The aim of this study was to determine the size and age compositions, growth
rates, reproductive biology, dietary compositions and mesh selectivity of the King
George whiting Sillaginodes punctata, the Australian herring Arripis georgiana, the
yelloweye mullet Aldrichetta forsteri and the sea mullet Mugil cephalus in Wilson Inlet.
These biological variables were chosen for analysis since the resultant data would be of
use for managing these commercially important marine species in this seasonally closed
estuary, which is located on the southern coast of Western Australia.
Samples of each of the above species were typically collected, in at least every
other month between April 1988 and April 1990, by using seine netting and gill netting
at several sites, distributed at regular intervals throughout the 48 km2 basin of Wilson
Inlet, and by gill netting at one site in each of two tributary rivers. The ages of
Sillaginodes punctata, Aldrichetta forsteri and Mugil cephalus were determined using
the annuli on scales, whereas those of Arripis georgiana were determined using the
annuli on otoliths. The annuli on the above hard structures of the four species were
shown to be formed annually before they were used for aging purposes.
Although at least four year classes of each species were found in Wilson Inlet,
the 0+ year class of none of the species was well represented in this estuary. The
paucity of this age class was probably attributable, in most cases, to the spawning
grounds of these species being located some distance away on the lower west coast of
Australia. However, the absence of any 0+ S. punctata in one year was attributed to the
mouth of Wilson Inlet being closed at a crucial time, which thereby prevented the
recruitment of this species into this estuary.
Virtually all of the S. punctata caught in Wilson Inlet were less than the LSo at
first maturity, which implies that none of this species return to this estuary after they
have emigrated to sea, matured and spawned. However, appreciable numbers of each of
the other three species were found at lengths greater than their respective Lsos. There
was clear evidence that large A. forsteri tended to leave the estuary soon after the bar at
the estuary mouth was breached. In contrast, length-frequency distributions indicated
that large A. georgiana often tended to remain in the estuary, even when they exceeded
the LSo at first maturity and the estuary mouth was open. The maximum lengths
attained by S. punctata, A. georgiana, A. forsteri and M. cephalus were 491,373,430
and 525 mm, respectively, while the maximum ages attained by these four species were
5+, 7+, 5+ and 4+, respectively. While there was no significant difference between the
growth rates of the two sexes of either S. punctata or M. cephalus, the overall growth
rates of the females of A. georgiana and A. forsteri were both greater than those of their
males.
Each fish was assigned a maturity stage using the criteria of Laevastu (1965).
The maximum maturity stage recorded for the gonads of S. punctata was IV (maturing).
The fact that no S. punctata with spawning (stage VI), spent (stage VII), recovering
spent (stage VIII) or resorbing gonads were found provides overwhelming evidence that
this whiting species neither spawns in Wilson Inlet nor returns to this estuary after it has
spawned at sea. Furthermore, spawning and recently-spent gonads were never found in
individuals of either A. georgiana, A. forsteri or M. cephalus, whch implies that these
species also do not spawn in the estuary. However, some A. georgiana and A. forsteri
with recovering spent gonads were found in Wilson Inlet, which indicates that some
individuals of these two species enter the estuary after spawning at sea. There was no
indication that this also applied with M. cephalus. Yet, the gonads of some individuals of this latter species and also of A. georgiana contained resorbing gonads, i. e gonads
that had reached a reasonably advanced stage, but not maturity, and which were now
undergoing regression. The fish with these gonads had presumably been trapped in the
estuary at the time their gonads were developing and could not therefore emigrate out to
their marine spawning areas.
The dietary compositions of the four species were shown to be significantly
different. Sillaginodes punctata and A. georgiana fed mainly on benthic
macroinvertebrates, while all but the smallest M. cephalus were detritivores and
A. forsteri was an omnivore. Furthermore, S. punctata fed mainly on errant polychaetes
and nemerteans, whereas A. georgiana ingested decapods, mysids and other teleosts.
The diets of each species underwent size-related changes. The differences in dietary
compositions among and within species would reduce the likelihood of inter- and
intraspecific competition for food resources.
Comparisons have been made between traditional methods for estimating the
selectivity of S. punctata, A. georgiana, A. forsteri and M. cephalus in gill nets and a
new method of analysis developed (with Dr. N. Hall), which incorporates the best
features of the traditional methods.
| Identifer | oai:union.ndltd.org:ADTP/221743 |
| Date | January 2000 |
| Creators | Pia Orr |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Pia Orr |
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