The fish faunas of south coast estuaries: Volume 2

This study was undertaken to determine the key biological characteristics of Black Bream, Estuary Cobbler, King George Whiting, Australian Herring, Yelloweye Mullet and Sea Mullet in selected south coast estuaries. These estuaries were the permanently-open Oyster Harbour, the seasonally-open Broke, Irwin and Wilson inlets and the normally-closed Wellstead Estuary and Stokes Inlet. The data were used to discuss the management implications for the above species of differences in the environmental characteristics of these estuaries and, where appropriate, also the impact of fishing on those species. Finally, a proposal is made for the ways in which data for a suite of selected fish species could be used to monitor the health of south coast estuaries. On the basis of the number of annually-formed growth zones in their otoliths (ear bones), the maximum ages of Black Bream ranged downwards from 13 to 15 years in Wilson Inlet, Wellstead Estuary, Culham Inlet and Stokes Inlet, to 9 years in Oyster Harbour and only 5 years in Irwin Inlet and Hamersley Inlet. The growth of Black Bream varied markedly among estuaries, presumably reflecting differences in the density of this species, quality/quantity of food and/or salinity regime. The width of the annual growth zones in otoliths, which is considered to reflect the annual growth of fish, varied among years, particularly in Stokes Inlet. In that estuary, the growth zones were widest in years of relatively high rainfall and thus when salinities would not have been elevated. Those environmental conditions are thus regarded as optimal for the growth of Black Bream. The trends exhibited throughout the year by gonadal development demonstrate that Black Bream spawns mainly in spring in the estuaries on the south coast of Western Australia. The “average” lengths at maturity of Black Bream in the four estuaries, from which substantial numbers of animals could be obtained, were similar, with those for females ranging only from 146 to161 mm. Although no Black Bream matured at the end of their first year of life in those estuaries, the majority (73-100%) in each estuary did become mature at the end of their second year of life. The vast majority or all fish were mature by 200 mm, which is well below the minimum legal length (MLL) of 250 mm for the retention of this 3 species in Western Australia. Thus, all fish that survive to reach the MLL have the potential to reproduce before being legally retained. Recruitment of Black Bream varied markedly among years and estuaries. Recruitment in the seasonally-open Wilson Inlet was greatest in years of below average rainfall and thus also of relatively elevated salinities (but still below that of seawater) and reduction in stratification and associated deoxygenation of the bottom water layer in the rivers. Although massive mortalities of Black Bream in two of the normally-closed estuaries, i.e. Hamersley and Culham inlets, prevented comprehensive biological comparisons across the three estuaries of this type, the data were sufficient to indicate that recruitment in these estuaries is strong in years of relatively high rainfall and thus when salinities are below that of fullstrength seawater. There were indications that total annual mortality of Black Bream was greatest in estuaries with the highest fishing pressure. The results obtained during the present and past studies emphasise that Black Bream is highly plastic in its biological characteristics, particularly in terms of growth and reproduction, which helps account for the great success of this species in a wide range of estuarine environments. Like Black Bream, the Estuary Cobbler that is found in estuaries, completes the whole of its life cycle in these systems. The Estuary Cobbler was most abundant in Irwin and Wilson inlets, followed by Oyster Harbour and Wellstead Estuary, while only two individuals were caught in Stokes Inlet and none was taken in Broke Inlet. The abundance of Estuary Cobbler in the estuaries in which this species was relatively abundant was greatest in summer and least in winter. In Wilson Inlet, the abundance and length of Estuary Cobbler were greater in the area closed to commercial fishing at the seaward end of the estuary than in the area open to fishing, strongly indicating that the closed area is acting as a refugium for this species. Growth of Estuary Cobbler was greatest in Irwin Inlet and least in Oyster Harbour, presumably reflecting differences in environmental characteristics such as the amount, species composition and density of seagrass. The average length of female Estuary Cobbler at first maturity was greatest in Irwin Inlet and least in Wilson Inlet, reflecting differences in growth. Total mortality was very high in Wilson Inlet (1.44 year-1) and even more particularly in Irwin Inlet (2.35 year-1). 