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Fishery Bulletin 102(1) 



ment level. Our results have demonstrated that there is 

 a strong spatial component in the recruitment dynamics 

 of bay anchovy. Although fish recruitment processes his- 

 torically have been difficult to understand, our six-year, 

 spatially extensive research has provided new insights into 

 processes that control bay anchovy recruitment. 



Ontogenetic migration pattern 



It is apparent that ontogenetic migration plays a role in 

 the spatial and temporal patterns in abundance, biomass, 

 and production of bay anchovy. There are several lines of 

 evidence. Rilling and Houde (1999a), in a baywide analy- 

 sis, reported that mean density of eggs and larvae in June 

 and July 1993 was very high in the lower Chesapeake Bay 

 compared to more upbay sites. Dovel (1971) and Loos and 

 Perry (1991) reported possible upbay or upriver migra- 

 tion of bay anchovy larvae and juveniles in the mainstem 

 and tributaries of the Bay. Recent otolith microchemical 

 analyses have strongly supported the hypothesis that 

 an upbay ontogenetic migration by small YOY anchovy 

 (>25 mm, late larvae and small juveniles) occurs (Kimura 

 et al., 2000). In the middle Hudson River estuary (Schultz 



April-May 



39°00' 



£ 38°00 



37°00 



39°00 



38°00 



37°00' 



30 40 50 60 70 80 90 100 



TL (mm) 



1995 1996 1997 1998 1999 2000 



Figure 2 



Abundance-weighted mean latitude of occurrence of bay anchovy 

 (Am hoa mitchilli) in Chesapeake Bay, 1995-2000. 



et al., 2000) and Chesapeake Bay (North and Houde, in 

 press), selective tidal-stream transport was suggested as 

 a mechanism for up-estuary movements of bay anchovy 

 larvae. Our conceptual model of the bay anchovy life cycle 

 includes migration patterns in the bay based on available 

 knowledge and evidence (Fig. 6). 



It is uncertain what benefits YOY bay anchovy derives 

 from upbay migration in summer and whether the migra- 

 tion is passive or active before a subsequent reverse migra- 

 tion in the fall. To explain upbay movements of estuarine 

 fishes, Dovel ( 1971 ) proposed that there is a "critical zone" 

 of low salinity and high prey production in the upper bay, 

 which is important as a nursery for bay anchovy and 

 other fish species. In late spring and early summer, age-1 

 and age-l+ bay anchovy mature and move upbay while 

 spawning, although the year 2000, when mean freshwater 

 streamflow during the previous fall-winter was lowest, was 

 an exception. Recruited YOY bay anchovy apparently over- 

 winter primarily, but not entirely, downbay until spring. 



There remains a possibility of significant immigration 

 to the bay by adult bay anchovy in some years from the 

 coastal ocean or tidal tributaries of the bay. Without such 

 immigration, baywide adult abundance would decrease 

 continuously during the April-October period through 

 natural mortality However, in two years of our six-year 

 study, 1995 and 1998, estimated adult abundance in- 

 creased substantially from April to July, and in 1999 

 adult abundance increased from June to October, 

 implying significant immigration to the bay in those 

 years (Jung, 2002). 



Recruitment control and regulation 



The modified Ricker recruitment model that included 

 SSB and AL as explanatory variables provided a good 

 fit to bay anchovy recruitments. Although the model 

 fitted well, there were only six years of data, and 

 the underlying mechanisms explaining relationships 

 between the distribution and level of SSB, hydro- 

 logical conditions, and density-dependent regulatory 

 processes in recruitment of bay anchovy are not yet 

 clear. Nevertheless, correlations and the recruitment 

 model clearly indicated a density-dependent effect of 

 SSB level and also implicated environmental factors 

 (at the mesoscale) that are related to mean DO concen- 

 tration, latitudinal distribution of SSB (AL), and the 

 recruitment level of bay anchovy (Fig. 4). 



The modified Ricker model for bay anchovy < Fig. 5) 

 indicates a density-compensatory stock-recruitment 

 relationship (Ricker, 1975). although we do not know 

 at what life stages density-dependent processes are 

 most important. Without accounting for the control- 

 ling effect of AL and mean DO on a regional scale, 

 the density-dependence might have gone undetected 

 (Fig. 4 1. Recent individual-based models suggest that 

 density-dependent processes during early-life stages 

 could stabilize bay anchovy recruitments (Wang et 

 al., 1997; Cowan et al., 1999; Rose et al, 1999). At the 

 small scales of several meters modeled by Wang et al. 

 (1997) and Cowan et al. (1999), larval-stage feeding 



