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Variations in abundance, feeding patterns and prey availability of post-settlement King George whiting, Sillaginodes punctata, in nearshore marine waters

Whitehead, Ayesha (2000) Variations in abundance, feeding patterns and prey availability of post-settlement King George whiting, Sillaginodes punctata, in nearshore marine waters. Honours thesis, Murdoch University.

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Abstract

Spatial variation in the recruitment of Sillagi,nodes punctata into five sheltered unvegetated sites in south-western Australia was determined through fortnightly sampling of post-settlement individuals, using a fine-mesh seine net, during the recruitment period (between early September and early December). The diets of these recruits were examined to determine whether there were spatial, temporal and ontogenetic variations in dietary composition. In addition, the invertebrates which constituted important prey for post-settlement S. punctata were sampled at the same locations using sediment corers and sweep nets to establish spatial patterns in prey abundance. These spatial patterns in abundance of prey were compared with the abundance and dietary composition of post-settlement S. punctata. Particulate organic matter and microphytobenthos were also sampled using cores to establish whether the spatial patterns in abundance of invertebrate prey were associated with the distribution of these plant materials.

Densities of post-settlement S. punctata differed significantly among sites, being highest at Mangles Bay, thus demonstrating that the recruitment of S. punctata into unvegetated habitats in nearshore marine waters varied on a spatial scale. At sites with lower densities of fish, S. punctata were present throughout the recruitment period at Point Peron and Safety Bay, whilst they were only present in the first half of the recruitment period at the other two sites. This result suggests the possibility of movement or mortality of recruits at these sites.

The overall diet of post-settlement S. punctata at the study sites was dominated by crustaceans, comprising cyclopoid, calanoid or harpacticoid copepods or gammarid amphipods. The dietary composition of post-settlement S. punctata varied spatially suggesting fish were feeding opportunistically. Fish at Mangles Bay consumed harpacticoid and calanoid copepods frequently and in large proportions, whilst those at Point Peron and Safety Bay consumed gammarid amphipods :frequently in large proportions. At Woodman Point and Rockingham, sites in which only small individuals were captured in the initial stages of sampling, the overall diet was dominated by cyclopoid copepods. Ordinations emphasised that the dietary composition of fish at Mangles Bay was distinct from that at other sites, primarily due to high proportions of harpacticoid and calanoid copepods in the diet of fish at this site and to high proportions of cyclopoid copepods and gammarid amphipods in the early and late stages of sampling, respectively, at the other sites.

With increasing size, post-settlement S. punctata at Mangles Bay consumed smaller proportions of harpacticoid copepods and larger proportions of calanoid copepods, while those at Safety Bay consumed lower proportions of cyclopoid copepods and increasing proportions of gammarid amphipods. The dietary composition of S. punctata caught from different sites exhibited temporal variation during the sampling period with respect to the proportions of the main prey types consumed. This temporal pattern was not consistent with size-related changes in dietary composition at Mangles Bay, where the diets of similar-sized fish varied with respect to the sampling occasion on which they were collected.

The densities of prey commonly consumed by S. punctata exhibited marked spatial variation, with densities ofharpacticoid and calanoid copepods being highest at Mangles Bay, and those of gammarid amphipods being highest at Point Peron and Safety Bay. Incidentally, the high densities of post-settlement S. punctata that were characteristic of Mangles Bay, coincided with high densities of harpacticoid and calanoid copepods. Furthermore, the low densities of fish that were recorded at Point Peron and Safety Bay throughout the sampling period, coincided with high densities of amphipods. At the sites where individuals were present only in the early phase of sampling, i.e. Woodman Point and Rockingham, low densities of all three prey commonly consumed by S. punctata were found.

Spatial variation in dietary composition of S. punctata was associated with that of the abundance of commonly consumed prey. Sillaginodes punctata at Mangles Bay consumed harpacticoid and calanoid copepods more frequently and in larger proportions to fish from the other sites and these prey were most abundant at this site. Similarly, S. punctata at Point Peron and Safety Bay, where amphipods were most abundant, consumed amphipods more frequently and in larger proportions to fish from Mangles Bay. The spatial patterns in abundance of prey were not associated with the mass of particulate organic matter or microphytobenthos, which were similar at all sites, except in the case of particulate organic matter, where Point Peron had slightly higher mass. In addition, a preliminary investigation of the stable isotopes, 13C/12C and 15N/14N, revealed that these sources of primary production were unlikely to be primary sources of carbon to this food chain. More likely sources could include rhodophytes and phaeophytes.

Item Type: Thesis (Honours)
Murdoch Affiliation: School of Biological Sciences and Biotechnology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Hyndes, Glenn
URI: http://researchrepository.murdoch.edu.au/id/eprint/41538
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