226 



Fishery Bulletin 100(2) 



Ralston, 1987) suggests that the precision of this estimate 

 of the instantaneous coefficient of natural mortality is like- 

 ly to be relatively low. It was therefore concluded that the 

 value for M derived from Ralston's equation represented 

 an overestimate. A more detailed examination of the catch 

 cun'e data suggested, but was unable to demonstrate con- 

 clusively, that the level of total mortality experienced by 

 the older fish when they were young was less than that 

 which is now being experienced by the population. Indeed, 

 if the decline in the estimated value for Z, displayed in Fig. 

 9, was extrapolated to an age of 40 years, the total mortal- 

 ity exhibited by the oldest age classes (when, as young fish, 

 they first became fully vulnerable to the fishery) would be 

 ca. 0.1/year. Such a value, which might be only slightly 

 greater than the natural mortality, matches the estimate 

 of Z calculated from the obsei-ved maximum age with Hoe- 

 nig's ( 1983) regi-ession equation. However, such agi'eement 

 may be fortuitous because the latter estimate should rep- 

 resent the total mortality experienced by the fish within 

 the sample, i.e. from age 9, rather than just the mortality 

 of the older fish. Nevertheless, if the level of natural mor- 

 tality, M, is ca. 0. l/year and the average level of instanta- 

 neous coefficient of total mortality, Z, from age 9 years is 

 ca. 0.21/year, the current level of fishing mortality, F. would 

 exceed 0.1 l/year. 



In the nearshore waters along the lower west coast of 

 Australia where this species is most heavily fished, the 

 abundance of G. hebraicum has declined to a level that 

 is of concern to fishermen. Numerous anecdotal reports 

 indicate that commercial and dedicated recreational fish- 

 ermen, such as those who provided the samples for this 

 study, now tend to move further offshore in order to obtain 

 catches of G. hebraicum comparable with those they used 

 to obtain in waters closer to the coast. However, many rec- 

 reational fishermen still continue to fish for G. hebraicum 

 (and other species) in the traditional areas where dhufish 

 were fished in the past. The expansion of the fishery for 

 dhufish to include waters farther offshore, allied with the 

 increasing use of global positioning systems (GPSs) to im- 

 prove fishing efficiency, is increasing the level of exploita- 

 tion of the stock as a whole. 



The fact that there are indications that the fishing-in- 

 duced mortality of dhufish may now exceed natural mor- 

 tality and that ongoing expansion in the extent to which 

 fishermen are moving offshore (and also, in the case of rec- 

 reational fishermen, in a northwards and southwards di- 

 rection from the main metropolitan region of Perth) will 

 further increase fishing pressure, is of concern to the man- 

 agers responsible for the fishery for G. hebraicum. How- 

 ever, because our sampling regime was not designed spe- 

 cifically at determining the levels of fishing mortality to 

 which G. hebraicum is being subjected, there is clearly a 

 need to undertake a study in which the main aim is to 

 achieve this objective. If such research were to confirm our 

 preliminary findings that fishing mortality is reaching an 

 unacceptable level, there will be an urgent need to use the 

 biological data produced during the current study to refine 

 the management plans designed to consei-ve this species. 



Female and male G. hebraicum first roach sexual matu- 

 rity at the end of their third year of life when they are just 



over 300 mm in length and they reach 500 mm, the MLL 

 for capture, when they are about 7 and 6 years old, respec- 

 tively. Thus, on average, the female and male dhufish that 

 live until they reach the MLL will have had the opportu- 

 nity to have spawned for four and three years, respectively, 

 before they can legally be retained following capture. On- 

 going research at the state fisheries laboratory in Western 

 Australia has indicated that ca. 50'^'i of fish caught in wa- 

 ters of 20-30 m depth die on being released back into the 

 water and that this percentage increases to ca. 95'r for 

 fish brought to the surface from depths greater than 40 m 

 (Moran-). Thus, the use of a MLL is likely to be of only lim- 

 ited value for consei-ving this species as fishing effort con- 

 tinues to increase. It is therefore important to introduce 

 measures that will conserve G. hebraicum by maintaining 

 the catches of this species at a level consistent with the re- 

 quirements for ecological sustainability. Examples of such 

 management controls might include closing areas to com- 

 mercial and recreational fishing ( particularly those around 

 reefs that are especially heavily fished) introducing quo- 

 tas for commercial fish catches, making adjustments to the 

 number of commercial licenses, further restricting the bag 

 limit for recreational fishermen, and limiting the number 

 of recreational fishermen that can fish in a given area. 

 Furthermore, because the Fremantle Maritime Centre has 

 successfully cultured G. hebraicum (Cleary et al.-^), there is 

 also now the potential for restocking this species in areas 

 in which it has become severely depleted. 



Acknowledgments 



Gratitude is expressed to those recreational and commer- 

 cial fishermen and fish processors and many friends and 

 colleagues for their help in the collection of fish and to 

 colleagues at the Centre for Fish and Fisheries Research 

 at Murdoch University for their help and advice. Grati- 

 tude is also expressed to Gavin Sarre for providing inde- 

 pendent counts of the number of translucent zones on 

 otoliths and to two anonymous referees for their construc- 

 tive comments on the original text. Funding was provided 

 by the Fisheries Research and Development Corporation 

 (FRDC). 



Literature cited 



Beamish, R. J. 



1979. Differences in the age of Pacific hake iMcrluccius pro- 

 ductus) using whole and sections of otoliths, J. Fish. Res. 

 Board Can. 36:141-151. 



- Moran, M. 2001. Personal commun. Western Australian Ma- 

 rine Research Laboratory, Fisheries Western Australia. PO Box 

 20, North Beach 6020, Western Australia. 



^ Cleary, J. J., G. I. Jenkins, and G. Partridge. 1999. Prelimi- 

 nary manual for the hatchery production of WA dhufish tGlauco- 

 soina hebraicum ). Interim report to FRDC ( Fisheries Research 

 and Development Corp.), 30 June 1999. (Project 96/308), 36 p. 

 Fremantle Maritime Centre. 1 Fleet St.. Fremantle. Western 

 Australia. 6160. 



