748 



Fishery Bulletin 88(4). 1990 



Jul 



Aug Sep 



Date 



Oct 



Figure 3 



Standardized abundance of age-0 weakfish (CPUE = number/ 

 2-minute tow) by station and date in Chesapeake Bay-York River, 

 VA. 



Multiple cohorts were apparent in lioth years. The 

 birthdate frequency distributions sliowed three cohorts 

 in 1983, and two cohorts in 1984 (Fig. 4). Cohorts were 

 defined in 1983 as follows: cohort 1 hatched before 16 

 July, cohort 2 between 16 July and 15 August, and 

 cohort 3 after 15 August. Another cohort may have 

 been present in 1983 before 18 June, but these fish 

 were relatively few in number and subsequently pool- 

 ed with cohort 1. In 1984 two cohorts 4 and 5 were 

 defined as those that hatched before 30 June and those 

 after, respectively (Fig. 4). 



Weakfish growth rates were significantly different 

 among cohorts within years, and between years (Table 

 2). Cohorts 1 and 4, both early-season cohorts, had the 

 lowest growth rates and were not significantly dif- 

 ferent from each other. Cohorts 2 and 3 had inter- 

 mediate growth rates, and cohort 5 had the fastest 

 growth rate compared with all other cohorts (Table 2). 



After separation of individual weakfish into their 

 respective cohorts, a similar pattern of movement by 

 each cohort was apparent: up the estuary with age and 

 season (Figs. 5, 6). For example, the youngest fish in 

 cohort 2 were first most abundant in the lower estuary 

 (stations 2 and 3), and as the season progressed older 

 fish from this cohort were more abundant up the estu- 

 ary at the same location where few individuals of this 

 cohort were collected earlier (station 4 in August vs 

 station 4 in September 1983; Fig. 5). Cohort 4 showed 

 the onlv difference from the above migration pattern. 



30 n 



15- 



> 

 o 



c 



0) 

 3 O 



a 



01 



15- 



1984 



N = 459 



May 



Jun 



' Jul 

 Month 



Aug Sep 



Figure 4 



Birthdate frequency distribution of age-0 weakfish cohorts in Chesa- 

 peake Bay-York River, VA, based on a 3-day moving average. 

 Numbers at the peaks of the distributions refer to cohorts (separated 

 by open and shaded bars). 



Table 2 



( '.rowth rates (mm/day) of individual age-0 weakfish coliorts. 

 Analysis of covariance of standard length on age with cohort 

 as the covariate (0.05 level). Student Newman Keuls test 

 (SNK-test) was used to show specific differences, denoted by 

 different letters (0.10 level). 



