562 



Fishery Bulletin 97(3), 1999 



June 



Log^ Abund = 2.69 - 41'Age 



r2 = b.96 S =0.02 



o 



-J 



2 

  



-2 

 -4 

 -6 



5 10 15 20 25 30 



Age (days) 



Figure 4 



Age-specific survival curves for bay anchovy larvae in Chesapeake 

 Bay. June and July 1993. The age-specific abundance estimates 

 for each day class were derived from age-length keys. 



Table 2 



Predicted cohort survivorships of bay anchovy larvae in 

 the upper, mid, and lower Chesapeake Bay in June and 

 July 1993. N^, = estimated number of smallest fully repre- 

 sented length class of larvae (3 mm SLl. M/G = ratio of 

 instantaneous mortality rate (/d ) to weight- specific growth 

 rate (/d), survival rate (S) = IW/W,,]-'""''' where W = dry 

 weight of a 12-mm-SL larva (1000 //g), W„ = dry weight of 

 a 3-mm-SL larva (10 mgl, and apparent survivorship at 

 12 mm = SxNg. 



Apparent 

 Ng survivorship 



(3-mm-SL Survival at 



Region larvae) M/G rate 12 mm SL 



June 



Upper bay 

 Mid bay 

 Lower bay 



July 

 Upper bay 

 Mid bay 



2.59 X IQi" 

 2.49 X lO* 

 7.28 X 10' 



.5.95 X 10» 

 2.08 X 10'" 



1.72 

 0.87 

 2.17 



0.35 

 0.62 



0.0004 

 0.0180 

 0.00005 



0.200 

 0.058 



Lower bay 11.72x10'" 1.19 0.004 



1,04 X 10' 

 4.48 X 10« 

 3.64 X 10'' 



1.19 X lO" 

 1.21 X 10» 

 4.69 X 10** 



Baywide, the mean density of zooplankton 

 that are potential prey of anchovy larvae 

 doubled between June and July (f-test, P<0.05) 

 (Table 1). Mean density of zooplankton was 

 highest in the upper bay in June (ANOVA, 

 P<0.05) and was higher in the upper and lower 

 bay than in the mid bay in July (ANOVA, 

 P<0.05). Nauplii of the copepod Acartia tonsa 

 were the single most abundant zooplankter col- 

 lected. Tintinnids, rotifers, and the cyclopoid 

 copepod Oithona sp. also were common. Mean 

 density of copepod nauplii, a major prey of bay 

 anchovy larvae, increased from 36.9/L in June 

 to 110.5/L in July (i-test, P<0.05). 



Correlations 



At the regional level, few correlations were 

 judged to be significant at the a = 0.05 level 

 between biological and environmental variables 

 (Table 4). The low degrees of freedom (/!=6) and 

 corresponding low power made it difficult to 

 reject null hypotheses. Several coefficients were 

 high enough to suggest possible correlations. 

 Anchovy larval abundances were positively cor- 

 related with egg abundances {r-+0.96, P<0.01) 

 and negatively correlated with gelatinous 

 predator biovolumes(r=-0.87,P<0.05) (Fig. 8A). 

 Larval growth rate was positively correlated 

 with temperature (r=-i-0.94, P<0.01) (Fig. 8B) 

 and possibly related to zooplankton density 

 (r=-i-0.72, P<0.11). Although not significant at a = 

 0.05, anchovy egg abundances and zooplankton den- 

 sities both may have been negatively correlated with 

 gelatinous predator biovolumes (r =-0.76, P=0.08). 



Predicting larval growth and mortality 



Regional variability in anchovy larval growth-in- 

 length rate for the combined June and July cruises 

 was explained reasonably well by a two-variable re- 

 gression model that included temperature and zoop- 

 lankton density (r-=0.93): 



g = 0.32 + 0.05X, + 0.004X^, 



where g = larval growth rate (mm/d); 



X, = log^, zooplankton density (organisms/L); 



and 

 X,, = temperature (°C). 



Larval growth rates increased with increasing tem- 

 peratures and zooplankton densities. Temperature 

 accounted for more of the variability in growth rate 

 than did zooplankton density. Observed and model- 



