324 



Fishery Bulletin 93(2), 1995 



cohorts), when mean water temperatures were rela- 

 tively warm, had within-cohort growth rates and 

 back-calculated lengths at 15 and 20 days posthatch 

 that were positively related to lengths at capture. 

 For example, individual growth rates of Upper Bay 

 larvae hatched on 14 May 1989 were not correlated 

 with their lengths at capture, suggesting that there 

 had been compensation for slow (or fast) growth at 

 earlier ages (Fig. 8). Growth rates of Upper Bay lar- 

 vae hatched on 23 May were positively related to their 



Hatched 14 May 





© 



o 



■s 



« 



3 

 tj 



"3 

 U 



u 



« 



pa 



Hatched 23 May 



10 



12 



Length (mm TL) at Capture 

 Figure 8 



Back-calculated growth rates (G, mmd* 1 ) of individual 

 striped bass, Morone saxatilis, larvae in relation to total 

 lengths (TL) at capture for Upper Bay larvae hatched on 

 14 May and on 23 May 1989. 



lengths at capture, indicating that initial growth rate 

 differences had been maintained (Fig. 8). 



Mortality rates 



Mortality rates of larval cohorts, which were highly 

 variable, were not correlated with initial cohort abun- 

 dance, growth rate, growth rate variability, stage 

 duration, or with any measured environmental fac- 

 tor. Mean mortalities of cohorts from the Potomac 

 River ranged from 21.9 to 23.9% d" 1 in 1987-89. Co- 

 hort-specific mortality rates of Potomac River lar- 

 vae ranged from Z = 0.05 to 0.92-d" 1 in 1987, from Z = 

 0.05 to 0.46-d" 1 in 1988, and from Z = 0.07 to 0.60-d" 1 

 in 1989 (Fig. 9). Temporal trends indicated that mor- 

 tality rates of Potomac River larvae were high in April, 

 lowest in early May, and apparently highest in late May 

 (Fig. 9). Larval cohort mortality rates in the Upper Bay 

 during 1989 ranged from Z = 0.02 to 0.28-d" 1 , averag- 

 ing 11% d _1 . There was no obvious temporal trend in 

 Upper Bay larval mortality rates (Fig. 9). 



G/Z ratio 



The cohort-specific G/Z ratios were variable, but the 

 median values in each year were positively related to 

 apparent year-class strength in the Potomac River. The 

 median ratios were 1.03 in 1987, 0.41 in 1988, and 0.94 in 

 1989 (Fig. 9). The Upper Bay median G/Z ratio in 1989 

 was 1.70, a value higher than the Potomac River me- 

 dian ratios because cohort mortality rates generally 

 were lower in the Upper Bay. In the Potomac River, the 

 cohort-specific G/Z ratios in each year were generally 

 highest for cohorts hatched during the last week of April 

 and the first 10 days of May, when growth rates were 

 increasing and mortality rates were lowest (Fig. 9). 



Productions of late-stage larvae 



Summed cohort productions of 8-mm-SL larvae were 

 correlated with juvenile recruitment indices in the 

 Potomac River, suggesting that recruitment is essen- 

 tially fixed during the larval stage. Summed cohort 

 productions were 190 million in 1987, 23 million in 

 1988, and 109 million in 1989. Potomac River indi- 

 ces of juvenile recruitment for those years were 6.4, 

 0.4, and 2.2, respectively (Schaefer et al. 1 ). Summed 

 production of Upper Bay larval cohorts at 8 mm SL 

 was 47 million in 1989. The Upper Bay's juvenile 

 recruitment index in 1989 was 19.4, a value 1.6 times 

 higher than the long-term mean (Schaefer et al 1 ). 



Most cohorts with larvae surviving to 8 mm SL were 

 hatched late in the spawning season when egg produc- 

 tion was low or declining (Fig. 10). In the Potomac River, 

 cohort productions of 8-mm-SL larvae were positively 



