560 



Fishery Bulletin 97(3), 1999 



to 



June 



L = 280 + 061d 

 n=55 S|, = 04 

 r^ = 83 



Upper Bay 



L=3.22+0.57d 

 n = 42 S^ = 04 



Mid Bay 



July 



10 15 20 25 



30 



Age (days) 



Figure 3 



Linear growth models for bay anchovy larvae in upper, mid, and lower Chesapeake Bay 

 regions, June and July 1993. L = standard length (mmi, d = age in days, estimated from 

 otolith-increment analysis. 



v-intercepts of the linear regressions ranged from 2.33 

 mm to 3.22 mm and did not differ significantly among 

 regions or between June and July (ANCOVA, P>0.05). 

 Growth in weight was rapid for surviving larvae. 

 Baywide, estimated weight-specific gi-owth rates G 

 {Id) for larvae <13 mm SL (Table 1) increased from 

 0.26 in June (29.7'7r/d) to 0.35 in July (41.9%^) 

 (ANCOVA, P<0.05 ). For the <13 mm larvae, the high- 

 est regional G was 0.40 in the upper bay in July 

 (49.2%/d), and the lowest G was 0.25 in the mid bay 

 in June (28.4%/d). In July, an estimated baywide G 

 for larvae >13 mm SL was only 0.11 (11.6%/d), a rate 

 much lower than that of smaller larvae. 



Mortality 



Larvae experienced high mortality rates. Baywide 

 instantaneous mortality coefficients M (/dl for lar- 

 vae in all age classes declined significantly from 0.41 

 (33.6%/d) in June to 0.23 (20.5%/d) in July (ANCOVA, 

 P<0.0001) (Fig. 4). When the data analysis included 

 only larvae <1 8-day s old (the oldest age represented 

 in June) estimated July M = 0.22 (19.7%/d), a rate 

 still significantly lower than the June rate (ANCOVA, 

 P<0.001). 



Larval mortality rates differed significantly among 

 regions in June and July ( ANCOVA, P<0.0001 ). High- 

 est mortality rates in each month were in the lower 

 bay. Regional daily rates ranged from 0.14 (13.1'^/d) 

 to 0.54 (Al.l'AIA) (Fig. 5). In June, the mid bay mor- 

 tality rate was significantly lower than upper and 

 lower bay rates. In July, when all regional mortality 

 rates differed significantly, the lower bay had the 

 highest mortality rate and the upper bay had the 

 lowest rate (Student-Neumann-Keuls test). 



Length-specific mortality rates (/mm ) declined dra- 

 matically as larval length increased (Fig. 6). In June, 

 the length-specific mortality rates ranged from 0.45/ 

 mm for 2-5 mm SL larvae to 0.07/mm for 10-13 mm 

 SL larvae. In July, the rates were lower, ranging from 

 0.33/mm for 2-5 mm SL larvae to 0.05/mm for 10-13 

 mm SL larvae. The highest regional length-specific 

 rate, 0.64/mm, occurred in the mid bay during July 

 for the 2-5 mm SL class. 



M/G ratio, production, and survival 



The baywide MIG ratio declined from 1.59 in June 

 to 0.67 in July (Fig. 7), indicating a nearly 70-fold 

 higher survival and biomass production of cohorts 



