Rilling and Houde: Variability in growth and mortality of Anchoa mitchilli 



567 



1997a). The baywide M IG ratio for bay anchovy lar- 

 vae decUned from 1.59 in June to 0.67 in July. The 

 low MIG ratio in July is a reflection of the coinci- 

 dent decline in larval mortality rate and increase in 

 growth rate that occurred between June and July. 

 The difference in MIG ratios between months im- 

 plies a 70-fold higher survival potential through the 

 larval stage for July-hatched cohorts. 



MIG ratios <1.0, signifying high regional produc- 

 tion potential, were observed in the mid bay in June 

 and in the mid and upper bay in July. Larvae in those 

 regions tended to have lowest mortality rates. An 

 abundance of large larvae generally indicates higher 

 survival rates of cohorts, but size-selective mortal- 

 ity or transport (or both) of larvae into a region 

 (Fortier and Leggett, 1982, 1985; Norcross and Shaw, 

 1984; Boehlert and Mundy, 1988) could have contrib- 

 uted to the relative abundances of large larvae and 

 low MIG ratios. In the Patuxent River subestuary 

 of Chesapeake Bay, progressive increases in larval 

 length upriver were reported by Loos and Perry 

 (1991), who hypothesized (with supporting evidence) 

 that transport of larvae was primarily responsible. 

 Similarly, MacGregor and Houde (1996) reported a 

 gradient in bay anchovy larval size on a cross-Bay 

 transect that was repetitively sampled; smallest lar- 

 vae were found offshore and largest larvae, inshore. In 

 the present study, selective up-bay transport of larx'ae 

 could have acted to reduce the MIG ratio in the upper 

 bay between June and July, but we cannot confirm it. 



In summary, mortality rates of bay anchovy early- 

 life stages were both temporally and regionally vari- 

 able at one-month temporal and at 60-km spatial 

 scales in Chesapeake Bay. Growth rates showed 

 strong temporal variation but no significant regional 

 differences. Stage-specific survival, which depended 

 upon both mortality and growth rates, was both size- 

 specific and growth-rate dependent. We found no 

 obvious indication of density-dependent mortality 

 (i.e. no correlations between egg or larval abundances 

 and mortality or growth rates), although recent in- 

 dividual-based modeling suggests that density-de- 

 pendence in early-life could be an important regula- 

 tor of bay anchovy recruitments in Chesapeake Bay 

 (Wang et al., 1997). The lower Chesapeake Bay in 

 July was the major source of potential recruits in 

 1993. Temperature, zooplankton prey, and gelatinous 

 predators all are believed to have contributed to tem- 

 poral and regional differences in growth and mortal- 

 ity of larvae. Further research is needed to define 

 scales and patterns of processes that control vari- 

 ability in production and recruitment of bay anchovy. 

 This will require coupled biophysical studies and 

 development of models that, up to now, have essen- 

 tially emphasized only biology. 



Acknowledgments 



Research was supported by National Science Foun- 

 dation grants OCE-92-03307 and OCE95-21512 to 

 E. D. Houde and by NSF Land Margin Ecosystem 

 grant DEB94-12113 to W. R. Boynton et al. We thank 

 the officers and crew ofRVHenlopen for capable re- 

 search-vessel support and numerous colleagues and 

 students who assisted in the research. We especially 

 thank S. Leach and L. Fernandez for assistance with 

 illustrations and preparation of the manuscript. 



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