Hesp et al.: Age and size composition, growth rate, reproductive biology, and habitats of Glaucosoma hebmicum 



223 



cohort, as they increase in size, from a hab- 

 itat that could be trawled to one where 

 reefs occur and where it was not possible to 

 trawl. This conclusion is supported by the 

 fact that the few dhufish of 150-300 mm 

 that were caught were collected from low- 

 lying reefs, i.e. reefs that contained rock 

 ledges up to 30 cm in height. In contrast, 

 G. hebraiciim >300 mm typically occupy 

 areas where there are substantial lime- 

 stone and coral reef formations and their 

 large size would make them less suscepti- 

 ble to predation in a habitat where large 

 predatory species, such as the Samson 

 fish iScriola hippos) and the pink snapper 

 {Pagrus auratus ) are found (Hesp, personal 

 obs.). 



Aging 



The trends exhibited by the marginal 

 increments on sectioned otoliths of G. 

 heb?-aiciim show that an opaque zone is 

 formed annually in the otoliths of this spe- 

 cies. However, comparisons between the 

 number of opaque zones on individual oto- 

 liths prior to and after sectioning demon- 

 strate that, after this species has reached 

 six years in age, one or more of these 

 zones often become visible only after the 

 otolith has been sectioned. This demon- 

 strates that earlier estimates of the age 

 of older G. hebraiciim, which were based 

 on the number of opaque zones visible in 

 whole otoliths (Sudemeyer et al.M, were 

 almost certainly often too low. 



An inability to detect all of the opaque 

 zones in the whole otoliths of older fish is 

 largely attributable to the fact that, as the 

 otolith increases in width, it becomes in- 

 creasingly difficult to distinguish between 

 the zones at the periphery of the otolith. 

 This problem parallels the situation re- 

 corded for several other medium-size to 

 large teleosts, such as Pacific hake (Mer- 

 luccius productus) (Beamish. 1979), starry flounder iPlaty- 

 ichthys stellatiis) (Campana, 1984) and blue-spotted flat- 

 head (Platycephalus speculator) (Hyndes et al., 1992). Our 

 results demonstrate that, although most G. hebraiciim are 

 less than 25 years old, some females and males live for 

 longer than 30 years and very occasionally for up to about 

 40 years. 



Other species that are typically caught by commercial 

 and recreational rod and hand-line fishermen when fish- 

 ing for dhufish include pink snapper iPagrus auratus), 

 Sampson fish (Seriola hipposK silver trevally iPseudo- 

 caranxdentexK breaksea cod iEpincphelides armatus). and 

 occasionally also King George whiting (Sillaginndes punc- 

 tata). The maximum total lengths and weights recorded 

 for these five species are 1300 mm and 19.5 kg for pink 



snapper, 1753 mm and 53.6 kg for Samson fish, 938 mm 

 and 10.0 kg for silver trevally, 550 mm and 2.9 kg for 

 breaksea cod, and 690 mm and 4.8 kg for King George 

 whiting, compared with 1219 mm and 25.8 kg for dhufish 

 (Hutchins and Thompson 1995). 



Although reliable data have been obtained for the age 

 and growth of a number of commercial and recreational fish 

 species that live in nearshore coastal or estuarine waters 

 in southwestern Australia (e.g. Chubb et al., 1981; Hyndes 

 et al., 1992, 1996; Hyndes and Potter, 1996, 1997; Lauren- 

 son et al, 1994; Fairclough et al., 2000; Sarre and Potter, 

 2000), comparable data for those species that are found in 

 and around reefs in deeper waters in southwestern Aus- 

 tralia are restricted to those recorded for G. hebraicum in 

 this paper and for the King George whiting Sillagitiodes 