4 Reasonable recruitment of Estuary Cobbler occurred in each year in Irwin and Wilson inlets, the main estuaries in which this species is targeted commercially. The abundance of Estuary Cobbler in Wilson Inlet was far lower during the present study (2005-08) than in a directly comparable study in 1987-89. Furthermore, the age structure of the gill net catches has shifted from comprising predominantly 2+ and 3+ year old fish in 1987-89 to largely 2+ fish in 2005-08, with very few fish older than 3+ years of age being recorded during the more recent period. In addition, the growth of Estuary Cobbler was greater and the age at maturity less in 2005-8 and estimates of total mortality were far greater in Wilson Inlet in 2005-08 (1.44 year-1) than in 1987-89 (0.79 year-1). While exhibiting considerable inter-annual variability, annual commercial catches have not undergone any conspicuous increase or decline between the above two periods The maintenance of commercial catches at an approximately constant level despite the reduction in abundance, and particularly of those of older year classes, and in the length and age at maturity, together with increases in growth and total mortality, indicate that, between 1987-89 and 2005-08, the commercially important Estuary Cobbler in Wilson Inlet has become more heavily exploited in that system. In contrast to the Black Bream and Estuary Cobbler in south coast estuaries, the King George Whiting, Australian Herring and the Yelloweye and Sea mullets all spawn in marine waters outside these systems. The length of all but one of the King George Whiting caught in estuaries was less than the average length of maturity, demonstrating that this species leaves the estuary before becoming fully mature and that it does not return after spawning. The abundances of King George Whiting varied markedly among estuaries, being greatest in Wilson Inlet and least in Broke Inlet and the recruitment of the 0+ juveniles of this species varied among estuaries and years. The relatively high numbers in Wilson Inlet are probably due to a combination of good recruitment into the estuary prior to and during the beginning of the study, when the estuary mouth was open for a protracted period, and their not having had the potential to leave the estuary for most of the study as a result of the estuary mouth being open for only 4 weeks during that period. The low numbers in Broke Inlet are likely to reflect the fact that, although the mouth was open for a protracted period in the 5 second year of the study, the amount of freshwater discharge was high in that year and led to reductions in salinity to levels that were very low and thus unlikely to be favourable to this marine species. Differences in the growth of King George Whiting in the various estuaries probably reflects differences in prey availability as a result of the divergent environmental characteristics in the different estuaries and/or the effects of density and thus of competition for resources. The lack of a conspicuous change in the growth of this species in Wilson Inlet between 1987-89 and 2005-08 indicates that environmental conditions in this estuary may have remained similar over nearly 20 years. Catch rates in Wilson Inlet in 2005-07 were approximately a third of those recorded in 1987-89, indicating a marked decline in abundance. The reduced abundance, however, did not apparently have an influence on the growth of this species. The results of this study emphasis that the fisheries for King George Whiting in south coast estuaries are based on its juveniles. Australian Herring was relatively abundant in the Broke, Irwin, and Wilson Inlets and Oyster Harbour, but not in Wellstead Estuary. The females of this species were relatively more abundant than the males in the catches taken in each estuary, which may reflect their greater voracity and movements. The presence of very few Australian Herring < 140 mm and < 2 years of age in the samples from any estuary reflects the fact that the larvae and juveniles spawned on the lower west and western south coasts are passively transported eastwards via the Leeuwin Current and wind-driven water movements and do not return as large juveniles until their third year of life when they migrate back towards their marine spawning grounds and often then enter estuaries. This pattern of age-related movements also accounts for the percentage of Australian Herring with lengths greater than the average length at maturity exceeding 96% in the seasonally-open Broke, Irwin and Wilson inlets and being as high as 83% in the permanently-open Oyster Harbour. This provides a stark contrast with the situation recorded for King George Whiting, which, in estuaries, is represented very largely by juveniles. Since the strength of the Leeuwin current is weakest during La Niña years, it is relevant that, during this study, the El Niño Southern Oscillation Index was relatively neutral 6 and that the strength of this current would thus have been relatively h...