320 



Fishery Bulletin 93(2). 1995 



Potomac River 1 987 



200 



Cladocera 



Copepod Adults & Copepodites 



Copepod Naupln 



Rotifera 



Upper Bay 1 988 



150 - 



1 10 20 30 10 20 30 9 



Potomac River 1 988 



= 300- 



Upper Bay 1 989 



1 10 20 30 10 20 30 9 



Potomac River 1 989 



200- 



100 



1 10 20 30 10 20 30 

 April May 



Date 



Figure 4 



Densities of zooplankton taxa estimated on each survey date in the Potomac River, 

 1987-89 and Upper Bay, 1988-89. Note that Y-axis scales change among years and 

 areas. 



sonally in each year. Cohorts that hatched late in 

 the season grew relatively fast, had shorter stage du- 

 rations (Fig. 5), and reached larger sizes by 20 days 

 posthatch than did cohorts hatched earlier, which 

 grew in cooler water. Both methods of estimating 

 growth produced similar rates, although estimates 

 from the aggregate method were much less precise 

 (Figs. 5 and 6). 



Aggregate-sample growth rates of Potomac River 

 larvae differed significantly among 3-day cohorts in 

 1987 and 1989. Cohort growth rates, after adjusting 

 for age by analysis of covariance, tended to increase 

 as the season progressed (Fig. 5). They ranged from 



0.19 to 0.44 mmd ' (mean ±95% CI=0.26 ±0.06 

 mmd" 1 ) in 1987, from 0.11 to 0.22 mmd" 1 

 (mean=0.18 ±0.04 mmd" 1 ) in 1988, and from 0.12 to 

 0.53 mmd" 1 (mean=0.24 ±0.07 mmd" 1 ) in 1989. The 

 regression fits of growth data were poor for some 

 Potomac River cohorts in 1988. The predicted mean 

 lengths at 20 days posthatch of larval cohorts were 

 variable, ranging from 7.7 to 11.7 mm SL in 1987, 

 from 6.9 to 9.0 mm SL in 1988, and from 4.8 to 11.1 

 mm SL in 1989. Predicted ages at 8.0 mm SL also 

 varied, ranging from 13.2 to 21.2 days in 1987, from 

 14.2 to 23.8 days in 1988, and from 14.6 to 31.6 days 

 in 1989 (Fig. 5). 



