82 



Fishery Bulletin 97(1), 1999 



nile striped bass from five sites along the Hudson 

 River (at river kilometers (rkm) 40, 63, 95, 114, and 

 201 ) during July (n =37), August («=88), and Septem- 

 ber (?! =93) ft-om state-run fishery surveys. Fish were 

 collected either with a 30.5-m (lOO-ft) beach seine 

 with 6.1-m bag, 0.64-cm mesh, or with a 61-m (200- 

 ft) beach seine with 1.27-cm stretch mesh (0.64 cm 

 square). Fish were held on ice in the field, then stored 

 ft^ozen. 



In the laboratory, juvenile striped bass were de- 

 fi-osted, and standard, fork, and total lengths (SL, 

 FL, and TL respectively) were measured. Sagittal 

 otoliths were removed from each fish, cleaned in 10% 

 sodium hypochlorite (bleach), and embedded in 

 Spurr's epoxy. One otolith from each pair was subse- 

 quently sectioned along the transverse axis and pre- 

 pared in thin section according to the method of Secor 

 et al. (1991). A total of 63 juvenile otoliths were suc- 

 cessfully prepared and read. 



Otolith microstructure analysis 



Otolith microstructure was examined with a light 

 microscope with a video attachment connected to an 

 image analysis system. Specimens were viewed in 

 the sagittal plane by using the digital image for pro- 

 jection. Most specimens were viewed at magnifica- 

 tions of 500-750X, but 1250x (lOOx objective) was 

 used to resolve the finest increments. Increments 

 were counted several times and averaged. Larval 

 otoliths were examined by one reader, and two inde- 

 pendent readers were used to cross-check the con- 

 sistency of increment counts for juvenile striped bass 

 otoliths. 



Ages were estimated by correcting for the effect of 

 temperature on the timing of first increment deposi- 

 tion (Houde and Morin^ ). Age at first increment depo- 

 sition was estimated as (11.56-0.45 T) for striped 

 bass and (9.05-0.327) for white perch, where T is 

 water temperature (Houde and MorinM. From our 

 measurements of water temperature («=30) from 14 

 April to 19 July 1994, we estimated a linear increase 

 in river temperatures over the study period with the 

 regression 



Water temperature - -17.6 -t- 0.23 x (day of year) 



[r2=0.98, P<10-*1. 



We used this equation to estimate water tempera- 

 tures on the day of the first increment and assumed 



little change in temperature back to the hatching day. 

 Ages were estimated as number of increments + tem- 

 perature adjustment; hatching dates were then 

 backcalculated from the date of capture. 



Image analysis was used to measure increment 

 widths. For larvae, widths of the total number of in- 

 crements on an otolith were measured and averaged 

 to obtain an average daily increment width (in mm). 

 In addition, when possible, widths of the first five 

 increments (11-15) and of the sixth and seventh in- 

 crements (16-17) were measured to obtain two stan- 

 dardized estimates of early growth. Increments Il- 

 ls exhibited slower average growth than did incre- 

 ments 16-17. In all, otoliths of 248 larval striped bass 

 and 526 larval white perch were examined. 



Somatic growth rates were calculated for larvae 

 and juveniles by the equation 



^^,-K 



)IAge, 



where L, = length at capture; and 

 L, = length at hatch. 



L^ is assumed to be 3.0 mm for white perch and 4.0 

 mm for striped bass (Mansueti, 1958; Lippson and 

 Moran, 1974). In addition, "standardized" growth 

 rates were calculated for 7-day-old larvae so that 

 comparisons of growth could be made before, dur- 

 ing, and after the zooplankton bloom. This was done 

 by determining the number of increments deposited 

 by age 7 (typically <5), multiplying that number by 

 the mean increment width of 11-15, adding the re- 

 sult to the otolith core radius, and then by using the 

 following regressions to calculate standard lengths 

 at age 7 days: 



SLgg = -0.37 -I- 1.95 (natural log of otolith width) 



[r2=0.83, «=210, P<10-*], 



SL 



UP  



-1.67 + 2.18 (natural log of otolith width) 

 [/-'■^=0.88, /!=522, P<10-*]. 



Houde, E. D., and L. G. Morin. 1990. Temperature effects 

 on otolith daily increment deposition in striped bass and white 

 perch larvae. International Council for the Exploration of the 

 Sea, Copenhagen, Denmark, Council Meeting 1990/M:5. 



The same procedure was used to backcalculate age 

 7-d somatic growth rates for juvenile striped bass. 



Mortality and recruitment potential 



For purposes of comparison with Chesapeake Bay 

 studies, mortality rates were estimated for fish lar- 

 vae collected at New Hamburg (rkm 105) by using a 

 method similar to that described in Secor and Houde 

 (1995). Cohort analyses were confined to the New 

 Hamburg station because we were able to collect both 

 species consistently in large numbers. All larvae in 

 these samples were counted, and lengths of at least 



