280 



Fishery Bulletin 90(2). 1992 



25 r 



y 20 



g 15 





10 



A O D 

 1987 



F A J 



1988 



Lower basin 

 Upper basin 



O D F A 



1989 



Figure 3 



Bottom-water temperatures recorded at sites at 

 lower and upper ends of the basin of Wilson Inlet, 

 Western Australia, August 1987-April 1989. 



corresponding cohort caught in December were taken 

 in gillnets which, because of the mesh sizes of these 

 nets, would have taken only the larger members of that 

 year-class. 



Larval P. speculator, ranging up to a length of 13 

 mm, were collected in plankton trawls in December 

 1987, February 1988, and February and April 1989 

 (Fig. 4). Otoliths of juveniles caught in April 1988 

 and 1989, and which from their lengths (23-89 mm) 

 clearly corresponded to the + age-class, did not have 

 a translucent zone. The group of fish representing 

 the 1988 year-class had reached 198-231 mm by 

 December 1988, and 258-323 mm by April 1989. 

 These length ranges are similar to those attained 

 by the previous (1987) year-class in December 1987 and 

 April 1988 (Fig. 4). The modal length of the 1 -t- age- 

 class in October 1988 ( = 1987 year-class) was identical 

 to that of the 1+ age-class in October 1987 (= 1986 

 year-class). Six year-classes were usually found in 

 samples from each month, and as many as eleven year- 

 classes were present in February 1988 (older year- 

 classes are not shown in Fig. 4). Maximum lengths for 

 each sex were 696mm for a lO-i- female and 545 mm 

 for a 12-^ male. 



Estimated ages and growth curves using 

 whole and sectioned otoliths 



All of the 197 otoliths which showed one translucent 

 zone (= annulus) in sectioned otoliths also displayed 

 a single zone prior to sectioning (Fig. 5). However, 44% 

 of otoliths with two or more translucent zones after 

 sectioning produced underestimates using whole oto- 

 liths. Between 23% and 57% of the otoliths with 2-4 



o 



B 



1987 

 Sepember 



October 



200 300 400 .SOO 600 

 Length (mm) 



Figure 4 



Length-frequency histograms for the 1985-89 year- 

 classes of Platycephalus speculator. 



annuli each showed one less translucent zone prior to 

 sectioning and thus underestimated ages by 1 year. 

 Discrepancies between the number of translucent zones 

 in sectioned and whole otoliths were even more marked 

 in otoliths with 5 or more translucent zones. Indeed, 

 the numbers of annuli were as many as 5 or 6 less in 

 whole otoliths than the 11 or 12 annuli observed in sec- 

 tioned otoliths (Fig. 5). The proportion of underesti- 

 mates using whole otoliths was greatest in spring for 

 those otoliths which showed 2 or 3 translucent zones 

 after sectioning (Fig. 6). 



