed information on fish distribution and movements. The sonic tagged fish were primarily age 2 or older fish ( mm TL, mean 594 mm, only one fish under 500 mm TL) since we needed fish large enough to carry the sonic tags without stress. About half (17) were wild juveniles and the 13 others were hatchery fish similar in size. From July through October 1997, the highest concentrations of both hatchery and wild juveniles were observed in north Haverstraw Bay (km 60-65). Although some juveniles made sporadic upstream migrations of as much as 64 km in a 1 to 3 day period, they typically returned to a 20 km section of the river between West Point (km 84) and North Haverstraw Bay. Most juveniles remained in this portion of the Hudson River throughout the study, however, occasional forays up to Newburgh Bay (km 92-98) were not uncommon. Some fish appear to have shed their telemetry tags because the tags remained stationary well into The others were regularly monitored in the river with absences in surveys being rare unless the fish left the system. Eight juvenile sturgeon ( mm FL) were regularly recorded in the Hudson River and they were found to be actively moving up to the end of the surveys in July Five juvenile sturgeon ( mm FL) appeared to have migrated out of the Hudson River in the fall of A final 4 juvenile sturgeon ( mm FL) were well monitored into the Spring of 1998 when it appeared they left the river system. Consequently, the telemetry records indicated that about 50% of the juvenile Atlantic sturgeon departed the Hudson River between fall 1997 and spring The other 50% remained as resident fish through the one year period. Size does not appear highly related to resident of marine migration behaviors. 18

19 Using the telemetry findings on the fairly large (age 2 and older) juvenile sturgeon, a concentration was located at km 62 near Haverstraw, NY. From 1 July through 1 August 1997, we captured 135 juvenile Atlantic sturgeon using targeted gill netting in a 1 km section of the river located near the shipping channel at km 62. This catch included primarily age 2 and older juvenile sturgeon composed of similar numbers and sizes of wild (averaged 685 mm TL; mm) and stocked (mean 603 mm TL, range ) fish. Although the abundance of juvenile Atlantic sturgeon was very low (Peterson et al. 1999), targeted gill netting at this location yielded catch rates at least 10 fold higher than in either of the previous two years, when random sampling was used (Table 3). Gear Evaluation Experiment Gill net sampling at km 62 near Haverstraw in the early summer of 1998 failed to capture juvenile Atlantic sturgeon in good numbers as seen in This change suggests that the group of sturgeon that were age 2 in 1997 may have emigrated from the Hudson River by 1998 and there were few sturgeon of that size in the river. The telemetry results showed about 50% of these size sturgeon may have left the river by summer of Consequently, the gear evaluation experiment was conducted in south Newburgh Bay (km range 93.3 to 95.7) where distribution results from 1995 and 1996 indicated small Atlantic sturgeon may be found. Six weeks of paired gear deployments were executed from 6 July 1998 to 14 August 1998 with good success capturing young sturgeon; primarily age 1. A total of 93 trawl samples captured 51 sturgeon and 87 gill net sets (one 10 and one 15 cm nets per set) captured 180 sturgeon (Table 4). The gears captured similar size sturgeon that were concentrated at sizes expected for age 1 fish (Figure 5). Trawl captured sturgeon that averaged 403 mm FL (range 84 to 593) and gill net captures were similar in size (average = 449 mm FL, range 220 to 820 mm). 19

20 Table 3. Catches of juvenile Atlantic sturgeon from the Hudson River in random and targeted gill net sampling. Year Total Catch Gill net sets CPUE Deployment Random Random Targeted 20

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23 The level of sampling effort was similar among gears (23.25 units trawling, units of gill netting) but not precisely equal as intended due to logistical limitations and sampling problems (snagged trawls, tide constraints on gill nets, etc.). The paired results by week (Table 4) varied considerably in catch rate but gill netting CPUE exceeded trawling CPUE in 5 of 6 periods. Gill net CPUE was significantly greater than trawling CPUE (Wilcoxon Signed Rank test, p ) although gill nets varied much more than trawling in catch rates (Figure 6). A paired t-test did not indicate a significant difference among gears (p = ) but the large variation in distribution between gears in CPUE supports the use of a non-parametric statistic (Wilcoxon Signed Rank) for this test. Mean gear CPUE was more different (8.7 versus 2.0 CPUE by gill netting and trawling respectively) than median CPUE (6.6 versus 1.3 as in Figure 6) differences. However, the 95% confidence interval about the mean paired differences included zero (15.2 to -1.8) while interquartile ranges about the medians did not overlap (Figure 6). Again, these findings suggest that the non-paramteric test provides the best evidence for differences in gear CPUE values. 23

