786 



Fishery Bulletin 98(4) 



markedly in their morphological and physicochemical 

 characteristics and in the composition of their biota (Sarre 

 et al., 2000). Thus, two of these water bodies are perma- 

 nently open to the sea, while one is intermittently closed, 

 another is normally closed, and one is permanently closed. 

 Furthermore, the regions where A. buicheri spawns in 

 these estuaries in the spring and early autumn range in 

 salinity from as low as 3.5-8.0'/« in the intermittently 

 open estuary to over 40%r in the normally closed estuary 

 and, as a result of their location at different latitudes, they 

 also differ in water temperature (Young et al., 1997; Sarre 

 and Potter, 1999). The differences in the biota of these sys- 

 tems are reflected in marked differences in the dietary 

 composition of A. butcheri, with, for example, the overall 

 contribution made by macrophytes to the volume of stom- 

 ach contents ranging from as low as 8.3')} in one popula- 

 tion to as high as 56.49f in another (Sarre et al., 2000). 



The aims of our study on A. butcheri were as follows: 

 1) to validate that the growth (opaque) zones visible on 

 sectioned otoliths of A. butcheri are formed annually; 2) 

 to compare the number of growth (opaque) zones in oto- 

 liths prior to and after sectioning in order to determine 

 whether otoliths always have to be sectioned to reveal 

 each of their opaque zones; 3) to ascertain whether the 

 number of annuli on scales corresponds to the number of 

 opaque zones on sectioned otoliths and can thus likewise 

 be used for aging this species; 4) to determine the age com- 

 position of A. butcheri in the above four estuaries and the 

 saline, coastal lake, in which the fishing pressure on black 

 bream varies from zero to substantial, and thereby ascer- 

 tain whether there is evidence that heavy exploitation of 

 this species can markedly reduce the proportion of older 

 fish; and 5) to determine the extent to which the growth 

 rates and length at age of A. butcheri differ amongst popu- 

 lations in the above five water bodies, which vary mark- 

 edly in their abiotic and biotic characteristics and amongst 

 which the dietary compositions of A. butcheri are signifi- 

 cantly different. 



Materials and methods 



Acanthopagrus butcheri was collected from the permanently 

 open Swan River and intermittently open Moore River 

 estuaries on the lower west coast of Australia (31-32°Si 

 and from the permanently open Nornalup Walpole and nor- 

 mally closed Wellstead estuaries (34-35°S) on the southern 

 coast of Western Australia (see Fig. 1 for locations of these 

 estuaries). Aco/;//!opa^'n/,s butcheri was also collected from 

 Lake Clifton, a landlocked saline, coastal lake. Because 

 the Department of Conservation and Land Management 

 (CALM) restricted the number of A. butcheri that could be 

 collected from this lake to 100, and because 85 of these 100 

 fish were males, emphasis was placed on the data obtained 

 for this sex in this lacustrine environment. 



Fish in estuaries were collected from over sand in near- 

 shore, shallow waters (<1.5 m depth) by using a 41-m 

 seine net with 9-mm mesh in the codend and from off- 

 shore, deeper waters (2-,5 m depth) by employing compos- 

 ite sunken gill nets containing eight 20 m long x 2 m high 



panels, each of which had a different mesh size, i.e. 38, 51, 

 63, 76, 89, 102, 115, or 127 mm. Sampling in the Swan 

 River Estuary commenced in September 1993 and was car- 

 ried out monthly until April 1995 with seine netting and 

 monthly until February 1995 with gill netting. The same 

 methods were used to sample A. butcheri in the Moore 

 River, Nornalup Walpole, and Wellstead estuaries between 

 the spring of 1993 and the summer of 1996-97 (December- 

 January). Sampling in the Swan River, Nornalup Walpole, 

 and Wellstead estuaries was carried out in the saline lower 

 reaches of the tributary rivers, i.e. upper estuary (Fig. 1), 

 which, for most of the year, contain the majority of the A. 

 butcheri found in those estuaries (Sarre and Potter, 1999). 

 In contrast, sampling was undertaken throughout the short 

 Moore River Estuary, which does not possess the large cen- 

 tral basins that are found in the other three estuaries (Fig. 

 1). The catches obtained by seine and gill netting in the 

 above four estuaries were supplemented by up to a further 

 1% by fish obtained with rod and line. A fine mesh seine 

 net, which was 5.5 m long and consisted of 1-mm mesh, was 

 used to collect small A. butcheri from extensive beds of the 

 macroalgae Gracilaria verrucosa in the downstream and 

 middle regions of the upper Swan River Estuary between 

 December 1998 and March 1999. (See "Results" section for 

 the reason for this additional sampling). The sample of 100 

 A. butcheri collected from Lake Clifton in November 1996 

 was obtained exclusively by rod and line, in accordance 

 with the conditions laid down by CALM. 



Both of the sagittal otoliths were removed from each 

 fish sampled from the four estuaries and Lake Clifton and 

 these otoliths were immersed in methyl salicylate solu- 

 tion. For sectioning, otoliths were mounted and embedded 

 in clear epoxy resin and cut into ca 0.5-mm transverse 

 sections with an Isomet low-speed diamond saw. Sections 

 were gi'ound on sequentially finer grades of carborundum 

 paper (400-1200 grade) and mounted on glass slides with 

 DePX mounting adhesive. Wliole otoliths and sectioned 

 otoliths were placed on a black surface and examined 

 microscopically under reflected light. 



Measurements were made of the distance between the 

 outer edge of the outermost opaque zone and the periphery 

 of the otolith in the case of the otoliths that were to be used 

 for aging fish in the Swan River Estuary This distance, i.e. 

 the marginal increment, was then expressed either as a 

 proportion of the distance between the primordium of the 

 otolith and the outer edge of the opaque zone, when only 

 one opaque zone was present, or as a proportion of the dis- 

 tance between the outer edge of the two outermost opaque 

 zones, when two or more opaque zones were present. All 

 measurements were recorded to the nearest 0.05 mm. As 

 with other sparids, a narrow opaque zone is laid down in 

 the otoliths of A. butcheri during the cool (winter) period 

 and a wide translucent zone is deposited during the warm 

 (summer) period (Johnson, 1983; Buxton and Clarke, 1991; 

 Francis et al., 1992; Booth and Buxton, 1997). 



Otoliths from 239 A. butcheri. collected from the Swan 

 River Estuary and covering a wide size range, were used 

 for comparing the number of narrow, opaque zones that 

 could be seen on this hard structure before and after sec- 

 tioning. The number of opaque zones visible in a subsam- 



