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Fishery Bulletin 97(1), 1999 



Fig. 1). There were significant site, species, and site 

 X species effects on mean somatic GRs (P<10~'*), with 

 highest GRs at the southernmost site (Haverstraw 

 Bay). Mean somatic growth rate for striped bass was 

 0.053, 0.232, and 0.238 mm/d at Kingston, New Ham- 

 burg, and Haverstraw Bay, respectively (greater than 

 an eightfold difference between highest and lowest; 

 it should be noted that the total number of fish 

 sampled at Kingston was very small). Growth rates 

 for white perch did not vary as greatly but did vary 

 in the same longitudinal direction (0.209, 0.215, and 

 0.245 mm/d at Kingston, New Hamburg, and 

 Haverstraw Bay). Time series of somatic GRs, aver- 

 aged over all sites for each species (Fig. 1), revealed 

 species-specific patterns of growth with respect to 

 the zooplankton bloom period. Striped bass GRs ac- 

 celerated more strongly during the bloom period than 

 did GRs of white perch. 



Somatic growth rates were related to water tem- 

 perature as 



GR = -0.050 -I- 0.013 X MWT 



[/•2=0.15, /i=774, P<10-*^], 



where MWT = mean water temperature during the 

 life of the individual larva (calculated as the tem- 

 perature midway through the larva's life). 



The effect of temperature was removed by calculat- 

 ing residuals of this regression and by using the re- 



siduals as a new data set ( temperature-adjusted GRs) 

 to assess growth relative to zooplankton dynamics. 

 Temperature-adjusted somatic GRs for white perch 

 showed little trend with respect to the zooplankton 

 bloom, whereas striped bass GRs peaked during the 

 bloom ( Fig. 2 ). This peak was not significantly greater 

 than the adjusted GRs after the bloom, but both dif- 

 fered significantly from the prebloom-adjusted GRs. 

 Our comparisons of somatic growth rates are based 

 on fish larvae of different sizes and ages. Compari- 

 sons of growth rates for a standardized age (we chose 

 age=7 days) can eliminate this potential source of 

 bias. Mean 7-d GRs (Fig. 3) showed a pattern simi- 

 lar to that for overall mean GR, indicating that 

 growth rates did not peak during the zooplankton 

 bloom. Although both species' 7-d GRs varied signifi- 

 cantly with respect to the bloom (with highest GRs 

 after the bloom), white perch 7-d GRs did not differ 

 significantly from striped bass 7-d GRs. 



Recruitment potential at New Hamburg 



Ten 6-d age cohorts, grouped arbitrarilv starting 1 

 May (cohort A) and extending through the period 

 beginning 24 June (cohort J), were identified in tin; 

 catches at New Hamburg (rkm 105). Insufficient 

 numbers prohibited estimation of Z for the last 

 striped bass cohort (J, 24-29 June) and the last two 

 white perch cohorts (I and J). Estimates of white 

 perch starting densities (natural log of A^,,) were high- 

 est in early cohorts (A and B; 

 Table 2). Striped bass initial 

 density estimates were highest 

 in later cohorts (E, G, and H; 

 Table 2). Earliest and latest 

 cohorts were poorly estimated 

 owing to low sample numbers. 

 Estimated mortality rates 

 were highest in cohort A for 

 both species (Table 2). Cohort 

 A (period starting 1 May) ap- 

 peared only in two samples; 

 presumably recruitment from 

 this group was very low. For 

 white perch, mortality rates 

 declined more or less mono- 

 tonically with time (and there- 

 fore also with increasing tem- 

 perature). Striped bass mortal- 

 ity rates were also lowest in 

 the latest cohorts, but there 

 did not appear to be a system- 

 atic decline (Table 2). 



Instantaneous growth rates 

 (G) in striped bass did not vary 



