796 



Fishery Bulletin 98(4) 



gill net sampling regimes, supplemented with a limited 

 amount of angling, any gross differences in the age struc- 

 ture of samples from populations in the different estuaries 

 almost certainly represent real differences. The percentage 

 of A. butcheri caught at >5 years of age in the Swan River 

 Estuary (5%) was far lower than in either the Moore River 

 Estuary (30%), approximately 100 km farther north on the 

 lower west coast of Australia, or the Nornalup Walpole 

 Estuary (45%) on the south coast of Australia. Note that 

 the estimate for the Swan River Estuary was restricted 

 to data collected during the main sampling period and 

 did not thus include the large samples of small fish that 

 were caught between December 1998 and March 1999. 

 The above differences in the proportion of older fish pre- 

 sumably reflect a greater "mortality" of older fish in the 

 Swan River Estuary than in the other estuaries. It thus 



appears highly relevant that the population of A. butcheri 

 in the Swan River Estuary is exposed to heavy fishing 

 pressure from the recreational sector throughout the year 

 and from commercial fishermen during winter and early 

 spring, whereas the population in the Moore River Estu- 

 ary is lightly fished and that in the Nornalup Walpole 

 Estuary is not exposed to commercial fishing (Sarre and 

 Potter, 1999). 



Although representatives of all age classes up to 15+ 

 were recorded for the populations of A. butcheri in each 

 of the above three estuaries, this was not the case with 

 the Wellstead Estuary, which is located 300 km to the east 

 of the Nornalup Walpole Estuary. Thus, in the samples 

 collected from this estuary in 1995 and 1996, the 1989, 

 1987, 1986, 1985, and 1984 year classes were not repre- 

 sented, and the 1988 year class was represented by only 

 two fish (Fig. 6). This strong implication that, 

 in the Wellstead Estuary, A. butcheri either does 

 not spawn or has very limited spawning success 

 in some years parallels the situation recorded 

 for this species in the Gippsland Lakes in east- 

 ern Australia (Morison et al.. 1998; Hobday and 

 Moran^ Coutin et al.^). The work of Morison 

 et al. (1998) demonstrated that, in that latter 

 estuary, the commercial catches of A. butcheri 

 between 1993 and 1996 were dominated by 

 two year classes and that there had been no 

 strong recruitment of A. butcheri since 1989. 

 The absence or weakness of certain year classes 

 in the Gippsland Lakes, and also in the Hopkins 

 River Estuary which is also in eastern Aus- 

 tralia, has been attributed to the detrimental 

 inflLience on spawning success of such unfavour- 

 able environmental conditions as heavy freshwa- 

 ter discharge or unsuitable salinities i Newton, 

 1996; Hobday and Moran'). Thus, in the context 

 of the absence of the 1984 year class in sam- 

 ples, it appears relevant that, in the Wellstead 

 Estuary, which has normally remained closed 

 during the last 30 years, there was, as a result 

 of "cyclonic rainfall,"" an extremely protracted 

 period of heavy freshwater discharge between 

 the early spring of 1984 and the autumn of 1985 

 (Hodgkin and Clark^'). This led to a very severe 

 scouring of the substrate and a breaching of the 

 bar at the mouth of this estuary, with the result 

 that this mouth remained open between Septem- 

 ber 1984 and May 1985. The heavy freshwater 

 discharge that occurred during and immediately 

 after the 1985 spawning season of A. butcheri 

 in the Wellstead Estuary would thus almost cer- 



' Coutin, P., S. Walker, and A. Mori.son. 1997. 

 Black bream— 1996. Compiled by the Bay & Inlet 

 Fisheries and Stock Assessment Group. Fisheries 

 Victoria Assessment Report 14. Melbourne. Victoria. 

 Australia, 89 p. 



Hodgkin. E. P., and R. Clark. 1987. Wellstead 

 Kstuary, the estuary of the Bremer River Estuarine 

 Study Series 1. Environmental Protection Authority. 

 Perth, Western Australia, 22 p. 



