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The fish faunas of south coast estuaries: Volume 1

Citation

Chuwen, BM and Hoeksema, SD and Hesp, SA and Hall, NG and Potter, IC, The fish faunas of south coast estuaries: Volume 1, South Coast Natural Resource Management Inc., 04SC1-05e (2009) [Contract Report]


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Abstract

The estuaries on the south coast of Western Australia are either permanently open, or, through the formation of sand bars at their mouths, are only open to the ocean when the volume of riverine discharge is sufficient to cause the prominent sand bars at their mouths to be breached, as occurs seasonally in some estuaries (seasonally open) and infrequently in others (normally closed). The fish faunas of the nearshore, shallow waters and the offshore, deeper waters of estuaries that encompassed each of those types, i.e. the permanently-open Oyster Harbour, the seasonally open Broke, Irwin and Wilson inlets and the normally-closed Wellstead Estuary and Stokes Inlet, were sampled each season for at least two years between summer 2002 and spring 2007. Environmental data were collected on each sampling occasion and additional seasonal measurements were taken in the normally closed Hamersley and Culham inlets between summer 2002 and spring 2004. Fish in nearshore, shallow waters were caught by seine netting, while those in offshore, deeper waters were obtained by gill netting. Sampling was undertaken in the basin and lower part of the main tributary of each of the estuaries and in three isolated upstream pools in the main tributary of Stokes Inlet. Salinities in the basins ranged from hyposaline in the seasonally-open Wilson Inlet to approximately that of sea water in the permanently-open Oyster Harbour to nearly 300 at the end of a protracted period of low rainfall and high evaporation in the normally-closed and shallow Culham Inlet. The extent of cyclical seasonal fluctuations in environmental conditions differed markedly among estuaries. This reflected differences in the relationship between the volume of fluvial discharge, which is determined by a combination of the amount of local rainfall, catchment size and degree to which native vegetation has been cleared, and the amount of marine intrusion, which is largely controlled by the size and duration of the opening of the estuary mouth. Pronounced haloclines and oxyclines were usually present in the riverine but not basin regions of seasonally-open and permanently-open estuaries. The use of multivariate statistics emphasised that the environmental characteristics of even the three seasonally open and three normally closed estuaries differed markedly. 3 Irrespective of the frequency, size and duration of the opening of the estuary mouth, the fish fauna in the nearshore, shallow waters of the basin of each estuary was numerically dominated by three atherinid species and three gobiid species (92.9-99.7%), each of which completes its life cycle within these estuaries. However, the ichthyofaunal compositions in the nearshore, shallow waters of the basins of the five estuaries were significantly different, reflecting differences in the relative abundances of the above six species and, to a far lesser extent, of certain marine species. The contribution of marine species was greatest in the permanently-open estuary and least in the normally-closed estuary. The species richness in nearshore, shallow waters was greatest in summer and least in winter in each estuary and differed markedly between years only in Wilson Inlet. The density of fishes was greatest in the most eutrophic estuary (Wellstead Estuary) and least in the most oligotrophic estuary (Broke Inlet) and only underwent marked seasonal variations in Wilson Inlet and Wellstead Estuary, in which densities fell to their minima in winter. The composition of the fish fauna in the nearshore, shallow waters of the basin of both Oyster Harbour and Wellstead Estuary differed from that of its riverine region, which, unlike that of the other estuaries, contained shallow, nearshore areas conducive to seine netting. Ichthyofaunal composition varied between years in the Broke and Wilson inlets and Wellstead Estuary, in each of which there was little or no connection with the ocean in one of those years. Species composition underwent progressive seasonal changes throughout the year in Wilson Inlet and Wellstead Estuary, due largely to interspecific differences in patterns of recruitment. Gill netting seasonally for two years at sites in the basin and saline lower reaches of the main tributary of the seasonally-open Broke, Irwin and Wilson inlets, the permanently-open Oyster Harbour and the normally-closed Wellstead Estuary yielded 22,329 fishes representing 58 species. Overall, and irrespective of estuary type, the species compositions of the offshore, deeper waters in the basins and rivers differed markedly. This was attributable to consistently greater abundances of Mugil cephalus, and usually also of Acanthopagrus butcheri, in the rivers of each estuary and to the restriction of a range of species largely to the basins. However, the compositions in the basins of the five estuaries varied markedly, reflecting differences in the 4 extent and duration of the opening of the estuary mouth and/or whether extensive growths of macrophytes were present. Changes in the ichthyofaunal composition in the offshore deeper waters of the normallyclosed Wellstead Estuary between the first and second years of the study were attributable, in particular, to the movement of two mugilid species into offshore waters as they increased in size. Cyclical changes in ichthyofaunal composition were conspicuous in both regions of the estuary that underwent the most pronounced seasonal variations in environmental conditions. In each estuary, species richness was greater in the basin than river, where salinities were more variable and fell to lower levels and were thus less conducive to the immigration of most marine species. Catch rates were least in Broke Inlet, which had the lowest primary productivity, and were particularly high in Wellstead Estuary, which is highly eutrophic. The results of this study emphasise that ichthyofaunal composition can vary greatly with region (basin vs river) in microtidal estuaries, a finding that is of direct relevance to managers as these systems are becoming increasingly degraded and yet still constitute important nursery areas for certain fish species and often support recreational and commercial fisheries. The compositions of the fish faunas of offshore, deeper waters of the seasonally-open Wilson Inlet and normally-closed Wellstead Estuary in 2005-07 differed from those recorded in those estuaries in 1987-89 and 1996-98, respectively. In the case of Wilson Inlet, the fish fauna of 2005-07 was distinguished from that of 1987-89 by consistently lower abundances of Cnidoglanis macrocephalus, Platycephalus speculator and Sillaginodes punctata and consistently greater abundances of Arripis georgianus and Pelates sexlineatus. The possible roles of such factors as fishing, increased macrophyte growths and conditions in spawning areas in producing those differences are discussed. In Wellstead Estuary, the same three species (Acanthopagrus butcheri, Aldrichetta forsteri and Mugil cephalus) dominated the catches in both 2005-07 and 1996-98. The catch rates were, however, far lower in 1996-98, which is almost certainly due, in part, to the inhibiting influence of highly elevated salinities in the basin during that earlier period. 5 The catches of fishes obtained during this preliminary study of the upstream pools that form in the tributaries of normally-closed estuaries along the central south coast of Western Australia during protracted dry periods indicate that they play a role in maintaining the integrity of the ichthyofaunas of those estuaries. In particular, they could play a crucial role as refugia for the iconic Acanthopagrus butcheri when the salinities in the downstream regions of a system exceed those in which this species can survive.

Item Details

Item Type:Contract Report
Keywords:microtidal estuaries; estuary mouth closure; ichthyofaunal characteristics; region of estuary; temporal variability; life-cycle guilds; environmental characteristics; fluvial discharge marine intrusion; Western Australia, south coast
Research Division:Agricultural and Veterinary Sciences
Research Group:Fisheries Sciences
Research Field:Aquaculture
Objective Division:Animal Production and Animal Primary Products
Objective Group:Fisheries - Wild Caught
Objective Field:Fisheries - Recreational
Author:Chuwen, BM (Dr Ben Chuwen)
ID Code:71865
Year Published:2009
Deposited By:Sustainable Marine Research Collaboration
Deposited On:2011-08-11
Last Modified:2011-08-11
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