257 



Abstract. -Eggs and yolk-sac lar- 

 vae of bay anchovy, Anchoa mitchilli, 

 were surveyed at seven sites in Chesa- 

 peake Bay on 12 days in July 1991 to 

 estimate abundances and mortality 

 rates of daily cohorts and the relative 

 biomasses of adults that spawned them. 

 An objective was to determine variabil- 

 ity in abundance and mortality rates 

 among sites and survey dates. Esti- 

 mated abundances of eggs spawned 

 each day and their hourly mortality 

 rates were considered in relation to 1) 

 in situ predator abundances, 2) envi- 

 ronmental factors, and 3) initial egg 

 and yolk-sac larval abundances. The 

 mean initial abundance of bay anchovy 

 eggs on each day during the 12 experi- 

 ments was 6,630 eggs/m 2 1427.0/m 3 ). 

 Mean initial abundance of yolk-sac lar- 

 vae was 385 larvae/m 2 ( 24.6/m 3 ). Mean 

 adult biomass at the survey sites, esti- 

 mated from the egg productions, was 

 18.0 g/m 2 (1.16 g/m 3 ). A correlation 

 analysis indicated that spawning by 

 bay anchovy may be most intense in 

 areas with high zooplankton biomass 

 and where the ctenophore Mnemwpsis 

 leidyi, a potential predator on eggs and 

 larvae, was least abundant. Mean co- 

 hort instantaneous egg mortality was 

 0.066 eggs/h; on average, 73% of 

 spawned eggs died before hatching. 

 Yolk-sac larvae incurred a mean cohort 

 instantaneous mortality of 0.053/h, i.e. 

 64% mortality during the first 24-h 

 posthatch. Together, the mean egg and 

 yolk-sac larval mortality rates indi- 

 cated that >93 f 7r of bay anchovy daily 

 cohorts die within 2 days after egg fer- 

 tilization and before larvae reach the 

 first-feeding stage. The range of cohort- 

 specific mortality rates at a single sta- 

 tion sampled on five consecutive days 

 was equal to that observed at the seven 

 sites. The high abundances, combined 

 with high and variable cohort mortal- 

 ity rates, emphasize the probable im- 

 portance of the egg and yolk-sac larval 

 stages in the recruitment process of bay 

 anchovy. 



Cohort abundances and daily 

 variability in mortality of eggs and 

 yolk-sac larvae of bay anchovy, 

 Anchoa mitchilli \n Chesapeake Bay 



Suzanne E. Dorsey 



Chesapeake Biological Laboaratory 

 Center for Environmental and Estuarme Studies 

 The University of Maryland System 

 Solomons. MD 20688-0038 



and 



Marine Sciences Research Center. 



State University of New York at Stony Brook 



Stony Brook. NY 1 1 794-5000 



Edward D. Houde 



Chesapeake Biological Laboratory 

 Center for Environmental and Estuarme Studies 

 The University of Maryland System 

 Solomons. MD 20688-0038 



John C. Gamble* 



Sir Alister Hardy Foundation for Ocean Science 

 The Laboratory, Citadel Hill 

 Plymouth PL I 2PB, England 



Manuscript accepted 11 December 1995. 

 Fishery Bulletin 94:257-267 (1996). 



Most marine and estuarine fishes 

 are highly fecund and spawn pelagic 

 eggs that suffer high mortalities 

 (Dahlberg, 1979; McGurk, 1986; 

 Pepin, 1991). Historically, an esti- 

 mation of in situ mortality rates of 

 fish eggs and yolk-sac larvae has 

 been problematic because of their 

 temporally and spatially patchy dis- 

 tributions (Fortier and Leggett, 

 1985; Heath, 1992). However, it is 

 important to obtain such estimates 

 because the magnitude and vari- 

 ability of mortalities potentially can 

 affect recruitment levels and con- 

 tribute to fluctuations in stock 

 abundance (Houde, 1989a). 



Bay anchovy, Anchoa mitchilli, is 

 the most abundant fish in estuar- 



ies and bays along the east coast of 

 the United States (Hildebrand and 

 Schroeder, 1928; Bigelow and Schroe- 

 der, 1953; Houde and Zastrow, 1991 ). 

 The species is euryhaline and occurs 

 in habitats as diverse as tidal fresh- 

 water tributaries and continental 

 shelf waters. In most estuaries, in- 

 cluding the Chesapeake Bay, the 

 bay anchovy is an important trophic 

 link between plankton and pisci- 

 vores (Baird and Ulanowicz, 1989; 

 Luo and Brandt, 1993). Its eggs and 

 larvae dominate summer ichthyo- 

 plankton collections from Chesa- 

 peake Bay, accounting for 99% of the 



Deceased. 



