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Fishery Bulletin 90(2). 1992 



not be representative of all groups. Furthermore, 

 trends shown by the marginal increment in some long- 

 lived fish become clear only after the otoliths have 

 either been sectioned or broken and burnt (Campana 

 1984, Collins et al. 1988). This accounts for estimates 

 of age sometimes being lower when whole otoliths have 

 been used than when either sectioned or broken and 

 burnt otoHths were employed (Beamish 1979a, Cam- 

 pana 1984, Collins et al. 1988). 



Although the Platycephalidae occurs along the coasts 

 and within estuaries throughout the Indo-west Pacific 

 region, the majority of the 41 species of flathead found 

 in Australia are restricted to its southern waters (Sri- 

 ramachandra-Murty 1975, Paxton and Hanley 1989). 

 Despite wide distribution and, in some cases, the com- 

 mercial and recreational importance of the Platycepha- 

 lidae, estimates of the age and growth of represen- 

 tatives of this family are limited to those obtained for 

 Platycephalus bassensis, P. castelnaui, and P. specu- 

 lator by Brown (1977) and for P. richardsoni by Cole- 

 fax (1934), Fairbridge (1951), and Montgomery (1985), 

 the populations of which were all located in south- 

 eastern Australia. The most abundant species of flat- 

 head on the temperate southern coast of Western 

 Australia is P. speculator, a species which has been 

 shown to breed within Wilson Inlet, the largest estuary 

 of this region (Hyndes et al. In press). 



Previous attempts to age platycephalids have used 

 whole sagittal otoliths (Colefax 1934, Fairbridge 1951, 

 Brown 1977, Montgomery 1985). However, a prelim- 

 inary investigation of the translucent zones in the sagit- 

 tal otoliths of P. speculator from southwestern Aus- 

 tralia showed that the outer opaque and translucent 

 zones on the otoliths of larger fish often became clear 

 only when the otoliths had been sectioned. 



The present study was undertaken to determine the 

 age structure and growth of P. speculator in Wilson 

 Inlet, where this species is abundant and contributes 

 to the local commercial and recreational estuarine 

 fisheries (Lenanton and Potter 1987). Emphasis has 

 been placed on elucidating the degree to which section- 

 ing the otoliths influences marginal increment trends, 

 age estimates, and growth equations. In addition, 

 marginal increment data were pooled for both whole 

 and sectioned otoliths to examine whether the resul- 

 tant overall annual marginal increment trends were 

 strongly influenced by that of a group(s) of otoliths with 

 a particular number(s) of translucent zones. 



Materials and methods 



Sampling locality and regime 



Wilson Inlet (117°25'E and 34°50'S) has a narrow en- 

 trance channel which opens into a wide basin (48 km^) 



supplied by two main tributary rivers. Water depth in 

 the basin is generally less than 2m. Platycephalus 

 speculator was collected monthly from within the basin 

 of Wilson Inlet between September 1987 and April 

 1989 using beach seines (mesh size in pocket 9.5 mm) 

 during the day and gillnets (six stretched-mesh sizes, 

 38- 102 mm), otter trawls (mesh size in pocket 25 mm) 

 and plankton trawls at night (mesh size 1 mm). 



Bottom water temperatures near the entrance chan- 

 nel of Wilson Inlet and 12 km further up the estuary 

 near the top end of the basin were recorded at the time 

 of sampling. 



Age determination 



Each fish was measured (total length) and weighed to 

 the nearest 1mm and O.lg, respectively. Sex was 

 recorded when the gonad could be identified as either 

 ovary or testis, which was usually possible in fish 

 > 100 mm in length. Both of the sagittal otoliths of 1305 

 juvenile and adult fish were cleaned, dried, and stored 

 in gelatin capsules. 



Whole otoliths were placed in methyl salicylate solu- 

 tion and examined microscopically under reflected light 

 against a black background. The marginal increment, 

 i.e., the distance between the outer edge of the outer- 

 most translucent zone and the periphery, was mea- 

 sured on one of the otoliths of each fish and expressed 

 either as (1) a proportion of the distance between the 

 focus and the outer edge of the translucent zone when 

 only one translucent zone was present, or (2) as a 

 proportion of the distance between the outer edges 

 of the two outermost translucent zones when two or 

 more translucent zones were present. Measurements 

 were always made along the same axis, to the nearest 

 0.05 mm (Fig. 1). The number of translucent zones on 

 each otolith was recorded. 



These otoliths were later mounted and embedded 

 in black epoxy resin (Bedford 1983, Augustine and 

 Kenchington 1987) and cut into 1.5-2 mm transverse 

 sections using the diamond saw described by Augustine 

 and Kenchington (1987). Sections were mounted on 

 glass slides and their surfaces ground on sequentially 

 finer grades (400-1200) of carborundum paper. Sec- 

 tions were then coated with clear nail polish and 

 examined microscopically under reflected light. Mea- 

 surements of the marginal increment and counts of the 

 number of translucent zones in these sectioned otoliths 

 were carried out in precisely the same manner as 

 described above for whole otoliths. 



Mean marginal increment values were plotted sep- 

 arately for both whole and sectioned otoliths with 

 1-4 and > 5 translucent zones to ascertain if they follow 

 a consistent annual trend and thus permit the trans- 

 lucent zones to be considered as annuli. Width and 



