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Fishery Bulletin 94(2). 1996 



eggs and -70% of the larvae (Dovel, 1971; Olney, 

 1983). 



Recruitment variability in anchovies is hypoth- 

 esized to result from variable stage-specific mortal- 

 ity (Lasker 1985b; Smith 1985). If bay anchovy egg 

 and yolk-sac larvae experience both high and vari- 

 able stage-specific mortalities, the effect on annual 

 recruitment could be substantial. Crude estimates 

 of daily mortality of eggs and yolk-sac larvae ranged 

 from 26% to 97% in Biscayne Bay, Florida, whereas 

 mortality rates of feeding-stage larvae were lower, 

 averaging 30%/d (Leak and Houde, 1987). To obtain 

 reliable estimates of mortality during the short egg 

 and yolk-sac larval stages (about two days total) of 

 bay anchovy, it is necessary to sample repeatedly a 

 daily cohort of eggs or yolk-sac larvae throughout a 

 24-h period. Our surveys followed that plan during 

 twelve days on Chesapeake Bay in which mortalities 

 were estimated in relation to biotic or abiotic factors 

 that may have affected the rates. By restricting the 

 study to cohorts of eggs and prefeeding yolk-sac lar- 

 vae, the calculated mortalities must, therefore, have 

 resulted primarily from causes other than starvation. 



In Chesapeake Bay, bay anchovy is sexually ma- 

 ture at 40 mm fork length (FL), 10 months after 

 hatching (Zastrow et al., 1991), and seldom lives be- 

 yond age 2 (Wang, 1992; Newberger and Houde, 

 1995). It is a serial spawner (Luo and Musick, 1991; 

 Zastrow et al., 1991) and has a reproductive season 

 in Chesapeake Bay that extends from May until Sep- 

 tember. Peak spawning occurs in July ( Dalton, 1987 ) 

 when virtually all females spawn nightly (Zastrow 

 et al., 1991). Age-1 females produce most eggs 

 (Zastrow et al., 1991), which hatch into yolk-sac lar- 

 vae at approximately 20-24 h after fertilization. The 

 yolk-sac larval stage also is short, with a duration of 

 about 24 h (Houde and Zastrow, 1991). 



The high abundances, serial spawning, and the 

 presence of a single cohort of eggs on each day for 

 this species allowed for repeated estimates of cohort- 

 specific mortality rates within one season and for 

 investigations into the factors that may affect vari- 

 ability in the rates. The four objectives of the study 

 were as follows: 1) to estimate daily-cohort abun- 

 dances and variability of bay anchovy eggs and yolk- 

 sac larvae in selected areas of Chesapeake Bay; 2) to 

 estimate mortality rates and variability of daily co- 

 horts of bay anchovy eggs and yolk-sac larvae; 3) to 

 relate densities of two potential predators — the 

 scyphomedusan Chrysaora quinquecirrha, and the 

 ctenophore Mnemiopsis leidyi (as well as environ- 

 mental variables) — to anchovy egg and yolk-sac lar- 

 val mortality rates; and 4) to estimate relative spawn- 

 ing-stock biomasses of bay anchovy at the experimen- 

 tal sites. 



Materials and methods 



Sampling design 



After determining that anchovy eggs were present 

 in test lifts of a plankton net, daily experiments were 

 conducted at seven sites (Fig. 1) to estimate mortal- 

 ity rates of bay anchovy. Once a site had been se- 

 lected, samples were obtained at 2-h intervals to es- 

 timate abundances of eggs and yolk-sac larvae. The 

 sites were defined by the drift patterns of concur- 

 rent free-drifting mesocosm studies (Houde et al., 

 1994), which were initiated when the sites were se- 

 lected. Each sample for this study was taken adja- 

 cent to free-drifting mesocosms. The mesocosms, 

 which were 5 m in length and 1 m in diameter, acted 

 as drogues and were assumed to drift with the water 

 mass that was originally sampled. Each experiment 

 consisted of 8 to 12 duplicate lifts of a plankton net. 

 Water depths at the seven sites generally ranged from 

 10 to 25 m. Based upon the mesocosm drift tracks 

 (Fig. 2), the mean area of the Bay included in an in- 

 dividual experiment was 59.8 km 2 . 



Figure 1 



Chesapeake Bay and the seven sampling sites. 



