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



283 



otoliths with four or more annuli were observed only 

 after otoliths had been broken and burnt (Campana 

 1984). Likewise, in the case of king mackerel Scom- 

 beromorus cavalla, the percentage of otoliths with an 

 opaque zone on their edge exhibited an annual trend 

 only after the otoliths were sectioned (Collins et al. 

 1988). 



The consistency in annual trends of marginal incre- 

 ments among sectioned otoliths of P. speculator, 

 despite differing numbers of translucent zones, ac- 

 counts for the clear annual trend in marginal incre- 

 ments when data for all otoliths in each of the monthly 

 samples were pooled. The contrast between the con- 

 spicuous trend shown in pooled data for whole otoliths, 

 and the relatively poor trend exhibited in whole otoliths 

 with two or more translucent zones, shows how trends 

 can be unduly influenced by those of a relatively large 

 sample size of otoliths exhibiting a particularly strong 

 annual trend, such as was present with those otoliths 

 having one translucent zone. For validation of the use 

 of translucent zones as annuli, it is thus important to 

 establish that the trends shown by the marginal in- 

 crements on otoliths with differing numbers of translu- 

 cent zones each follow a consistent annual trend 

 (Johnson 1983, Maceina et al. 1987, Potter et al. 1988, 

 Beckman et al. 1989). 



Age and growth estimates 



Our results demonstrate that, while age estimates be- 

 tween sectioned and whole otoliths corresponded when 

 one translucent zone was present, ages of older fish 

 were underestimated by 2-4 years or as much as 5 or 

 6 years using whole otoliths. Increased resolution of 

 the growth zones after sectioning is reflected in the far 

 lower variability between age estimates made by two 

 independent readers using sectioned otoliths. 



Although otoliths with two or three translucent zones 

 frequently yielded counts of one less zone prior to sec- 

 tioning, many underestimates occurred with the oto- 

 liths taken from fish between mid-spring and early 

 summer (October-December). In other words, they 

 were collected during the period when sectioning en- 

 abled the new opaque zone to be detected approximate- 

 ly 2 months earlier than was possible with whole 

 otoliths. 



Our inability to detect all of the translucent zones in 

 whole otoliths can in part be attributed to the growth 

 pattern of the otolith. Whereas the first translucent 

 zone can be easily detected in whole otoliths, the dis- 

 proportionate increase in otolith thickness relative to 

 its width results in the translucent zones becoming 

 increasingly more closely apposed and therefore diffi- 

 cult to distinguish from one another. This parallels the 

 situation recorded by Beamish (1979a, b) for Pacific 



hake Merluccius productus, and for several species of 

 rockfish (Sebastes), and also by Campana (1984) for 

 starry floimder Platychthys stellatus. 



Despite the fact that a large proportion of ages were 

 underestimated using whole otoliths, the von Berta- 

 lanffy growth curves derived from data using whole 

 otoliths, particularly of males, did not differ markedly 

 from those obtained using sectioned otoliths. This can 

 be attributed to the fact that approximately 74 and 65% 

 of growth for males and females, respectively, occurred 

 in the first 3 years of life when underestimates of age 

 were limited. 



Implications for management 



The vast majority of male P. speculator reach sexual 

 maturity at the end of their first year of life (Hyndes 

 et al. In press).Since males have attained only 190-310 

 mm by this time (Fig. 8), they will only occasionally 

 have reached 300 mm, the minimum legal length for 

 capture of this species. However, the majority of 

 females do not first attain sexual maturity until they 

 are 2 years old (Hyndes et al. In press), by which time 

 they have reached 250-400 mm. Thus, the females of 

 P. speculator can be exploited before they have had the 

 opportunity to spawn. 



In summary, this study has demonstrated that, in the 

 case of the flathead P. speculator, it is crucial to sec- 

 tion its otoliths in order to obtain an accurate estimate 

 of age. Sectioning reduces the problems of distinguish- 

 ing between peripheral translucent zones which, due 

 to the growth pattern of the otolith, become increas- 

 ingly more closely apposed with increasing size. While 

 the results presented in this paper refer only to P. 

 speculator, they parallel in some respects those ob- 

 tained for Platychthys stellatus and Scomberomorus 

 cavalla (Campana 1984, Collins et al. 1988). Such age 

 underestimates have obvious implications in estimating 

 mortalities for use in fisheries management. Our re- 

 sults also demonstrate the importance of plotting 

 marginal increments for otoliths with different num- 

 bers of translucent zones, to establish that such zones 

 are laid down annually on the otoliths of fish repre- 

 senting each presumed age-group. Since the females 

 of P. speculator are being caught before they have 

 spawned for the first time, there is a case for increas- 

 ing the minimum legal size for capture. 



Acknowledgments 



We thank F. Baronie, D. Gaughan, P. Geijsel, P. Hiun- 

 phries, L. Laurenson, and F. Neira for their assis- 

 tance with sampling. The sectioning saw was kindly 

 provided by S. Blaber of the CSIRO Marine Labora- 



