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



69 



DO during that same period in the middle bay (/•-=(). 75, 

 P=0.02;Fig. 3C). 



Correlations 



Correlation analyses suggested that regional mean DO 

 concentrations are the most important environmental 

 correlate associated with spatial distribution of SSB and 

 recruitment processes of bay anchovy. The mean locations 

 (latitudes of occurrence), abundances, and biomasses for 

 YOY and adult bay anchovy were analyzed with respect 

 to environmental variables (Table 5). Recruitment levels 

 (YOY abundance) in October were consistently inversely 

 correlated with DO concentrations in the lower and 

 middle bay in June-August (/-=-0.13 to -0.89). Biomass- 

 weighted mean latitude of SSB (age 1+) in April-May was 

 consistently and positively correlated with regional salini- 

 ties in April-May (r=0.30 to 0.88). On the other hand, in 

 June-August, surface-layer mean salinity in the lower Bay 

 and subpycnocline-layer mean DO in the lower and middle 

 bay were significantly and positively correlated with mean 

 latitude of SSB or AL (r=0.82 to 0.91). Baywide SSB in 

 April-May and June-August tended to be negatively cor- 

 related with water temperature in April-May (r=-0.45 to 

 -0.90). 



Recruitment model 



Although SSB alone did not correlate significantly with 

 recruitment level, mean DO in June-August was signifi- 



cantly related to the mean latitude of SSB in June-August 

 (or AL) and bay anchovy recruitment level in October (Figs. 

 3C and 4). AL was selected as the explanatory variable, 

 rather than DO, because DO data were uncalibrated in 



1999 and 2000. The correlation observed between AL and 

 DO ( Fig 3C ) suggested that AL can serve as a proxy for DO 

 in the stock-recruitment model. Including AL and SSB for 

 April-May in a modified Ricker model provided a good fit 

 to bay anchovy recruitment levels observed from 1995 to 



2000 (Fig. 5). The model is 



R v = 365 S exp (-0.19 S 1.35 AL) (modified Ricker model). 



In the model, if AL is held constant, predicted recruitment 

 level of bay anchovy is maximum when baywide SSB in 

 April-May is approximately 5.3 x 10 3 tons. Collinearity and 

 influence diagnostic statistics did not indicate collinearity 

 between the two independent variables (S and AL), or that 

 an observation in any year had a dominating influence on 

 parameter estimates. 



Discussion 



Complex environmental processes and biological interac- 

 tions control bay anchovy recruitment in Chesapeake Bay. 

 Dissolved oxygen (DO), freshwater flow, salinity, and tem- 

 perature acting on prerecruits and adults are important 

 factors affecting bay anchovy distribution and levels of 

 recruitment. Spawning stock size also is related to recruit- 



