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The fish faunas of south coast estuaries: Volume 2
Citation
Chuwen, BM and Hoeksema, SD and Hesp, SA and Hall, NG and Coulson, PG and Crisafulli, B and Rodwell, TM and Potter, IC, The fish faunas of south coast estuaries: Volume 2, South Coast Natural Resource Management Inc., 04SC1-05e (2010) [Contract Report]
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Abstract
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
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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).
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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
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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
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and that the strength of this current would thus have been relatively high and therefore
presumably also the transport of the larvae and juveniles of this species.
As Australian Herring do not spawn in estuaries, the gonads of those of its mature
individuals that are prevented from migrating to their marine spawning grounds by the
formation of a sand bar at the mouth of an estuary are resorbed. The fact that the growth of
Australian Herring was least in Irwin Inlet and Oyster Harbour, which were open to the ocean
during the spawning period of this species, may thus reflect a greater allocation of resources
to reproductive output than to somatic growth than occurs in those estuaries in which this
species is prevented from migrating to its marine spawning areas. While the growth of this
species in Wilson Inlet is very similar to that recorded nearly 20 years ago, the presence of
larger and older individuals during the current than past studies may reflect the fact that the
estuary mouth remained closed for extended periods during the present study and would thus
have prevented those individuals from migrating to the sea.
Although Yelloweye and Sea mullet were both caught in appreciable numbers in each
estuary, they were least abundant in the permanently-open Oyster Harbour. The females of
both of these mullet species reached larger sizes and grew to older ages than their males.
While the 0+ juveniles of both species can only be recruited into seasonally-open and
normally-closed estuaries when the mouths of those estuaries are open at the end of the
spawning period and during the ensuing months, the presence of an open estuary mouth at
those times does not always result in the recruitment of the 0+ juveniles of these species.
A lack of such recruitment into an estuary was presumably due either to spawning not
occurring in nearby marine coastal waters and/or to conditions being unfavourable for the
transport of larvae and juveniles to the region of the estuary mouth.
The growth of Yelloweye Mullet varied markedly among the estuaries, with, in the
case of females, growth being greatest in Broke Inlet, followed by Wilson Inlet, Wellstead
Estuary, Irwin Inlet and finally Oyster Harbour. These differences are likely to reflect the
influence of differences in such factors as prey abundance, quality of habitat and density.
Although the growth of Sea Mullet also differed among estuaries, the pattern of differences
between estuaries differed from that for Yelloweye Mullet. Thus, for example, growth was greater in Wilson Inlet and Oyster Harbour than in Broke Inlet, presumably reflecting the
presence of far less macrophyte growth and thus of detritus, the food source of Sea Mullet.
The growth curves of both the females and males of Yelloweye Mullet in Wilson Inlet in
2005-07 were remarkably similar to those of the corresponding sexes of this species in that
estuary in 1987-89. In contrast, the growth of Sea Mullet is less in the more recent than earlier
period, possibly due to an increase in the density of this species and/or a decline in the quality
of the benthic environment.
A suite of fish species, that comprises three atherinids, i.e. Elongate Hardyhead,
Western Hardyhead and Silverfish, and three gobies, i.e. Bluespot Goby, Southern Longfin
Goby and Longheaded Goby, is proposed as a suitable indicator of estuarine health for
estuaries on the south coast of Western Australia. These species were considered appropriate
as 1) they typically dominate the fish faunas of nearshore, shallow waters in estuaries along
the length of the south coast, 2) they complete their life cycles within these systems, 3) they
are not subjected to recreational or commercial fishing pressure and 4) the biology of each
species is well known. The potential of the suite of indicator species to reflect the health of
estuaries on the south coast is emphasised by the fact that the characteristics of that suite
exhibited essentially the same trends as those of the entire fish fauna and could readily be
correlated with certain environmental variables. Furthermore, the suite of indicator species
can be consistently targeted in the basins of all estuaries along the south coast in a costeffective
manner.
A regime to monitor the fish faunas and biological characteristics of key fish species
in south coast estuaries is proposed. This regime includes the collection of baseline data for
all south coast estuaries and on-going fishery-independent monitoring of their fish faunas,
which will complement the types of fishery-dependent and environmental data that are
currently collected. Monitoring of the above-mentioned indicator suite of species will also
provide cost-effective and valuable information on the ecological condition of south coast
estuaries.
Item Details
Item Type: | Contract Report |
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Keywords: | estuaries; Acanthopagrus butcheri; Cnidoglanis macrocephalus; Sillaginodes punctata; Arripis georgianus; Aldrichetta forsteri; Mugil cephalus; indicator species monitoring; south coast, Western Australia. |
Research Division: | Agricultural, Veterinary and Food 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 freshwater |
UTAS Author: | Chuwen, BM (Dr Ben Chuwen) |
ID Code: | 71866 |
Year Published: | 2010 |
Deposited By: | Sustainable Marine Research Collaboration |
Deposited On: | 2011-08-11 |
Last Modified: | 2011-08-11 |
Downloads: | 0 |
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