The aims of the studies undertaken for this thesis on the black bream
Acanthopagrus butcheri, a species which is confined to estuaries, were to determine the
following. (1) The age compositions, growth rates, reproductive biology and diets of the
populations of this species in four different estuaries (Swan River, Moore River,
NomaluplWalpole and Wellstead estuaries) and a landlocked saline lake (Lake Clifton)
and (2) the seasonal and regional distributions of this species within one estuary (Swan
River Estuary). Acanthopagrus butcheri were collected at regular intervals from
nearshore, shallow (< 2 m) and offshore, deeper (> 2.5 m) waters of the permanently open
Swan River Estuary and intermittently open Moore River Estuary on the lower west coast
of Australia and from the permanently open Nomalup/Walpole Estuary and normally
closed Wellstead Estuary on the southern coast of Western Australia. One hundred
A. butcheri were also obtained from a landlocked, coastal saline lake (Lake Clifton), 90 km
south of the Swan River Estuary. Sampling employed seine nets, composite gill nets and
rod and line.
In the Swan River Estuary, black bream typically occur in the saline reaches of the
tributary rivers which constitute the upper estuary. However, during heavy freshwater
discharge in winter, many individuals are swept downstream into the basins that constitute
the middle estuary. These fish migrate back into the upper estuary in spring and the larger
fish spawn in this region between the middle of spring and early summer. Although
smaller fish tend to remain in the upper estuary during summer as salinities increase, the
larger fish migrate further upstream where salinities are lower. The salinities in which
A. butcheri spawned in the different systems ranged from as low as 5.5 - 6.8 %CJin the
Moore River Estuary to as high as 40.7 - 45.2 %O in the Wellstead Estuary.
The use of marginal increment analyses demonstrated that the opaque zones
revealed in otoliths by sectioning are formed annually and could thus be used for ageing
individual fish and that the opaque zones visible in whole otoliths prior to sectioning could
be used for ageing fish up to six years old. The number of annuli on scales did not
provide a reliable estimate of age. The structure of the age compositions in the four
estuaries varied, presumably reflecting differences in fishing pressure and, in one case, the
lack of recruitment in some years. The growth rates of A. butcheri in the four estuaries
and landlocked lake differed, which is probably related to variations in one or more of the
following; water temperature, density of fish, salinity and the type of food available.
The monthly trends exhibited by gonadosomatic indices and the prevalence of
different gonadal maturity stages and mature oocytes demonstrate that spawning typically
occurs in spring and early summer. The frequent occurrence of yolk vesicle, yolk granule
oocytes and post-ovulatory follicles in the ovaries of some mature fish provides strong
circumstantial evidence that A. butcheri is a multiple spawner, i.e. spawns more than once
during each breeding season. Estimates of the minimum total fecundity ranged between
9.07 x lo4 and 7.09 x lo6, with a mean of 1.58 x lo6. Variations amongst the lengths and
ages at first maturity in three of the estuarine populations of A. butcheri could apparently
be attributed to the influence of variations in growth rate.
Female and male Acanthopagrus butcheri both possess an ovotestis, a feature
characteristic of the Sparidae. There is strong circumstantial evidence that, once a member
of this species reaches maturity, it can be considered a rudimentary hermaphrodite, i.e. it
possesses either functional ovaries and far smaller and immature testes or functional testes
and immature ovaries of variable size. There was no evidence that this species undergoes
either a protogynous or protandrous sex change.
Acanthopagrus butcheri can consume various benthic and epibenthic prey,
including crustaceans, polychaetes, molluscs and teleosts, and can also ingest considerable
volumes of algae. However, the dietary compositions of A. butcheri in the four estuaries
and Lake Clifton differed markedly, whch, together with information on the biota in those
systems, indicate that A. butcheri feeds on those prey items that are most abundant in their
environment. Yet, there is also evidence that, in any given system, A. butcheri will focus on
a particular prey, even when other prey, which are regularly consumed in considerable
volume in other systems, are abundant. The dietary compositions of A. butcheri in each
estuary underwent ontogenetic changes, which would reduce the potential for intraspecific
competition for food resources.
| Identifer | oai:union.ndltd.org:ADTP/221745 |
| Date | January 1999 |
| Creators | G A Sarre |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright G A Sarre |
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