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Resource partitioning by demersal teleosts and elasmobranchs in temperate coastal waters of south-western Australia

Platell, Margaret Ellen (1998) Resource partitioning by demersal teleosts and elasmobranchs in temperate coastal waters of south-western Australia. PhD thesis, Murdoch University.

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Demersal teleosts and elasmobranchs were collected by trawling over sandy substrata at nine sites in shallow (5 - 15 m in depth) and deeper (20 - 35 m) waters in four regions along ca 200 km of the inner continental shelf of south-western Australia. Sampling at these sites was carried out at night in each season between summer 1990/91 and winter 1992. The distributions and dietary compositions of the more abundant of the benthic carnivorous fish have been determined and compared to elucidate whether resource partitioning is occurring within and between species, thereby reducing the potential for intra- and interspecific competition. The dietary compositions were analysed using measures of dietary breadth and dietary overlap, and by classification and nonmetric ordination, and took into account any ontogenetic changes in diet that occurred within a species. The dietary compositions have also been related to mouth morphology, body form and feeding behaviour in an attempt to elucidate the bases for any differences in diets amongst species.

The gerreid Parequula melbournensis and the carangid Pseudocaranx wrighti are morphologically similar, attain comparable sizes and co-occurred in abundance at seven of the nine sites. The dietary compositions of these two species differed significantly, due mainly to the fact that P. melbournensis consumed large amounts of partially-burrowed polychaetes, and particularly onuphids, whereas P. wrighti ingested a variety of epibenthic prey, i.e. bivalves, gastropods, nereid polychaetes and echinoderms, which also included more mobile taxa, such as mysids and amphipods. These differences in the types and overall range of prey consumed by these two species, together with the ingestion of far less sediment by P. melbournensis than P. wrighti, suggest that the former species targets its prey more precisely, a characteristic that would presumably be facilitated by its far more protrusible mouth.

The second comparison involved two ambush predators, namely the scorpaenid Maxillicosta scabriceps and the platycephalid Platycephalus longispinis, which grows to a larger size. The environment of these two species was partially partitioned, with M. scabriceps being more abundant than P. longispinis at sites where there were greater amounts of rocks and seagrass. This habitat difference probably reflects the fact that scorpaenids, such as M. scabriceps, lie in wait for their prey on the substrate surface, and thus rely on a heterogeneous habitat to help provide camouflage, whereas platycephalids, such as P. longispinis, typically bury themselves in the substrate prior to ambushing their prey. The dietary compositions of these two species were significantly different, even when the species were of comparable size, reflecting the tendency for P. longispinis to ingest large volumes of relatively mobile teleosts, and for M. scabriceps to consume prey which are less mobile than teleosts, such as mysids, decapods and crabs. The above differences occurred despite the fact that the former species has a relatively smaller mouth. The ingestion of other fish by P. longispinis is presumably related to the ability of this species to swim rapidly after its prey.

Although Upeneichthys stotti and Upeneichthys lineatus, the two most numerous species of mullid, were almost invariably both abundant in deeper waters, only U. lineatus was also abundant in shallow waters. The dietary compositions of these two mullids were only significantly different when these species co-occurred, when U. stotti ingested relatively more mobile prey, i.e. mysids, cumaceans and carid decapods, and U. lineatus consumed larger and slower-moving prey, i.e. bivalves, onuphid polychaetes and crabs. Since these interspecific dietary differences occurred despite the possession by both species of virtually identical mouths, they presumably reflect differences in foraging behaviour.

The elasmobranch family Urolophidae was represented by four abundant species, i.e. Urolophus lobatus, Urolophus paucimaculatus, Trygonoptera personata and Trygonoptera mucosa. Although these species frequently co-occurred, they were each abundant in at least two sites where one or more of the other species was not present. Differences between dietary compositions were much greater between than within genera, due mainly to the far greater consumption of mysids, amphipods and carid decapods by the two Urolophus species and of polychaetes by the two Trygonoptera species. These differences indicate that the latter two species forage within the substrate. The intrageneric differences were related to the additional consumption of fish by U. lobatus and of errant polychaetes and penaeid decapods by U. paucimaculatus, and the ingestion of mainly errant polychaetes by T. personata as opposed to sedentary polychaetes by T. mucosa.

