Hyndes et al.: Age and growth estimates of Platycephalus speculator 



281 



Number of annuli using sectioned otoliths (O ) 

 234 5678 9 10 11 



12 



c aw 

 u 2 ^ 



t/i .5 o 



-3 - 





^ ° 



-6 



Figure 5 



Number of otoliths with 1-12 annuli base(i 

 on sectioned sagittal otoliths oi Platycephxdus 

 speculator, and underestimates of the num- 

 ber of annuli observed on the same whole 

 sagittal otoliths. Numbers in parentheses in- 

 dicate the number of fish of different ages 

 based on sectioned otoliths, while numbers 

 above the closed circles indicate the percent- 

 age of underestimates using whole otoliths. 



The coefficient of variation for 

 replicate age estimates between 

 readers was far less for sectioned 

 otoliths (1.2%) than for the same 

 otoliths prior to sectioning 

 (8.7%). While the estimated age 

 varied by only 1 year for each of 

 the six sectioned otoliths for 

 which there was disagreement, 

 the estimated ages varied by as 

 much as 3 years for the 53 whole 

 otoliths for which there were 

 discrepancies. 

 The relationship between width and thickness of 

 sagittal otoliths of P. speculator is curvilinear, demon- 

 strating that width does not increase proportionately 

 with thickness (Fig. 7). The relationship between otolith 

 width (W) and otolith thickness (T) is described by the 

 following polynomial equation: 



W = -0.283 H- 4.635T - 1.175T2 (r'^ 0.82, n 123). 



The von Bertalanffy growth parameters for both 

 male and female flathead were initially determined 

 using individual lengths at estimated age (Table 1). 

 Examination of the length-at-age plots showed that the 

 curve for both sexes fell below the majority of the 

 points for fish >5 years old, i.e., the asymptote was 

 too pronounced to accommodate lengths of the older 

 fish. Individual lengths of the fish were grouped into 

 intervals of 0.1 years and the curves determined again 

 by weighting the data by the inverse of the sample size 

 for each age interval (Beckman et al. 1990). This pro- 

 cedure resulted in a better fit of the curve (Fig. 8). 

 Although the values for to for males and females were 

 shifted slightly away from zero (namely from -0.134 

 to -0.332 and from -0.056 to -0.423, respectively), 

 differences in the lengths of males and females at 



Figure 6 



Seasonal incidence of underestimates by one or more an- 

 nuli when using whole vs. sectioned sagittal otoliths of 

 Platycephalus speculator. 



