555 



Abstract.— Temporal and spatial vari- 

 ability in growth and mortality rates 

 of bay anchovy, Anchoa mitchilli. lar- 

 vae was analyzed in Chesapeake Bay. 

 Larvae were collected in cruises dur- 

 ing June and July 1993. on transects 

 spaced at 18.5-km (10 nmi) intervals 

 over the entire bay. Growth and mor- 

 tality rates were estimated in lower, 

 mid, and upper bay regions and ana- 

 lyzed in relation to environmental vari- 

 ables, predators (biovolumes of the 

 scvphomedusa Chn'saora quinquecirrha 

 and the ctenophore Mnemiopsis leidyi). 

 and larval prey (zooplankton abun- 

 dances). Otolith increment analysis in- 

 dicated that the mean baywide growth 

 rate of larvae increased significantly 

 from 0.59 mm/d in June to 0.72 mm/d 

 in July. The baywide mortality rate of 

 larvae declined from 0.41 (33.6'^f/d) in 

 June to 0.23 (20.5'^f/d) in July In each 

 m.onth. regional mortality rates were 

 highest in the lower bay. Regionally, 

 mortality ranged from a low of 0.14 

 (13.1'^?-/d I in the upper bay in July to a 

 high of 0.54 (41.7%^) in the lower bay 

 in June. Mortality rates declined with 

 increasing larval size. Stage-specific 

 survival was both size-specific and 

 growth-rate dependent as indicated by 

 trends in mortality (M), weight-specific 

 growth iGl, and the M/G ratio. Growth 

 rates were positively correlated with 

 temperature and zooplankton abun- 

 dance. Larval abundances, but not mor- 

 tality rates, were negatively correlated 

 with gelatinous predator biovolumes. 

 Recruitment potential of bay anchovy 

 was judged to be highest in July in the 

 lower third of Chesapeake Bay. Al- 

 though lower, production of anchovy 

 prerecruits in June and in other Bay 

 regions was substantial and contrib- 

 uted significantly to prerecruit abun- 

 dances in 1993. 



Regional and temporal variability in 

 growth and mortality of bay anchovy, 

 Anchoa mitchilli, larvae in Chesapeake Bay 



Gene C. Rilling 



Edward D. Houde 



University of Maryland Center for Environmental Science 



Chesapeake Biological Laboratory 



PO Box 38 



Solomons, Maryland 20688-0038 



Present address (for G C Rilling); Connecticut Department of Environmental Protection 



Office of Long Island Sound Programs 

 79 Elm Street, Hartford, Connecticut 06106-5127, 



E-mail address (for G C Rilling) chris rilling apostate ct us 



Manuscript accepted 13 July 1998. 

 Fish. Bull. 97:555-569 ( 1999). 



In highly fecund fishes that spawn 

 serially over a protracted season 

 and a broad geographic range, vari- 

 able patterns of cohort successes 

 and failures may result that not 

 only lead to fluctuating recruit- 

 ments but that are difficult to de- 

 tect in the absence of sampling pro- 

 grams that are temporally and spa- 

 tially intensive. We estimated tem- 

 poral and regional variability in lar- 

 val-stage growth and mortality of 

 bay anchovy {Anchoa mitchilli). an 

 abundant (Hildebrand and Schroe- 

 der. 1928) and highly productive 

 species (Newberger and Houde, 

 1995; Wang and Houde, 1995) in 

 Chesapeake Bay. In exploratory 

 analyses and simulations, Houde 

 (1996, 1997b) demonstrated that 

 variability in recruitment success of 

 bay anchovy can occur when stage- 

 specific mortality rates vary during 

 early life. Recently, individual- 

 based models of bay anchovy dy- 

 namics in Chesapeake Bay have 

 demonstrated how stage-specific 

 mortality and growth processes may 

 operate and how density-dependent 

 regulation could dampen fluctua- 

 tions in abundance iWang et al., 

 1997). 



Bay anchovy are an important 

 component of the Chesapeake Bay 

 food web. They are not commer- 

 cially exploited but are a major prey 



of harvested species such as blue- 

 fish (Pomatomus saltatrix), weak- 

 fish (Cynoscion regalis), and striped 

 bass (Morone saxatilis) (Hartman 

 and Brandt, 1995) and may repre- 

 sent up to 90*^ of piscivorous fish 

 diets seasonally (Baird and Ulano- 

 wicz, 1989). Spawning by bay an- 

 chovy is widespread in the Bay and 

 occurs over a broad range of tem- 

 peratures and salinities (Dovel, 

 1971; Houde and Zastrow, 1991). 

 Bay anchovy is a pelagic, serial 

 spawner (Luo and Musick, 1991; 

 Zastrow et al., 1991) that spawns 

 most intensively from May through 

 August in Chesapeake Bay, where 

 it may account for 96-99^)?^ of fish 

 egg and 67-88T of larval catches 

 (Olney, 1983). During peak spawn- 

 ing, densities of eggs frequently 

 range from 10 to >1000/m'^ and den- 

 sities of larvae from 1 to >100/m'^ 

 (Olney, 1983; Dalton, 1987; Dorsey 

 et al., 1996; MacGregor and Houde, 

 1996; Rilling and Houde, manu- 

 script in review). Within a spawn- 

 ing season, regions of highest egg 

 and larval abundances may shift, as 

 they did from the upper to the lower 

 bay between June and July 1993 

 (Rilling and Houde, manuscript in 

 review). Such shifts may have im- 

 portant repercussions for bay an- 

 chovy production and for production 

 of other fish species and inverte- 



