Hesp et a\ Age and size composition, growth rate, reproductive biology, and habitats of Glaucosoma hebraicum 



215 



data employed to determine the age compositions and 

 fiirowtli rales ot female and male G. hebraicum. The repro- 

 ductive variables were also used to help ascertain where 

 (1. hcbr'ciiciim spawns and whether this species is a mul- 

 tiple spawner sciisu deVlaming (1983), i.e. whether indi- 

 vidual females release eggs on more than one occasion in 

 a spawning season. The lengths of both sexes at first ma- 

 turity were calculated to determine whether they lay be- 

 low the minimum legal length (MLL) of 500 mm and were 

 thus appropriate for helping to conserve this species. The 

 ages at which females and males mature were also deter- 

 mined in order to elucidate whether fish might spawn in 

 one or more spawning seasons before they attain the MLL. 

 Attempts were also made to ascertain the types of habitat 

 occupied by G. hebraicum at different stages during its life 

 cycle and to obtain preliminary mortality estimates which 

 could be used as an indicator of whether this species is 

 being lightly or heavily fished. Finally, the data collected 

 during this study were used to discuss ways in which the 

 fishery for G. hebraicum might be managed most appro- 

 priately in the future. 



Materials and methods 



Glaucosoma hebraicum, that were less than the MLL of 

 500 mm total length (TL), were collected between May 

 1996 and June 1999 by commercial trawls, hand-lines, 

 and a recreational spear diver under a research collection 

 permit issued by Fisheries Western Australia — the gov- 

 ernment agency responsible for managing the fishery for 

 this species. Filleted carcasses of G. hebraicum >500 mm 

 TL, together with their gonads, were obtained monthly 

 between May 1996 and April 1998 from commercial fish- 

 processing plants and weigh-ins at local recreational fish- 

 ing club competitions. These fish had been caught by 

 commercial or recreational rod and hand-lines along the 

 lower west coast of Australia between Mandurah (32°32'S) 

 and the Houtman Abroholos <28°35'S), i.e. within that 

 part of the distribution of G. hebjxiicum where this spe- 

 cies is considered to be most abundant and is most heavily 

 fished. 



The total length of each fish was measured to the near- 

 est 1 mm and the weight of each fish <500 mm was 

 weighed to the nearest 1 g. The weights of 334 females and 

 442 males >500 mm TL were weighed to the nearest 10 g 

 prior to filleting. The relationship between total length (L) 

 in mm and total wet weight ( W) in g of each sex was 



Females logW = logO.0000417 + 2.859 logL 



(/(=486, r'^=0.995) 



Males logW = logO.0000322 -f- 2.898 logL 



(w=572, r2=0.995). 



These relationships were then used to estimate the weights 

 of the female and male fish that had been filleted but not 

 weighed. Note that all of the logarithm values recorded in 

 this paper are natural logarithms. 



On several occasions, a video camera, attached by cable 

 to a television monitor and video recorder, was lowered 



over the substrata during commerical hand-line fishing for 

 dhufish. Video footage of the substrate over which dhufish 

 were caught was later examined to determine the types of 

 habitat occupied by this species. 



Age determination 



The two sagittal otoliths of each fish were removed, cleaned, 

 dried, and then stored in paper envelopes. All sagittal oto- 

 liths were sectioned, except for those which, when placed 

 in methyl salicylate and examined microscopically under 

 reflected light against a black background, could clearly 

 be seen to possess either no opaque zones or only a single 

 opaque zone. However, because the opaque zones in the 

 whole otoliths of large fish were so numerous and closely 

 spaced that they were often difficult to distinguish from 

 one another and because previous estimates of the age of 

 dhufish were based on counts of opaque zones in whole 

 otoliths (Sudmeyer et al.'), the number of opaque zones 

 visible in 100 otoliths, obtained from a wide size range of 

 fish, were compared prior to and after sectioning to ascer- 

 tain whether sectioning increased one's ability to detect 

 the opaque zones. 



For sectioning, the otoliths were mounted in clear ep- 

 oxy resin and cut into 500 pm sections with a low-speed 

 diamond saw (Buehler). The sections were cleaned and 

 mounted on slides with DePX mounting medium and ex- 

 amined under reflected light with a dissecting microscope 

 attached to a video camera (Panasonic WV-CD20). The im- 

 age was analyzed by using the computer imaging package 

 Optimas 5 (Optimas, 1995). The number of opaque zones 

 in each otolith was always counted twice and on different 

 days and without knowledge of either the date of capture 

 or the size of the fish from which the otolith came, and 

 also, in those cases where the two counts differed, on a 

 third occasion. Although the number of times that a third 

 count did not agree with either of the two previous counts 

 was negligible for otoliths with less than 15 opaque zones, 

 such disagreement increased to ca. 10% for otoliths with 

 15-25 opaque zones and ca. 30% for those with more than 

 25 opaque zones. When a third count was necessary and 

 was not the same as either of the two previous counts, fur- 

 ther counts were made until successive counts did not dif- 

 fer by more than two opaque zones. On such occasions, the 

 final count was recorded. 



An independent reader counted the number of opaque 

 zones on 110 otoliths from a wide size range offish. Eighty 

 four percent of the counts of the number of opaque zones 

 made by this independent reader were the same as those 

 of the senior author for 50 sectioned otoliths that had been 

 judged by the senior author to have up to 10 such zones 

 and, in those cases where there were discrepancies, the dif- 

 ferences were never more than one opaque zone. Eighty 

 percent of the counts made by the independent reader of 

 the number of opaque zones on 50 sectioned otoliths re- 

 corded as possessing between 11 and 25 such zones by the 

 senior author were the same or differed by only one from 

 those of the senior author and, where there were discrepan- 

 cies, these rarely exceeded three opaque zones. In the case 

 of ten otoliths with >25 opaque zones, the maximum dis- 