24 CATCH PER UNIT EFFORT Trawls Gill Nets SAMPLING GEAR Figure 6. Catch of juvenile Atlantic sturgeon per unit of gill netting and trawling effort paired by sampling week (n= 6 per gear). The plots show the range, interquartile range, and median values. 24

25 Discussion The unusually low abundance of juvenile Atlantic sturgeon in the Hudson River during this study complicated our assignment to define the best methods for sampling and population monitoring. However, in some ways the low numbers of fish helped because small and geographically distinct groups of sturgeon could be distinguished by age and size. The summer occurrence of mostly 1 year old Atlantic sturgeon in south Newburgh Bay offers the best opportunity for clear whole-population monitoring. This group of fish is the largest sized sturgeon with no evidence of emigration from the Hudson River, and they are found at a location distinct from that used by older fish that have begun emigrating. Gill nets are effective for capturing these sturgeon, and they are easier and less expensive to use than trawling. Our main finding then is that standardized gill netting in south Newburgh Bay during summer will provide data on the relative status of recruitment and population trend for the Hudson River Atlantic sturgeon population. Habitats of the Hudson River that support juvenile Atlantic sturgeon are primarily concentrated in the lower half (south of about Poughkeepsie NY) of the tidal Hudson River. The Highlands Gorge section appears to be the most heavily used region, however the telemetry results indicate this river section is a movement corridor between other high use habitats. Also, the Highlands Gorge section includes the major concentration site for marine migrant juveniles (>630 mm FL, Bain 1997) that are not of interest for population monitoring, but these fish were recorded in the large-scale sampling. Dispersed sampling in 1995 included a relatively large group of age 1 juveniles and another group of mixed age, migratory juveniles. The center of distribution in 1995 was well upriver in freshwater where the small sturgeon were most numerous. The 1995 peak of small yearling sturgeon included a large portion of stocked fish, so this apparent abundance peak was in part created by stocking Atlantic sturgeon of the 1994 year class. Sampling in 1996 produced a much different picture. The abundance peak was on age 2 fish with hatchery source 25

26 juveniles accounting for a large portion. The abundance of large, migratory juveniles dropped and the center of distribution for all juvenile sturgeon moved downriver into brackish waters. Overall, the distribution pattern of juvenile Atlantic sturgeon matches the pattern reported from observations in other river systems (Merrimack River [Kieffer and Kynard 1993], Cape Fear River NC [Moser and Ross 1995]): a gradual downriver shift in distribution with increasing age, size, and presumably saltwater tolerance with an affinity for low salinity habitats at larger sizes. The largescale sampling and catch data defines juvenile sturgeon habitat in large and general terms, and it confirmed other accounts of shifts associated with size and age. Ultrasonic telemetry of 30 juvenile Atlantic sturgeon resolved some key details of the biology of juvenile Atlantic sturgeon. Telemetry and targeted gill netting in 1997 revealed a concentration of age 2 and older Atlantic sturgeon in north Haverstraw Bay. Again using the telemetry results, about half of these fish were found to remain in the Hudson River well into the summer of The other half of the marked fish appear to have left the river in the fall of 1997 or the spring of Finally, attempts in 1998 to repeat the locally high catch of juvenile Atlantic sturgeon seen in 1997 failed. Taken together, these observations indicate that a significant number of juvenile Atlantic sturgeon emigrate from the Hudson River in the year they reach about 630 mm FL as Dovel and Berggren (1983) concluded. This size is associated with age 2 fish, and emigration can occur in the spring and fall. Length at emigration apparently varies since telemetry marked fish smaller and larger than this length both emigrated from the Hudson and remained resident for another year. Unlike Dovel and Berggren (1983), we conclude that the distribution of age 2 and older sturgeon and their emigration behavior precludes confident use of sturgeon catch in Haverstraw Bay as an index of recruitment and population trend. Catch rates in the brackish, lower Hudson should fluctuate with changing overall abundance of Atlantic sturgeon, but the portion of the population being sampled will include several age groups and mostly the fish that did not yet emigrate. 26

