63 



Abstract— Recruitment of bay anchovy 

 {Anchoa mitchilli) in Chesapeake is 

 related to variability in hydrologi- 

 cal conditions and to abundance and 

 spatial distribution of spawning stock 

 biomass (SSB I. Midwater-trawl surveys 

 conducted for six years, over the entire 

 320-km length of the bay, provided 

 information on anchovy SSB, annual 

 spatial patterns of recruitment, and 

 their relationships to variability in 

 the estuarine environment. SSB of 

 anchovy varied sixfold in 1995-2000; 

 it alone explained little variability in 

 young-of-the-year (YOY) recruitment 

 level in October, which varied ninefold. 

 Recruitments were low in 1995 and 

 1996 (47 and 31xl0 9 ) but higher in 

 1997-2000 (100 to 265 xlO 9 ). During 

 the recruitment process the YOY popu- 

 lation migrated upbay before a subse- 

 quent fall-winter downbay migration. 

 The extent of the downbay migration 

 by maturing recruits was greatest in 

 years of high freshwater input to the 

 bay. Mean dissolved oxygen (DO) was 

 more important than freshwater input 

 in controlling distribution of SSB and 

 shifts in SSB location between April- 

 May (prespawningl and June-August 

 (spawning) periods. Recruitments of 

 bay anchovy were higher when mean 

 DO was lowest in the downbay region 

 during the spawning season. It is 

 hypothesized that anchovy recruit- 

 ment level is inversely related to mean 

 DO concentration because low DO is 

 associated with high plankton produc- 

 tivity in Chesapeake Bay. Additionally, 

 low DO conditions may confine most 

 bay anchovy spawners to the downbay 

 region, where production of larvae and 

 juveniles is enhanced. A modified Ricker 

 stock-recruitment model indicated den- 

 sity-compensatory recruitment with 

 respect to SSB and demonstrated the 

 importance of spring-summer DO levels 

 and spatial distribution of SSB as con- 

 trollers of bay anchovy recruitment. 



Recruitment and spawning-stock biomass 

 distribution of bay anchovy (Anchoa mitchilli) 

 in Chesapeake Bay* 



Sukgeun Jung 

 Edward D. Houde 



University of Maryland Center for Environmental Science 



Chesapeake Biological Laboratory 



1 Williams St., P.O. Box 38 



Solomons, Maryland 20688 



E-mail address (for S Jung): iung@cbl.umces.edu 



Manuscript approved for publication 

 30 September 2003 by Scientific Editor. 



Manuscript received 20 October 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:63-77 (20041. 



Recruitment for marine fishes is vari- 

 able and is regulated or controlled by a 

 combination of density-dependent and 

 density-independent processes. It has 

 been hypothesized that density-inde- 

 pendent processes dominate from the 

 egg to larval stages whereas density- 

 dependent control by predation may be 

 more important in the juvenile stage 

 (Sissenwine, 1984; Houde, 1987). Den- 

 sity-dependent processes may be stock 

 dependent, regulated by adult abun- 

 dances, or dependent on abundances of 

 the early-life stages (Ricker, 1975). In 

 estuarine systems, where hydrological 

 conditions (e.g. dissolved oxygen, tem- 

 perature, and circulation) vary widely, 

 the roles of density-independent physi- 

 cal factors on fish recruitments may 

 be dominant, making it difficult, but 

 still important, to partition density- 

 dependent and density-independent 

 processes, particularly for short-lived 

 small pelagic fishes such as anchovies 

 and sardines. 



Bay anchovy {Anchoa mitchilli) (En- 

 graulidae) is a coastal species distrib- 

 uted broadly in the western Atlantic 

 from Maine to Mexico. This small fish is 

 the most abundant and ubiquitous fish 

 in Chesapeake Bay, the largest estu- 

 ary on the east coast of North America 

 (Houde and Zastrow, 1991; Able and 

 Fahay, 1998). It is not fished, yet there 

 is evidence that recruitment is variable 

 (Newberger and Houde, 1995). It feeds 

 on zooplankton — primarily copepods and 

 other small Crustacea — and is a major 

 prey of piscivores, including several eco- 

 nomically important fishes (Baird and 

 Ulanowicz, 1989; Luo and Brandt, 1993; 



Hartman and Brandt, 1995). Male and 

 female bay anchovy in Chesapeake Bay 

 mature at 40^15 mm fork length (44-50 

 mm total length) at about 10 months 

 of age, and peak spawning occurs in 

 July (Zastrow et al.. 1991). Most eggs 

 are produced by age-1 individuals (Luo 

 and Musick, 1991; Zastrow et al., 1991). 

 Bay anchovy may survive to age 3+ and 

 reach approximately 100 mm length and 

 5 g wet weight ( Newberger and Houde, 

 1995; Wang and Houde, 1995). 



Newberger and Houde (1995) noted 

 large differences in annual survey 

 abundances of bay anchovy that appar- 

 ently resulted from variability in an- 

 nual recruitments. In Chesapeake Bay, 

 abundance, growth, and mortality rates 

 of bay anchovy eggs and larvae vary 

 temporally and spatially (Dorsey et 

 al, 1996; MacGregor and Houde, 1996; 

 Rilling and Houde, 1999a, 1999b). Indi- 

 vidual-based models were developed to 

 test the hypothesis that recruitment of 

 bay anchovy is determined by variable 

 growth and mortality during early-life 

 stages that are regulated by density-de- 

 pendent processes (Wang et al., 1997; 

 Cowan et al., 1999; Rose et al„ 1999). 

 In previous research, there was little 

 knowledge of levels of spawning stock 

 biomass or density-independent envi- 

 ronmental factors that may control re- 

 cruitment through their effects on spa- 

 tial and temporal variability in growth 

 and mortality of prerecruit anchovy. 



* Contribution 3696 of the University of 

 Maryland Center for Environmental Sci- 

 ence, Chesapeake Biological Laboratory, 

 Solomons, MD 20688. 



