785 



Abstract— Af^e compositions and growth 

 rales liave been determined for popu- 

 lations of Acanthopagrus butcheri in four 

 estuaries and a saline, coastal lake, all 

 of which differ markedly in their mor- 

 phological, physicochemical. and biotic 

 characteristics. Because the opaque 

 zones in otoliths were shown to form 

 annually, the number of these zones 

 could be used to age individual fish. 

 However, the otoliths of fish that were 

 more than six years old had to be sec- 

 tioned in order to consistently reveal 

 all opaque zones. The number of annuli 

 on scales did not provide reliable esti- 

 mates of age. Acanthopagrus butcheri, 

 which typically completes its life cycle 

 in estuaries, was represented in each 

 of the five water bodies by fish >15 

 years old and lengths and weights >365 

 mm and >860 g. respectively. The maxi- 

 mum length and weight of A. butcheri 

 recorded in any of the five water bodies 

 were 485 mm and 2196 g. respectively. 

 The values for L. in von BertalanfTy 

 growth equations differed significantly 

 between females and males in three of 

 the four estuarine populations (P<0.001 

 or <0.01), whereas those for both k and 

 ty differed significantly between the 

 sexes in only one population and then 

 only at P < 0.05. The values for k and 

 L „ in the von Bertalanffy gi-owth equa- 

 tions differed significantly among both 

 females and males in the four estuaries 

 at either P < 0.001 or P < 0.05. These 

 parameters also differed significantly 

 between the males in Lake Clifton and 

 the males in each estuary, except the 

 Swan River Estuary. Growth rates in 

 two of the more northern water bodies 

 were greater than those in the two 

 southern and cooler estuaries. The pat- 

 tern of growth in the Moore River Estu- 

 ary, as reflected by changes in length 

 with time, differed from that in the 

 other four water bodies in that it was 

 initially slower and subsequently did 

 not show such a marked tendency to 

 form an asymptote. The slow initial 

 rate of increase in length in the Moore 

 River Estuary may be related to par- 

 ticularly high densities of juvenile A. 

 butcheri in nearshore, shallow water, as 

 well as to a relatively lower abundance 

 of appropriate food or very low salini- 

 ties, or to both of the latter The per- 

 centage contribution made by fish >5 

 years was the lowest by far in the Swan 

 River Estuary, which was subjected to 

 the greatest fishing pressure. 



Manuscript accepted 22 March 2000. 

 Fish. Bull. 98:785-799 (2000). 



Variation in age compositions and growth rates 



of Acanthopagrus butcheri (Sparidae) 



among estuaries: some possible contributing factors 



Gavin A. Sarre 



Ian C. Potter 



School ol Biological Sciences and Biotechnology 



Murdoch University 



Western Australia 6150, Australia 



E mail address (for G A Sarre) sarreig'central murdoch edu au 



The black hream, Acatithopcignis butch- 

 eri, which completes its hfe cycle within 

 estuaries (Potter and Hyncies, 1999; 

 Sarre and Potter, 1999) is one of the 

 most important recreational and com- 

 mercial fish species in the estuaries 

 of southern Austraha (Lenanton and 

 Potter, 1987; Kailola et al., 1993). The 

 fact that the genetic compositions of the 

 populations of this species in the differ- 

 ent estuaries of southwestern Australia 

 vary, suggests that, although some A. 

 butcheri are occasionally flushed out of 

 estuaries during those winters when 

 freshwater discharge is particularly 

 heavy, the population in an estuary 

 remains essentially discrete from those 

 in other estuaries (Chaplin et al., 1998; 

 Potter and Hyndes, 1999). The confine- 

 ment of each population of A. butcheri 

 to its natal environment means that, if 

 fishing pressure is sufficiently high in 

 any one estuary, the population in that 

 estuary cannot be replenished natu- 

 rally by immigration from outside that 

 system. Indeed, there is good evidence 

 that the abundance of this sparid in 

 the Blackwood River Estuary in south- 

 western Australia declined markedly 

 between the 1970s and 1990s as a 

 result of a combination of commercial 

 and recreational fishing activities (see 

 Valesini et al., 1997; Lenanton et al., 

 1999; Lenanton';Valesini-). 



The increasing potential for A. butch- 

 eri to become overexploited as recre- 

 ational fishing in estuaries increases 

 means that it is now important to have 

 information on the age compositions of 

 this species in the various estuaries in 

 order to ascertain whether the older 

 age classes are becoming excessively 

 depleted in some of these estuaries. 



Such data are dependent on accurate 

 estimates of the age of fish. In the 

 past, such estimates for A. butcheri have 

 typically been based on the number 

 of annuli on scales (Butcher, 1945; 

 Thomson, 1957; Weng, 1971; Hobday 

 and Moran-^). However, no attempt was 

 made in any of these studies to use 

 traditional methods to validate that 

 the gi'owth zones (annuli) on that hard 

 structure are formed annually — a proce- 

 dure now considered essential in aging 

 fish (Beamish and McFarlane, 1983). 

 Although Morison et al. (1998) have 

 recently used the number of opaque 

 zones on otoliths as a criterion of age, 

 their approach to validating that those 

 zones were formed annually was based 

 on the obsei-vation that the number of 

 opaque zones on the otoliths offish in the 

 two cohorts that were the most strongly 

 represented in length-frequency data 

 for four consecutive years increased by 

 one in each successive year. 



Recent work on A. butcheri in south- 

 western Australia has focused on pop- 

 ulations in four estuaries and a land- 

 locked saline, coastal lake, which vary 



' Lenanton, R. C. J. 1977. Aspects of the 

 ecology of fish and commercial crusta- 

 ceans of the Blackwood River Estuary, 

 Western Australia. Report 19, Depart- 

 ment of Fisheries and Wildlife. Perth, West- 

 ern Australia, Australia, 72 p. 



- Valesini, F. J. 1995. Characteristics of 

 the ichthyofaunas of the Blackwood River 

 Estuary and Flinders Bay. L^npublished 

 honours thesis, Murdoch Liniversity. Perth, 

 Western Australia. Australia, 67 p. 



3 Hobday, D., and Moran, M. 1983. Age, 

 growth and fluctuating year class strength 

 of black bream in the Gippsland Lakes, 

 Victoria. Report No. 20. Marine Science 

 Laboratories, Victoria, Australia, 17 p. 



