798 



Fishery Bulletin 98(4) 



ing waters of the other three estuaries, can be gauged from 

 the fact that, during summer, the densities in such waters 

 sometimes reached 234 fish per 250 m^ in the Moore River 

 Estuary, whereas they never exceeded 50 fish per 250 m^ in 

 any of the other three estuaries. Because A. butcheri tends 

 to move into offshore and deeper waters as it increases in 

 size, as is the case with several other fish species in south- 

 western Austrahan estuaries (Chubb et al., 1981; Chrystal 

 et al., 1985; Potter et al., 1988; Wise et al., 1994), it then 

 becomes more widely dispersed. 



Although the high densities of juvenile A. butcheri in 

 nearshore, shallow waters of the Moore River Estuary 

 could have contributed to the initially slow rate at which 

 length increased early in life, it also seems possible that 

 the low salinities, i.e. generally <7%o (Young et al., 1997), 

 and quality of food in this estuary may also have had an 

 inhibiting influence on the rate at which length increased. 

 The view that low salinity has had such an effect is based 

 on a combination of the results of detailed laboratory 

 trials, which demonstrated that A. butcheri did not gi-ow 

 as well at 0%^ and 12%f as at 24%r (Sarre et al.'') and the 

 fact that the upstream regions of other estuaries, where 

 juvenile A. butcheri are located between late spring and 

 early autumn when most growth occurs, are characterized 

 by elevated salinities (Potter and Hyndes, 1994; Sarre, 

 1999). Although low salinities may restrict growth, it is 

 worth noting that gi'owth in the Wellstead Estuary, which 

 was the only estuary to become hypersaline, was greater 

 than in the Nornalup Walpole Estuary, which is likewise 

 located on the south coast of Western Australia. However, 

 as mentioned earlier, growth in the Wellstead Estuary was 

 not as great as in the Swan River Estuary and Lake Clif- 

 ton farther to the north. 



In the context of potential food, it may be relevant that 

 juvenile A. butcheri feed to a greater extent on algae 

 in the Moore River Estuary than in other estuaries. As 

 pointed out by Blaber (1974) during his study of another 

 sparid, Rhabdosargus holubi, which likewise ingests a 

 large amount of algal material, the volume of digestible 

 material consumed is small. However, as A. butcheri 

 increased in size in the Moore River Estuary, it fed to an 

 increasing extent on whole large bivalves (Sarre et al., 

 2000), a food source that has a particularly high energy 

 content (Whitfield, 1980). 



The age compositions recorded in this paper for A. butch- 

 eri in different water bodies have been combined with data 

 on reproductive biology to determine the lengths and ages 

 at which black bream typically reach maturity in these 

 systems, i.e. the Lr,„ and A,,,, (Sarre and Potter, 1999). The 

 resultant data showed that, amongst the estuarine popu- 

 lations, the Ar,|, for female A. butcheri was lowest in the 

 Swan River Estuary (2.2 years), in which the gi-owth was 

 greatest, and greatest in the Nornalup Walpole Estuary 



" Sarre, G. A., G. J. Partridge, R. C. J. Lcnanton, G. I. Jenkins, 

 and I. C. Potter. 1999. Elucidation of the characteristics of 

 inland fresh and saline water bodies that influence growth and 

 survival of black bream. Fisheries Research and Development 

 Corporation. Research Report. Project 97/309. Canberra, ACT, 

 Australia, 68 p. 



(4.3 years), in which early growth was relatively slow 

 and the asymptotic length the least. The minimum legal 

 length (MLL) for the capture of A. butcheri in southwest- 

 ern Australia, i.e. 250 mm, is attained as early as 2.7 years 

 in the Swan River Estuary and as late as 6.5 and 6.8 

 years in the Nornalup Walpole and Moore River estuar- 

 ies. Because the MLL is well above the L^^ for females and 

 males of A. butcheri at first maturity in each system, it 

 allows a substantial number of the members of each popu- 

 lation to reach maturity before they are likely to be caught 

 by either recreational or professional fishermen. However, 

 the relatively small contribution made by A. butcheri >5 

 years old in the heavily fished Swan River Estuary empha- 

 sizes the need to keep the upper part of that estuary closed 

 to commercial fishing and suggests that similar measures 

 may be necessary in other estuaries as they become more 

 heavily fished in the future. 



Acknowledgments 



We thank numerous people, particularly D. Mead-Hunter, 

 G. Richard, and D. Tiivel for help in collecting black bream, 

 and N. Hall and M. Platell for helpful comments on the 

 manuscript. Financial support was provided by the Aus- 

 tralian Fisheries Research and Development Corporation, 

 Fisheries Western Australia, and Murdoch University. 

 Gratitude is also expressed to two anonymous referees for 

 constructive criticism of our paper. 



Literature cited 



Beamish, R. J., and G. A. McFarlane. 



1983. The forgotten requirements for age validation in fish- 

 eries biology. Trans. Am. Fish. Soc. 1 12:735-743. 

 Blaber, S. J. M. 



1974. Field studies of the diet of Rhabdosargus holubi 

 (Pisces: Sparidae). J. Zool. iLond.) 173:407-417. 

 Booth. A. J., and C. D. Buxton. 



1997. The biology of the panga, Pterogymnus lanianus 

 (Teleostei: Sparidae), on the Agulhas Bank, South Africa. 

 Environ. Biol. Fish. 49:207-226. 



Butcher, A. D. 



194.5. The Gippsland Lakes bream fishery. Australian 

 Fisheries Newsletter 4:2-8. 

 Buxton, C. D., and J. R. Clarke. 



1991. The biology of the white musselcracker Sparodon 

 durbancnsis (Pisces: Sparidae! on the eastern cape coast. 

 South Africa. S. Afr J. Mar Sci. 10:28.5-296. 

 Cerrato, R. M. 



1990. Interpretable statistical tests for gi-owth comparisons 

 using parameters in the von Bertalanffy equation. Can. J. 

 Fish. Aquat. Sci. 47:1416-1426. 

 Chaplin, J. A., G. A. Baudain.s. H. S. Gill, R. McCulloch, and 

 I. C. Potter 



1998. Are assemblages of black bream LAcantliopagrus 

 butcheri ) in different estuaries genetically distinct'.' Int. J. 

 Salt Lake Res. 6:303-321. 



Chrystal, P. J., I. C Potter, N. R. Loneragan, and C. P Holt. 



1985. Age structure, growth rates, movement patterns and 

 feeding in an estuarine population of the cardinalfish 

 Apogon rueppellii. Mar Biol. 85:185-197. 