27 Our observations, results, and conclusions about the behavior and distribution of juvenile Atlantic sturgeon complement similar investigations in other systems. Lazzari et al. (1986) reported that small Atlantic sturgeon were concentrated in the freshwater, tidal portion of the Delaware River during summer. The size composition of their sturgeon catch (mostly mm FL) suggests that many of the fish were 1 and 2 years old and not yet large enough to begin river emigration. The sharp drop on juvenile sturgeon catch at sizes greater than 700 mm FL also suggested that the sizes associated with river emigration were similar to the Hudson River. Extensive up and downriver movement of juvenile Atlantic sturgeon in the Delaware River reported by Brundage and Meadows (1982) appears similar to what we observed with our telemetry marked fish. However, once size is used to partition behavior patterns, the distribution reported by Lazzari et al. (1986) becomes much like what we describe for the Hudson River. In the Saint John River (New Brunswick), Atlantic sturgeon less than 450 mm FL were primarily located in freshwater and larger fish were concentrated in the lower, brackish river (Pottle and Dadswell 1979) much as we describe for the Hudson River. Recently (Rochard 1992; Rochard et al. 1999), the ecology of juvenile European Atlantic sturgeon (Acipenser sturio ) has become well documented because of a large, successful spawning in the Gironde Estuary of France during This species is closely related to A. oxyrinchus, and the movements and distribution of juveniles closely matches our description of the juvenile Atlantic sturgeon in the Hudson River. Age 1 fish (42-55 cm TL, summer) occupy the upper, freshwater habitat of the Gironde system, and these fish gradually redistribute in the lower, brackish waters of the estuary at age 2 and sizes of cm TL. Some fish were captured in commercial fishing gear at the mouth of the estuary. These cm TL Atlantic sturgeon were regarded as newly emigrating fish. While the biology of young Atlantic sturgeon has received little attention in comparison to adults, the information and data that is available suggests a very similar pattern of distribution and movements to our findings for the Hudson River. 27

28 Our comparison of sturgeon catch rates and sampling efficiency for the commonly used gill nets and often used trawling gear indicated a significant but minor catch advantage with gill nets. The average difference was small though, and had we changed the gear deployment details we may have found no significant catch rate differences among these gears. Accepting the two forms of sampling as roughly comparable, gill nets would be preferred due to less demanding boat capabilities, crew expertise, and equipment cost. Compared to trawls, gill nets are cheap, easily mastered by summer field staff, repairable, and usable from small boats. Where trawling is a permitted fishing gear for Atlantic sturgeon, gill nets are used by commercial fisherman and the catch exceeds trawl captures by a wide margin (Collins and Smith 1997). Trawls (similar size gear as ours) were used extensively in the recent juvenile sturgeon research in the Gironde estuary France (Rochard et al. 1999). Their average catch (0.47 fish/trawl, 30 minute trawl) which was about equal to our lowest weekly average for shorter (10 minutes) trawl hauls suggesting our CPUE values were very good compared to another experienced field crew. Also, the use of our short trawl hauls follows the recommendations of Bagenal (1979) that many, short trawl samples are preferred to fewer, longer hauls based on extensive gear evaluation experiments. Overall then, gill nets would seem the preferred sampling gear despite good success with trawls in past research on sturgeon in the Hudson River (Dovel and Berggren 1983) and elsewhere (Rochard et al. 1999). Monitoring Recommendations Annual monitoring of the Hudson River Atlantic sturgeon population could be conducted best by standardized gill net sampling of age 1 juvenile fish with gill nets set in south Newburgh Bay (km range 93 to 96). Targeting the age 1 sturgeon holds the promise of assessing the whole Hudson population by annual tracking of a single critical year class. Methodological details can be the same as those describe in this report for gill net sampling. We recommend computing annual CPUE as done in this study, with attention given to the size structure of the catch as an indication 28

29 of age composition. Adequate sample size will be difficult to judge until results are obtained from years with more normal sturgeon abundances. Our 1998 gill net catch varied greatly by week, and we would expect this same pattern as long as the fish are few and likely patchy in distribution. In the event that annual sampling as we recommend fails to record many or any small Atlantic sturgeon, effort should be shifted to north Haverstraw Bay following our methods of Low recruitment would be indicated by sparse captures of age 1 fish in Newburgh Bay, and captures of older fish in Haverstraw Bay could act as a check that other age groups are present or that behavior has not changed. The practice of tagging of all captured Atlantic sturgeon with durable marks like PIT tags should also be maintained because a mark and recapture estimate of age 1 cohort size may be achievable when Atlantic sturgeon are in greater abundance. We feel the case is quite clear that estimates of age 1 fish abundance can be obtained with standard mark and recapture estimators and confidently applied to the whole Hudson River Atlantic sturgeon population - a quantitative measure of an entire cohort. However, some substantial increase in sturgeon abundance will be needed before such an estimate can be done at a practical level of field effort. That is, relative CPUE monitoring is practical and meaningful now, and when the population recovers from current low levels a precise population estimate should be easily obtained. 29

