214 



Abstract— Samples of the commer- 

 cially and recreationally important West 

 Australian dhufish iGlaucosoma Iwbra- 

 icum) were obtained from the lower 

 west coast of Australia by a variety of 

 methods. Fish <300 mm TL were caught 

 over flat, hard substrata and low-lying 

 limestone reefs, whereas larger fish 

 were caught over larger limestone and 

 coral reef formations. Maximum total 

 lengths, weights, and ages were 981 

 mm, 15.3 kg, and 39 years, respectively, 

 for females and 1120 mm, 23.2 kg, 

 and 41 years, respectively, for males. 

 The von Bertalanffy growth curves for 

 females and males were significantly 

 different. The values for L^, k, and t„ in 

 the von Bertalanffy growth equations 

 were 929 mm, 0. Ill/year, and -0.141 

 years, respectively, for females, and 

 1025 mm, 0.111/year, and -0.052 years, 

 respectively, for males. Preliminary 

 estimates of total mortality indicated 

 that G. hebraicum is now subjected 

 to a level of fishing pressure that 

 must be of concern to fishery man- 

 agers. Glaucosoma hebraicum, which 

 spawns between November and April 

 and predominantly between December 

 and March, breeds at a wide range 

 of depths and is a multiple spawner 

 The /.^(I's for females and males at first 

 maturity, i.e. 301 and 320 mm, respec- 

 tively, were attained by about the end 

 of the third year of life and are well 

 below the minimum legal length (MLL) 

 of 500 mm. Because females and males 

 did not reach the MLL until the end 

 of their seventh and sixth years of life, 

 respectively, they would have had. on 

 average, the opportunity of spawning 

 during four and three spawning sea- 

 sons, respectively, before they reached 

 the MLL. However, because G. hebra- 

 icum caught in water depths >40 m 

 typically die upon release, a MLL is 

 of limited use for consei-\'ing this spe- 

 cies. Alternative approaches, such as 

 restricting fishing activity in highly 

 fished areas, reducing daily bag limits 

 for recreational fishermen, introducing 

 quotas or revising specific details of cer- 

 tain commercial hand-line licences (or 

 doing both) are more likely to provide 

 effective conservation measures. 



Age and size composition, growth rate, reproductive 

 biology, and habitats of the West Australian dhufish 

 (Glaucosoma hebraicum) and their relevance to the 

 management of this species 



Sybrand A. Hesp 



Ian C. Potter 



Norman G. Hall 



Centre for Fish and Fisheries Research 



School ol Biological Sciences and Biotechnology 



Murdoch University 



South Street 



Murdoch, Western Australia 6150, Australia 



Email address (for I C Potter, contact author) i-potlenSpossum murdocti edu au 



Manuscript accepted 3 October 2001. 

 Fish. Bull. 100:214-227 (2002). 



The West Australian dhufish (Glauco- 

 soma hebraicum ), also known as the 

 Westrahan jewfish (McKay, 1997), is 

 one of the most commercially valuable 

 and recreationally sought after finfish 

 in Western Australia ( Sudmeyer et al. ' ). 

 This species is confined to southwest- 

 ern Australia, where its distribution 

 ranges southwards from Shark Bay at 

 26°00'S, 113°00'E down the west coast 

 and then eastwards along the south 

 coast to the Recherche Aj-chipelago at 

 34°10'S, 122n5'E(HutchinsandThomp- 

 son. 1995). Glaucosoma hebraicum is 

 one of four members of the monogene- 

 ric family Glaucosomatidae, which also 

 includes the pearl perch (Glaucosoma 

 scapulare) that is fished commercially 

 and recreationally in eastern Australia 

 (McKay. 1997). 



Despite the high quality of the flesh of 

 Glaucosoma species and the commercial 

 and recreational importance of G. hebra- 

 icum in particular, there are no refer- 

 eed papers on the biology of any species 

 in this genus. Furthermore, the results 

 of undergi-aduate studies on the biology 

 of G. hebraicum, which were collated by 

 Sudmeyer et al.,' included estimates of 

 age that were based on the number of 

 growth zones in whole otoliths, an ap- 

 proach that would almost certainly have 

 underestimated the age of many older 

 fish (see "Results" section). 



Numerous commercial and recre- 

 ational fishermen report that they now 

 fish much farther offshore in order to 

 obtain catches of dhufish comparable 

 with those they had previously been 



able to obtain from nearer the coast. 

 This consistent circumstantial evidence 

 strongly suggests that the abundance 

 of G hebraicum in more inshore waters 

 has declined in recent years and that 

 this is particularly the case in areas 

 near the city of Perth where this spe- 

 cies has been targeted by recreational 

 fishing crews. The indications that the 

 abundance of dhufish in nearshore wa- 

 ters was declining led the Australian 

 Fisheries Research and Development 

 Corporation to fund the current study, 

 with a view to producing biological da- 

 ta for managing G hebraicum . 



During the present study, we first 

 determined whether the otoliths of G 

 hebraicum had to be sectioned to de- 

 tect all of their opaque zones and we 

 then validated that these opaque zones 

 are formed annually and could thus be 

 used to age this species. As accurate 

 estimates of the age of a fish are de- 

 pendent on a reliable birth date, the 

 trends exhibited by reproductive vari- 

 ables were used to estimate the dura- 

 tion of the spawning period and, in par- 

 ticular, when spawning activity peaked. 

 The age of each fish was then deter- 

 mined and the resulting length-at-age 



Sudmeyer, J. E.. D. A. Hancock, and R. C. J. 

 Lenanton. 1992. Synopsis of Westralian 

 jewfish iGlaucosoma hebraicum^ (Richard- 

 son, 1845 1 (Pisces: Glaucosomatidae). Fish- 

 eries Research Report 96, Western Austra- 

 lian Marine Research Laboratory, Fisheries 

 Western Australia, PO Box 20, North Beach 

 6020, Western Australia. 



