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Fishery Bulletin 90(2). 1992 



Transverse 

 Section 



Figure 1 



Schematic diagram showing the translucent and opaque zones 

 in a whole and sectioned sagittal otolith of Platycephalus 

 speculator. Broken line represents the axis along which 

 measurements were taken. 



thickness of the whole otoliths of 123 fish, covering the 

 full range of sizes, were measured to the nearest 0.01 

 mm to examine the relationship between otolith width 

 and thickness. The number of translucent zones in 140 

 otoliths, of which up to 20 otoliths came from each age- 

 class (estimated from sectioned otoliths), were counted 

 in whole and sectioned otoliths by a second 'reader' who 

 had no previous experience in examining otoliths of this 

 species. The reproducibility of age estimates for each 

 method was determined by using the coefficient of 

 variation tSokal and Rohlf 1981, Chang 1982). 



Von Bertalanffy growth curves were fitted to 

 individual lengths of males and females at the esti- 

 mated age-at-capture by a nonlinear technique (Gal- 

 lucci and Quinn 1979) using a nonlinear subroutine 

 in SPSS (SPSS 1988) and assuming a 'birth date' of 

 1 January. This date corresponds approximately to 

 the midpoint of the period when, on the basis of 

 gonadosomatic indices and trends shown by oocyte 

 development, P. speculator exhibited peak spawning 

 activity in Wilson Inlet (Hyndes et al. In press). The 

 von Bertalanffy equation is Lt = L^ [l-e^^('^'"'], 

 where Lt is the length at age t (yr), L„ is the mean 

 asymptotic length predicted by the equation, K is the 

 growth coefficient, and to is the hypothetical age at 

 which fish would have zero length if growth followed 

 that predicted by the equation. Comparisons have 

 been made between the age estimates and von Ber- 

 talanffy growth curves, calculated from data obtained 

 using whole and sectioned otoliths and assuming that, 

 in both cases, the translucent zones correspond to 

 annuli. 



Results 



Marginal increments 



Annual trends in the mean marginal increments for 

 whole and sectioned otoliths with one translucent zone 

 were similar (Fig. 2). However, the sharp decline which 

 occurred in the marginal increment after the winter 

 (June- August) of 1988 was detected earlier in sectioned 

 otoliths (October) than in whole otoliths (December). 

 Although the data for 1987 were not as extensive, they 

 still exhibited a similar marked decrease at the same 

 time of year. In both years, the marginal increment on 

 both whole and sectioned otoliths subsequently rose 

 consistently through the summer, before leveling off 

 in the late autumn and winter (Fig. 2). 



Annual trends in mean marginal increments of sec- 

 tioned otoliths with two, three, and four translucent 

 zones parallel those in sectioned otoliths with one 

 translucent zone, with marginal increments falling 

 sharply in the spring (October) of both 1987 and 1988 

 (Fig. 2). Although the marginal increment on whole 

 otoliths with two, three, and four translucent zones 

 also declined in spring, the decrease was far less pro- 

 nounced and the trends less consistent. 



Since the number of otoliths with five or more 

 translucent zones was small, values for the marginal 

 increments on all such otoliths were pooled. Although 

 seasonal trends shown by the marginal increment in 

 sectioned otoliths with >5 translucent zones were 

 slightly less consistent than in those with 1-4 such 

 zones, they still followed a similar annual trend (Fig. 

 2). Furthermore, the translucent zones were still clearly 

 visible and had the same appearance as those in otoliths 

 with 1-4 translucent zones. No clear annual trend could 

 be seen in the marginal increments of whole otoliths 

 displaying >5 translucent zones (Fig. 2). 



The above trends in marginal increments of sectioned 

 otoliths (with a sharp decline only occurring at one time 

 of the year, i.e., in the spring) show that the first four 

 translucent zones on otoliths of P. speculator are laid 

 down annually. Since the same trends were exhibited 

 in pooled data for the fifth and subsequent translucent 

 zones, these zones were presumably also, at least in 

 most of these cases, laid down annually. We thus con- 

 sider the translucent zone on sectioned otoliths as an 

 annulus which can be used for ageing P. speculator 

 from Wilson Inlet. The data also show that the outer 

 opaque zone starts to form when water temperatures 

 are rising from their winter (July) minima of about 

 11°C towards their summer (December-February) 

 maxima of ~22°C (c.f. Figs. 2,3). 



The annual trend shown by the mean marginal in- 

 crement based on all sectioned otoliths, irrespective 

 of the number of translucent zones, was essentially 



