Powell et al.: Growth, mortality, and hatchdate distributions for Cynoscion nebulosus 



145 



Distal Edae 



Ventral 



Counting Path 2 

 (21 days to capture) 



Figure 2 



Transverse polished section of a spotted seatrout ( Cynoscion nebulosus) ( 18 mm SL; age 48 days) otolith 

 showing the counting paths. 



lengths. In all samples, the 21 st increment could easily be 

 traced in both measuring paths and in all samples the first 

 21 increments could be measured within the same image. 

 Increment widths were averaged over a 7-day period. Age 

 estimates were also obtained and we eliminated any oto- 

 lith used to measure increment widths if the difference in 

 total increment count between the two methods ( counts ob- 

 tained directly from the microscope versus those attained 

 by image analysis) was greater than 7 days or 10%. On 

 this basis, 117 otoliths were removed from the increment 

 width analysis. 



We believed counts obtained directly from the microscope 

 were more accurate than those obtained by summing the 

 number of increments measured on the computer moni- 

 tor with the image analysis system. Counting increments 

 directly through the microscope lens allows the reader to 

 optically section the otolith (by varying the focus), which 

 helps in detecting daily increments. "Frozen" multiple im- 

 ages are a result of using the image analysis; hence optical 

 sectioning is not possible. 



Data analysis Data from all years and sources were used 

 for 1) overall growth (i.e. larval and juvenile); 2) juvenile 

 growth; and 3) estimates of juvenile mortality. Data from 

 NOAA larval and juvenile collections were used to estimate 

 a body-length-otolith-radius relationship. Data from 1995 

 FMRI and NOAA collections, which was the most com- 

 plete data set, were used for growth comparisons between 

 cohorts, and hatchdate distributions. Data from 1995 FMRI 

 collections were used for 1) growth comparisons between 

 geographical subdivisions; 2) estimating a wet-weight-age 

 relationship to compute the ratio of wet-weight specific- 



growth to mortality (G:M ratios), which assesses the rela- 

 tive recruitment potential of individual cohorts (Houde, 

 1996; Rilling and Houde, 1999; Rooker et al., 1999); and 3) 

 determining the influence of temperature on otolith incre- 

 ment width. We used the FMRI data set exclusively for 

 the above analyzes because collections were spatially more 

 localized and wet weights were available. 



Natural mortality (M) estimates were derived by regress- 

 ing log. unadjusted numbers on age classes (5-day bins); 

 the resulting slope provided an estimate of total mortality 

 (Ricker, 1975). However, on the basis of the age-frequency 

 distributions (Fig. 3), we considered juveniles a40 days old 

 fully recruited to our gear and juveniles >90 days old ap- 

 peared to avoid our gear. Hence, only juveniles between 40 

 and 90 days old were used to calculate mortality. 



Hatchdate distributions were computed on a weekly ba- 

 sis and adjustments for mortality were made on individual 

 juveniles by the equation 



N =N,/e-z<, 



where N = estimated number at hatching; 



N t = number at time t (N t =l because N was calcu- 

 lated for each individual fish); 



Z = instantaneous daily mortality coefficient; 

 and 



t = age in days. 



Spotted seatrout cohorts were divided into weekly units, 

 but comparisons between cohort growth was done on a 

 monthly basis because of inadequate numbers for weekly 

 comparisons. A test of heterogeneity of slopes was imple- 