30 References Atlantic States Marine Fisheries Commission Fishery management plan for Atlantic sturgeon. Atlantic States Marine Fisheries Commission, Fishery Management Report 17, Washington, DC, USA. Atlantic States Marine Fisheries Commission ASMFC recommends two-rear moratorium for Atlantic sturgeon. News Release March 20, 1996, Washington, DC. Atlantic States Marine Fisheries Commission Amendment 1 to the interstate fishery management plan for Atlantic sturgeon. Atlantic States Marine Fisheries Commission, Washington, DC, USA. Bagenal, T. B EIFAC fishing gear intercalibration experiments. Food and Aqriculture Organization of the United Nations, European Inland Fisheries Advisory Commission, EIFAC Technical Paper 34, Rome, Italy. Bain, M. B Atlantic and shortnose sturgeons of the Hudson River: common and divergent life history attributes. Environmental Biology of Fishes 48: Bain, M. B., N. Haley, D. Peterson, J. Waldman, and K. Arend Harvest and habitats of Atlantic sturgeon (Acipenser oxyrinchus ) in the Hudson River estuary: lessons for sturgeon conservation. Instituto Español de Oceanografía Publicaciones Especiales, Madrid, Spain. [in review] Boreman, J Sensitivity of North American sturgeons and paddlefish to fishing mortality. Environmental Biology of Fishes 48: Brundage, H. M., III, and R. E. Meadows The Atlantic sturgeon, Acipenser oxyrhynchus, in the Delaware River estuary. Fisheries Bulletin 80: Coch, N. K., and H. J. Bokuniewicz Oceanographic and geologic framework of the Hudson system. Northeastern Geology 8(3):

31 Collins, M. R., and T. I. J. Smith Distribution of shortnose and Atlantic sturgeons in South Carolina. North American Journal of Fisheries Management 17: Dovel W. L,. and T. J. Berggren Atlantic sturgeon of the Hudson Estuary, New York. New York Fish and Game Journal 30: Haley, N., J. Boreman, and M. Bain Juvenile sturgeon habitat use in the Hudson River. Section VIII: 39 pp, in J. R. Waldman and E. A. Blair, editors. Final Reports of the Tibor T. Polgar Fellowship Program, Hudson River Foundation, New York, New York. Johnson, J. H., D. S. Dropkin, B. E. Warkentine, J. W. Rachlin, and W. D. Andrews Food habits of Atlantic sturgeon off the central New Jersey coast. Transactions of the American Fisheries Society 126: Kieffer, M., and B. Kynard Annual movements of shortnose and Atlantic sturgeon in the Merrimack River, Massachusetts. Transactions of the American Fisheries Society 122: McEnroe, M., and J.J. Cech, Jr Osmoregulation in white sturgeon: life history aspects. American Fisheries Society Symposium 1: Moser, M. L., and S. W. Ross Habitat use and movements of shortnose and Atlantic sturgeons in the lower Cape Fear River, North Carolina. Transactions of the American Fisheries Society 124: New York State Department of Environmental Conservation Hudson River Estuary Management Program News. Hudson River Estuary Quarterly Issues Update and State of the Hudson Report 3(4):6. New York Department of Environmental Conservation New York Department of Environmental Conservation announces emergency moratorium on Atlantic sturgeon fishing. New York State Department of Environmental Conservation, Press Release, 20 March, Albany, NY. 31

32 Peterson, D. L., M. B. Bain, and N. Haley Evidence of declining recruitment of Atlantic sturgeon in the Hudson River. North American Journal of Fisheries Management (in press). Pottle, R., and M. J. Dadswell Studies on larval and juvenile shortnose sturgeon (). Literature review prepared for the Northeast Utilities Service Company by Washburn and Gillis Associates, Fredericton, New Brunswick, Canada. Rochard, E Mise au point d'une méthode de suivi de l'abondance des amphihalins dans le système fluvio- estuarien de la Gironde, application à l'étude écobiologique de l'esturgeon Acipenser sturio. Thèse de doctorat, Université de Rennes I, France. Rochard, E., M. Lepage, S. Tremblay, and C. Gazeau Spatial distribution of juveniles Acipenser sturio in the Gironde estuary (France) before their first journey to sea. Pages in B. Elvira (editor). Symposium on Conservation of the Atlantic sturgeon Acipenser sturio in Europe. Publicaciones de la Estación de Ecología Acuática, University Complutense of Madrid, Madrid, Spain. Smith, T. I. J The fishery, biology, and management of Atlantic sturgeon, Acipenser oxyrhynchus, in North America. Environmental Biology of Fishes 14: Stevenson, J. T Life history of Hudson River Atlantic sturgeon and a model for fishery management. Masters Thesis, University of Maryland, College Park, Maryland, USA. Stevenson, J. T., and D. H. Secor Age determination and growth of Hudson River Atlantic sturgeon (Acipenser oxyrinchus ). Fishery Bulletin (in Press). Van Eenennaam, J. P., and S. I. Doroshov Effects of age and body size on gonadal development of Atlantic sturgeon. Journal of Fish Biology 53: Van Eenennaam, J. P., S. I. Doroshov, G. P. Moberg, J. W. Watson, D. S. Moore, and J. Linares Reproductive condition of the Atlantic sturgeon (Acipenser oxyrinchus ) in the Hudson River. Estuaries 19:

