Limburg et al.: Growth, mortality, and recruitment of larval Morone spp. 



83 



100 individuals were measured unless fewer than 

 100 occurred in the sample. Densities (numbers of 

 fish per 1000 m^) by 0.5-mm size class were com- 

 puted and corrected for differences in day and night 

 catch efficiencies, and for extrusion of smallest indi- 

 viduals, by using regressions in Houde et al.^ 



Age distributions offish collected on different sam- 

 pling dates were estimated from the otolith-aged fish. 

 Rather than estimate variance from an age-length 

 regression and use it to assign probabilities of age- 

 at-length (Secor and Houde, 1995), we estimated the 

 mean and variance in ages for each 0.5-mm size group 

 and used those parameters to assign age-at-length 

 probabilities (z-scores). 



The corrected catch densities, separated into 0.5- 

 mm size classes, were multiplied by the appropriate 

 size-based age distributions to obtain numbers offish 

 of different ages. Following Secor and Houde (1995), 

 hatching dates were calculated and fish were grouped 

 into 6-d cohorts. In some of the cohorts, the age-esti- 

 mation technique resulted in very small initial num- 

 bers of fish. If these numbers were tenfold smaller 

 than the maximum calculated densities of a cohort, 

 they were deleted from the data set. Then mortality 

 rates (Z) of the cohorts were estimated by fitting an 

 exponential decay model to abundances offish within 

 specific cohorts over time. 



We used an index (Rutherford and Houde, 1995; 

 Secor and Houde, 1995) of mean instantaneous 

 growth (estimated for an individual fish as G = In 

 (W/Wq)//, where t = age at capture, W, = weight at 

 capture, and W^ = weight at hatch) over instanta- 

 neous mortality rate iG/Z) to compare the benefits 

 accrued in growth versus the costs expressed as mor- 

 tality for larvae occurring before, during, and after 

 the zooplankton bloom. Weights were estimated from 

 lengths by the equation 



Table 1 



Mean (± SD) somatic growth rates (GR, mm/d) of larval 

 white perch and striped bass, by site and date, 1994. (n=526 

 white perch and 248 striped bass; n.d.=no data; — = zero 

 fish in samples.) 



White perch 



Striped bass 



Date 



MeanGR SD n Mean GR SD 



Haverstraw Bay (rkm 65-70) 



18 May 



24 May 



3 June 



7 June 



10 June 



13 June 



22 June 



29 June 



6 July 



0.181 0.067 



0.186 

 0.409 

 0.211 

 0.382 

 0.427 

 0.313 

 n.d. 



0.054 

 0.215 

 0.086 



0.199 



New Hamburg (rkm 105) 



18 May 



24 May 



3 June 



7 June 



10 June 



13 June 



22 June 



29 June 



6 July 



0.155 

 0.192 

 0.218 

 0.236 

 0.210 

 0.265 

 0.306 

 0.331 



Kingston (rkm 148) 



18 May — 



24 May 0.205 



3 June 0.167 



7 June 0.200 



10 June 0.201 



13 June 0.196 



22 June 0.245 



29 June 0.300 



6 July n.d. 



0.037 

 0.060 

 0.070 

 0.085 

 0.081 

 0.083 

 0.049 

 0.052 



0.061 

 0.053 

 0.053 

 0.089 

 0.086 

 0.113 

 0.099 



15 

 



17 

 9 

 8 

 1 

 3 

 1 





 37 

 47 

 42 

 35 

 29 

 11 

 11 



9 





 9 

 52 

 47 

 59 

 35 

 12 

 37 



0.072 0.069 



0.185 

 0.286 

 0.178 

 0.250 

 0.293 

 0.284 

 n.d. 



0.069 

 0.162 

 0.216 

 0.247 

 0.224 

 0.207 

 0.252 

 0.264 



0.044 

 0.032 

 0.146 

 0.017 



n.d. 



0.111 

 0.128 

 0.111 

 0.046 

 0.088 

 0.007 



0.022 

 0.127 

 0.084 

 0.030 

 0.039 

 0.056 

 0.084 

 0.062 



0.027 

 0.054 

 0.189 



9 





 29 

 25 



8 

 17 

 36 



3 







3 



11 



14 



4 



8 



9 



20 



40 





 3 

 6 

 2 

 1 

 

 

 



W = 3.763 X 10-1 ^ ^(4 2879) 



[n=67, r2=0.92, P<10-6]; 



where W = mg wet weight; and 



L - length in mm (Limburg et al. 



1997). 



Finally, we estimated the dates of first-feeding for 

 striped bass larvae and juveniles with respect to the 

 zooplankton bloom. We assumed that fish begin to 

 feed at day 5 after hatching. We used an age-length 



regression from the 63 juvenile striped bass to esti- 

 mate ages of the remaining juveniles. We assumed 

 that little or no out-migration would be occurring in 

 July and August (Dovel, 1992), so that fish collected 

 during these months would show the effect of differ- 

 ential recruitment without the confounding process 

 of out-migration. 



Results 



2 Houde, E. D., E. J. Chesney R. M. Nyman, and E. S. Ruther- 

 ford. 1988. Mortality, growth and growth rate variability of 

 striped bass larvae in Chesapeake subestuaries. Interim Re- 

 port to Maryland Department of Natural Resources. Chesapeake 

 Biological Laboratory, Solomons, MD. Ref No. [UMCEES]CBL 

 88-96, 126 p. [Available: Chesapeake Biological Laboratory, Box 

 38, Solomons, MD 20688.) 



Growth rates 



Mean somatic growth rates (GRs) of both species of 

 Morone tended to be lowest prior to the bloom (be- 

 fore 3 June); rates were higher after the bloom (22 

 June and later), especially for white perch (Table 1; 



