Secor et al.: Dispersal and growth oi Aapenser oxynnchus 



807 



(61%), followed by isopocis {Cyathu?-a polita and Cyathura 

 sp., 23%), amphipods (Leptocheirus plumulosus and Gam- 

 mariis sp., 10% i, chironomid larvae (1.6%), and mysids (Neo- 

 inysis americana, 1.5%). Occurrence data among yearlings 

 indicated about 50% probability of finding all major food cat- 

 egories — worms, amphipods, and isopods — in feeding fish. 



Discussion 



Failure to capture a single wild yearling (<70 cm TL; see 

 Peterson et al., 2000, for size criterion ) during the course of 

 our study is a strong indication of the Atlantic sturgeon's 

 endangered status in Chesapeake Bay. In the fall of 1997, 

 the floating carcass of a 2.6-m-TL female Atlantic stur- 

 geon was observed in the James River.* Also, wild year- 

 lings (<50 cm TL) were observed during 1998 in Virginia 

 tributaries (Musick'') suggesting that some reproduction 

 was still occurring in lower Chesapeake Bay. On the other 

 hand, absence of yearling Atlantic sturgeon during Vir- 

 ginia Institute of Marine Science trawl monitoring over 

 the past 15 years led Grogan and Boreman (1998) to pre- 

 dict >95% probability of extirpation in Virginia. Although 

 this prediction has proven false, it still provides strong 

 evidence of extremely depressed reproduction rates. 



Results from our study indicated that the Chesapeake 

 Bay can support nursery functions for juvenile Atlantic 

 sturgeon. High capture rates by fishermen suggest high 

 survival of released fish and high vulnerability to the gear 

 types deployed. Wide dispersal and positive growth rates 

 indicate that juveniles dispersed to areas that supported 

 consumption and metabolic needs. Indeed, mean weight 

 increased nearly 20-fold over the first year after release. 

 Sizes at the end of the second year of life (summer 1997) 

 were intermediate between size estimates for 2-year-old 

 juvenile sturgeons from the Hudson River (Dovel and 



Spells, A. 1997. Personal common 

 Road, Charles City, VA, 23030. 

 ' Musick, J. 1999. Personal commun 

 Marine Science, Gloucester, VA. 



USFWS, 11110 Kimages 

 Virginia Institute of 



Berggren, 1983) and those from southern populations 

 ( Table 4 ). Thus, hatchery-produced fish had grown at levels 

 that might be expected for juveniles naturally occurring in 

 the Chesapeake Bay. 



Although limited by a small sample size, our diet analy- 

 sis seems consistent with the few published reports on food 

 habits of juvenile Atlantic sturgeon. Aquatic insects, amphi- 

 pods, isopods, and both polychaete and oligochaete worms 

 are the most common items previously reported for juve- 

 niles residing in fresh and brackish waters (Vladykov and 

 Greeley, 1963; Smith, 1985; Moser and Ross, 1995; Haley, 

 1998). Contrary to Vladykov and Greeley (1963) and Moser 

 and Ross (1995), we found no evidence of juvenile sturgeon 

 predation upon mollusks, which represent the highest bio- 

 mass of benthic invertebrates in the Chesapeake Bay. 



We failed to detect any significant selection among tracked 

 individuals for certain water temperatures or dissolved 

 oxygen conditions. Overall, tracked fish did not encounter 

 severe hypoxia or extremely high temperatures (Table 3, 

 Fig. 4). However, on at least one occasion, other than the 

 day of release, yearlings encountered dissolved oxygen satu- 

 rations close to 50% (4.3 mg/L, 25.9°C) — a level that might 

 reduce growth and survival rates for this species (Secor 

 and Gunderson, 1998). Preferential use of cooler areas or 

 deep thermal refuges, or both, has been observed in field 

 studies on juvenile Atlantic (Moser and Ross, 1995) and 

 Gulf (Clugston et a!., 1995) A. oxyrinchus desotoi sturgeons. 

 Such refuges are unavailable in the Nanticoke River, a shal- 

 low system (Table 3) with little thermal stratification. 



Low rates of use of habitats of salinity below 1 ppt agree 

 with past observations that yearling Atlantic sturgeon 

 principally use brackish water habitats (Dadswell, 1979; 

 Brundage and Meadows, 1982; Smith, 1985; Moser and 

 Ross, 1995; Haley et al., 19961. Observed down-estuary 

 movement is consistent with seasonal movements reported 

 for juvenile Atlantic sturgeon responding to summer peak 

 temperatures in the Hudson River (Dovel and Berggren, 

 1983). Use of areas below rkm 10 was probably underes- 

 timated because river width substantially exceeded the 

 detection range of ultrasonic transmitters and shallow 

 reefs or sills probably obstructed detection. 