33 Vladykov, V.D., and J.R. Greeley Order Acipenseroidei. Pages in Fishes of the North Atlantic, Pt. III, Memoir Sears Foundation for Marine Research #1, Yale University, New Haven, CT. Waldman, J. R., J. T. Hart, and I. I. Wirgin. 1996a. Stock composition of the New York Bight Atlantic sturgeon fishery based on analysis of mitochondrial DNA. Transactions of the American Fisheries Society 125: Waldman, J. R., K. Nolan, J. Hart, and I. I. Wirgin. 1996b. Genetic differentiation of three key anadromous fish populations of the Hudson River. Estuaries 19: Waldman, J. R., and I. I. Wirgin Status and restoration options for Atlantic sturgeon in North America. Conservation Biology 12:

34 Table 1. Observed and expected capture counts (presence) and total chi-square for a test of the distribution of juvenile Atlantic sturgeon within the Hudson River. The distribution of fish was significantly (X2 (df = 6) = 33.37; p < 0.001) different from a uniform distribution. River strata Observed Expected Wide estuary Highlands gorge Wide river Narrow river Meandering channel-shoal Bifurcating channel-shoal Confined and dredged channel 0 4.1

35 Table 2. Summary statistics on the river distribution of small (< 600 mm FL) wild and stocked Atlantic sturgeon by sampling period. Sampling Period Sturgeon Group N 1 Mean ± S.D. (rkm) Median (rkm) Range (rkm) 1995 Wild 6 95 ± Stocked ± Wild 5 92 ± Stocked ± & Wild ± Stocked ± N = number of unique capture stations

36 Table 3. Catches of juvenile Atlantic sturgeon from the Hudson River in random and targeted gill net sampling. Year Total Catch Gill net sets CPUE Deployment Random Random Targeted

37 Table 4. Summary of paired trawl and gill net sampling effort, catch, and catch per unit effort grouped by week in Catch was juvenile Atlantic sturgeon in south Newburgh Bay (km ). Trawling Gill nets Week Units Catch Gill Units Catch of Trawls Sturgeon of per unit nets Sturgeon of per unit sampling fished caught effort effort fished caught effort effort July July July July August August Totals

38 NY Troy Albany K M FRESHW ATER BRACKISH SEASONALLY M ILE Catskil Kingston W estpoint Dam BRACKISH MA CT Hyde Park Poughkeepsie Croton Point Long Isl.Sound M ay - June - June - August Estuaries and Rivers Marine Waters June - M ay - August June - August Spring & Fa l& Summer NJ October October October Figure 1. Life cycle New and York distribution of Atlantic sturgeon in the Hudson River estuary. Late Non- Fem ale Male Egg & Early Winter Atlantic Ocean Juvenile Spawning Spawners Spawners Larval Juvenile Adults

39 246 CONFINED AND DREDGED CHANNEL NY Albany Troy Dam BIFURCATING CHANNEL-SHOAL 172 MA MEANDERING CHANNEL-SHOAL Catskil Hudson CT NARROW RIVER 124 Saugerties WIDE RIVER HIGHLANDS GORGE WIDE ESTUARY Kingston Newburgh Poughkeepsie WestPoint NJ Haverstraw Peekskil Croton-on-Hudson Km Atlantic Ocean New York Figure 2. Morphometric strata of the Hudson River estuary using channel and geologic division of Coch and Bokuniewicz (1986). 10

40 Number 10 0 < Fork Length (mm) Number 10 0 < Fork Length (mm) Figure 3. Atlantic sturgeon length frequency distributions for 53 juvenile collected in 1995 and 36 collected in

41 60 40 Number 20 0 Wide Estuary Highlands Gorge Wide River Narrow River Meandering Bifurcating Confined & Dredged Channel-shoal Channel-shoal Channel River Strata Figure 4. Number of juvenile Atlantic sturgeon captured in each stratum, data combined. 16