Jung and Houde: Recruitment and spawning-stock biomass distribution of Anchoa mitchilli 



73 



larval production there; and 3) density-depensatory 

 predator satiation occurs when concentrations of bay 

 anchovy larvae and juveniles at the mesoscale ( 10-100 

 km ) are high in relation to satiation potential of preda- 

 tors, which favors larval production and high anchovy 

 recruitments. 



First, averaged DO level in the bay or its regions 

 may be an indicator of ecosystem metabolism and sec- 

 ondary production. DO level in the subeuphotic layer 

 is an indicator of respiration and secondary produc- 

 tion by planktonic and benthic communities (Kemp 

 and Boynton, 1980; Kemp et al., 1992). Recruitment 

 levels of bay anchovy increased substantially in 1997 

 and in subsequent years. We speculate that enhanced 

 detrital production potentially increased zooplankton 

 prey abundances in the subsequent year and that asso- 

 ciated elevated levels of respiration by detrital micro- 

 organisms and zooplankton contributed to low mean 

 DO. Increased zooplankton prey abundances, in turn, 

 may have promoted production of larval and juvenile 

 bay anchovy in 1997 and 1998. Thus, increased prey 

 availability, associated with low mean DO concentra- 

 tion, could have enhanced recruitment (Fig. 4). 



The second hypothesis proposes that spatial restric- 

 tion of SSB by low DO is a factor controlling bay anchovy 

 recruitment. Based on our results, hypoxic conditions in 

 the bay appear to define the distribution and potential for 

 upbay migration of bay anchovy SSB (Fig. 3C). In years 



when the baywide subpycnocline mean DO level was low, 

 spawning bay anchovy tended to be most concentrated in 

 the lower bay (Table 5, Fig. 3, A and C), possibly because 

 hypoxia in deeper waters of the mid-bay region discouraged 

 upbay migration. The region selected by adult anchovy as 

 the predominant spawning area and its variability played 



R = 365 Sexpf-O.l - S - 1.354Z.) 



r 2-- 



2.0 



Figure 5 



Stock-recruitment model (modified Ricker model). R = baywide number of recruits in 

 October (xlO 9 ). AL = location of bay anchovy iA?iclioa mitchilli) spawning stock biomass 

 in June-August in relation to the baseline latitude at the mouth of the bay, 37°00'N. S = 

 baywide spawning stock biomass (SSB xlO 3 metric tons for April-May 1. Balloon symbols 

 are observed data from 1995 to 2000. 



