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Distributions, size compositions and diets of different species of Odacidae in south-western Australia: Evidence for resource partitioning among species

MacArthur, Lachlan (1997) Distributions, size compositions and diets of different species of Odacidae in south-western Australia: Evidence for resource partitioning among species. Honours thesis, Murdoch University.

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Members of the teleost family Odacidae were sampled seasonally in five different habitats in the offshore waters near Fremantle, on the lower west coast of Australia, using a small and large beam trawl. Three habitats comprised meadows of seagrass, which consisted predominantly either of Posidonia sinuosa, Posidonia coriacea or Amphibolis grifjithii, at depths of 4-9 m on Success and Parmelia banks. These seagrass meadows form adjacent meadows of 75-100, 25-50 and 75- 100% cover, respectively, and also differ in complexity, with A. grifjithii providing the most complex habitat and P. coriacea the least complex habitat. The two remaining habitats comprised shallow sand areas on Success and Parmelia banks (4-9 m deep), and deeper sand areas in Owen Anchorage and Gage Roads (12-15 m). Four replicate trawls were carried out in each habitat using both a small (1.0 x 0.5 m mouth, 2.5-1.0 mm mesh) and large (2.5 x 1.5 m mouth, 25-10 mm mesh) beam trawl during the day and night in summer, autumn and winter 1997 .. Additional replicated sampling, using the small beam trawl, was carried out in nearshore meadows (2.0-2.5 m deep) of mixed P. sinuosa and Posidonia australis seagrass in Cockburn Sound and Safety Bay in autumn and winter 1997.

A total of 559 fish representing eight species of odacids was collected using the small trawl, and a total of 967 fish representing eight species was collected using the large trawl. Siphonognathus radiatus was by far the most abundant odacid species caught in both the small and large trawls, contributing 87.3 and 63.2 % to the total catch in those trawls, respectively. Results of this study show that odacids in the slightly offshore Fremantle waters were found almost entirely in seagrass rather than over bare sand. Furthermore, S. radiatus was found in significantly higher densities in meadows of P. sinuosa and A. griffithii, than in those of P. coriacea or in bare sand regions. Odax acroptilus was found in much greater densities in the meadows of A. grifjithii than in any other habitat. While Odax cyanomelas was caught in much lower numbers, it too was found predominantly in A. grifjithii meadows. In contrast, Neoodax balte.atus and Haletta semifasciata were present in far higher densities in P. sinuosa meadows than other habitats, while S. attenuatus was found almost exclusively in meadows of P. coriacea.

Dietary studies showed that 0. acroptilus and S. radiatus, which co-occurred in relatively high densities in meadows of A. griffithii, displayed marked differences in their diets. Odax acroptilus consumed large amounts of red algae (mainly the erect coralline algae, Haliptilon roseum), and to a lesser extent, seagrass, whereas S. radiatus consumed mainly crustaceans and gastropods. Indeed, Schoener's overlap indices and classification and ordination showed that the diets of 0. acroptilus differed markedly from that of S. radiatus in all three seagrass habitats, as well as from those of N balteatus and H. semifasciata, which were found mainly in P. sinuosa meadows. These differences were due mainly to the ingestion of algae by 0. acroptilus. Siphonognathus radiatus (120-169 mm in length), N balteatus (20-69 mm in length) and H. semifasciata (220-269 mm in length), which occurred in relatively high densities in meadows of P. sinuosa, all consumed large quantities of gastropods. However, S. radiatus consumed larger gastropods than N balteatus, and a different family of gastropod (Columbellidae) than H. semifasciata. Furthermore, unlike these two other species, H. semifasciata ingested seagrass and greater quantities of polychaetes. Thus, the potential for competition for food resources among the abundant odacid species in offshore waters near Fremantle waters is low. Principal Components Analysis showed that the differences in the diet between 0. acroptilus and both S. radiatus and N balteatus may, at least in part, be attributable to the larger head and wider mouth and longer operculum to dentary length of 0. acroptilus. The ablity of this species to close its premaxilla over its dentary, combined with a fused beak dentition that is characteristic of this family, presumably allows 0. acroptilus to remove pieces of seagrass using a "scissor-like" motion. Thus, unlike many other fish in temperate seagrass meadows, 0. acroptilus and H. semifasciata consumed considerable amounts of seagrass. However, it is not known whether carbon from seagrass is assimilated by these fish.

For the odacids that were found in all three habitats, gastropods were consumed in greater volumes in P. sinuosa meadows, while crustaceans were consumed in greater volumes in A. griffithii meadows. Ordination revealed that for each odacid species that occurred in all three seagrass habitats, dietary compositions were more similar between fish from P. coriacea and A. griffithii meadows than between these habitats and P. sinuosa meadows, which is presumably due to the presence of P. coriacea plants in A. griffithii meadows and A. griffithii plants in P. coriacea meadows.

The diets of S. radiatus undergo marked ontogenetic change, with small fish consuming mainly copepods and other unidentifiable crustaceans, while gastropods, isopods and polychaetes increased in importance in the diets of larger fish. Ontogenetic differences in the diets of 0. acroptilus were less distinct, primarily due to the large amounts of algae consumed by all fish. However, bryozoans were only consumed by the smaller 0. acroptilus, whereas seagrass was consumed only by larger fish of this species, which may indicate that there is a mechanical limitation of fish below a certain size to remove pieces from relatively tough seagrass leaves. The lower gut fullness and higher contribution of unidentified material to the diets of particularly the smaller S. radiatus during the night, indicates that this species forages during the day. Since this species was found in lower densities using the small beam trawl during the night, the reduced feeding activity of S. radiatus at night appears to correspond with a vertical movement of fish at this time. Similarly, 0. acroptilus was found in lower densities at night, indicating that this species also forages in the seagrass meadows only during the day.

Sampling in nearshore seagrass meadows showed that the densities of odacid species were lower in these waters than in the deeper and more offshore waters. This feature, combined with the fact that the size distribution of odacids were similar in both regions, indicates that, unlike many other temperate marine teleosts, odacids do not use nearshore waters extensively as nursery areas. The new O+ recruits of S. radiatus, 0. acroptilus and N balteatus settled into the seagrass meadows between spring and autumn. Length data indicated that at least 2-3 cohorts of S. radiatus and 0. acroptilus were present in the offshore seagrass meadows. Furthermore, since mature (stage V) gonads were found in fish during the winter, those odacid species would appear to spend their entire life-cycle in seagrass meadows.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): 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: Thank you.
Supervisor(s): Potter, Ian and Hyndes, Glenn
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