Secor et al.: Dispersal and growth o\ Aapenser oxyrinchus 



809 



forage conditions. This is supported by the relatively low 

 rates of dispersal (0-0.4 krn/d; 0.1 BL/s), low horizontal 

 gradient observed in abiotic parameters, and a lack of cor- 

 relation between relocation probability and water quality 

 data. More intensive sampling and benthic resource map- 

 ping in Chesapeake Bay could confirm this speculation. 



A moderate reward system was highly effective in help- 

 ing to evaluate the release of hatchery sturgeon. Although 

 we expect more juveniles may have actually been cap- 

 tured than reported, fishermen were keenly interested in 

 the program and very cooperative with USFWS agents in 

 their collection of biological measures. High capture rates 

 of yearling Atlantic sturgeon by fishermen showed that 

 small sturgeons were highly vulnerable to gill and pound 

 nets, as found by Collins et al. (1996). Due consideration 

 needs to be given to incidental catches of juvenile-stage 

 sturgeon in recovery programs. 



Seasonal and spatial patterns of captures by fishermen 

 were the result of both seasonal dispersal and distribu- 

 tion of fishing effort in Chesapeake Bay. In winter and 

 early spring months, drift and anchor gill nets were set 

 for striped bass, white perch, and catfish in the upper 

 bay and in tributaries. Later in the spring and summer, 

 pound nets were set for menhaden, croaker (Micropogo- 

 nias iindulatus). and spot (Leiostomus xanthuj-us), which 

 tend to occur down-estuary. Therefore, the apparent dis- 

 placement of captures from the upper to lower Chesa- 

 peake Bay may be related to seasonal changes in fishing 

 effort. In addition, Virginia began a reward program in 

 February 1997; therefore captures were less likely to occur 

 prior to that date. 



The dispersal of released fish into Chesapeake Bay 

 mainstem by fall 1996 and reported captures in North 

 Carolina by the end of the winter also are consistent 

 with seasonal movement patterns described for Atlantic 

 sturgeon in other systems. Seasonal migration in juvenile 

 Atlantic sturgeon seems primarily regulated by changes in 

 temperature gradients between fresh and brackish waters 

 (Van Den Avyle, 1984). Juveniles tend to use brackish 

 waters close to estuary mouths during the colder months 

 as reported in Hudson River (Dovel and Berggren, 1983; 

 Gilbert, 1989), Delaware River (Brundage and Meadows, 

 1982; Lazzari et al., 1986), and the Winyah Bay system 

 (Smith et al., 1982). In the Hudson River, juveniles begin 

 moving to saltier waters by July (Dovel and Berggren, 

 1983), whereas in the Delaware River many fish remain 

 in tidal water until January (Lazzari et al., 1986). A differ- 

 ent pattern was observed by Moser and Ross (1995), who 

 noted that juvenile Atlantic sturgeon in the Cape Fear 

 River kept the same center of distribution all year round. 



High dispersal rates of released yearlings and absence 

 of schooling behavior were somewhat unexpected. Hatch- 

 ery-produced striped bass and Pacific salmon iOncorhyn- 

 chus spp. ) juveniles are known to school and remain in 

 restricted regions for weeks and months following release 

 (Nickelson et al., 1986; Hume and Parkinson, 1987; Dora- 

 zio et al., 1991; Nagata et al., 1994; Andreasen, 1995). 

 Recent genetic studies indicate strong population structur- 

 ing within the geographic range of the Atlantic sturgeon 

 (Waldman and Wirgin, 1998), which suggests high fidelity 



to natal estuaries. Gene flow studies also support con- 

 sistent homing behavior by the Gulf subspecies (Wirgin 

 et al., 1997). However, the rapid dispersal we observed 

 might suggest that yearlings had insufficient opportunity 

 to imprint to the Nanticoke River and thus might be 

 unlikely to home to it in future spawning migrations. 

 Apart from work on salmonids, little is known on homing 

 behavior in anadromous fishes. In Pacific salmon, the 

 window of imprinting occurs during the premigratory 

 smolt stage (Hasler and Scholz, 1983). Stocking young-of- 

 the-year Atlantic sturgeon in any future hatchery-based 

 restoration program would be prudent because it might 

 increase the duration of exposure to imprinting stimuli. 



This study demonstrates that the Nanticoke River and 

 Chesapeake Bay can continue to support nursery roles for 

 Atlantic sturgeon yearlings. Still, several questions remain 

 before we embark on a program of restoration through 

 hatchery-based reintroduction. What are the main factors 

 defining suitable habitat for Atlantic sturgeon juveniles? 

 Would available habitat support historical abundances? 

 Can we obtain sufficient numbers of brood stock from 

 the Hudson River or elsewhere to ward against inbreed- 

 ing depression? A critical question is whether released 

 juveniles will return and find spawning habitat. Spawn- 

 ing habitats are probably quite degraded from siltation 

 and sedimentation over the past two centuries. Unfortu- 

 nately, evaluating whether such habitats remain or can be 

 restored will be nearly impossible until an adult biomass 

 can be restored. 



Acknowledgments 



We are grateful to personnel at the U.S. Fish and Wildlife 

 Service Northeast Fishery Center fort their efforts in pro- 

 ducing juvenile sturgeon. E. Zlokovitz and E. Sadler pro- 

 vided assistance in the field and in collection of data. This 

 research was supported by the National Biological Service, 

 Maryland Department of Natural Resources, U.S. Fish and 

 Wildlife Senice, and the Chesapeake Bay Foundation. 



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