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The biology of four commercial fish species in a seasonally closed estuary

Orr, Pia (2000) The biology of four commercial fish species in a seasonally closed estuary. PhD thesis, Murdoch University.

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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.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Biological Sciences and Biotechnology
Supervisor: Potter, Ian
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