Since each of the above pairs or groups of species feed mainly during daylight (including dusk), they are potentially competing for prey resources at the same time. However, evidence is presented that the pempheridids Pempheris klunzingeri and Parapriacanthus elongatus, which are abundant in south-western Australia, feed mainly at night. Yet, previous data from elsewhere indicate that pempheridids consume similar prey to triglids, which feed mainly during the day and are represented in south-western Australian waters by Lepidotrigla papilio and Lepidotrigla modesta. However, in southwestern Australia, the above two pempheridid species feed mainly on certain small crustaceans, i.e. ostracods, and polychaete prey, which emerge from the substrate into the water column at night, while the above two triglids feed mainly on prey which occur on or close to the substrate surface, i.e. amphipods and mysids. Such differences can be related to the fact that the downwards-pointing mouth of triglids is better suited to feeding on prey living on the substrate surface beneath the fish, while the upward-pointing mouth of pempheridids would be better adapted to feeding on prey just above the fish. Although these species frequently co-occurred at several sites, each species attained its highest density at different sites, a feature which thereby represented a partial segregation of these species by habitat. Although the dietary compositions of each of the triglid species differed significantly, and the same was true for the two pempheridid species, this was only the case with the two triglid species when they co-occurred.

The patterns of ontogenetic change in the diets of the above species differed. The range in prey (dietary breadth) of P. melboumensis, P. wrighti, U. stotti, U. lineatus, U. lobatus, U. paucimaculatus and P. klunzingeri generally increased with increasing body size. Consequently, on those occasions when significant intraspecific dietary overlap occurred, it was mainly between successive size classes. The prey typically consumed by the small representatives of these species included, in order of importance, mysids, amphipods, carid decapods, cumaceans, ostracods and copepods. Although the larger of these crustaceans were still consumed as the above fish species increased in size, other prey, such as polychaetes, bivalves, crabs and isopods, began to be ingested in greater amounts. In contrast to the above trends, P. longispinis and T. personata increasingly focused on teleosts and errant polychaetes, respectively, as they increased in size, thereby leading to a decrease in their dietary breadths. Furthermore, M. scabriceps, T. mucosa and P. elongatus fed on similar prey throughout life. However, the first and third species consumed a wider variety of prey, including in particular mainly mysids, amphipods and crabs, and amphipods and errant polychaetes, respectively, whereas T. mucosa always consumed mainly sedentary polychaetes.

Finally, the data for the dietary compositions of each of these teleost and elasmobranch species, which are all relatively abundant over sandy substrates in the coastal waters of south-western Australia, were compared collectively. These comparisons also incorporated dietary data from the only other abundant benthic carnivorous teleosts in the study area, i.e. members of the Sillaginidae (Sillago bassensis, Sillago burrus, Sillago robusta and Sillago vittata). These analyses demonstrated that the compositions of the diets of sixteen of the eighteen most abundant demersal fish species on the lower west coast of Australia differed significantly from all other species, irrespective of whether or not these species were co-occurring. Although the diets of the two triglid species (L. modesta and L. papilio) were not shown to be significantly different from each other, it should be remembered that comparisons restricted to occasions when these species co-occurred, showed that their diets were significantly different under those circumstances. The above analyses thus show that the dietary resources are always partitioned to some extent between each of the eighteen species that were abundant in the study area, a process which, at least in some cases, is presumably facilitated by differences in their morphology and/or feeding behaviour. In the context of the latter point, is thus relevant that a collective principal components analysis of the head and mouth measurements of each of the teleost species demonstrated that there were significant differences between each of the species, except in the case of U. stotti and U. lineatus. The lack of a difference between these two mullid species is consistent with analyses of covariance of the same measurements for these two species.

The species compositions of the fish faunas at sites were shown to be significantly different from each other. This finding is consistent with the observation that there was generally some degree of habitat partitioning, even amongst morphologicallysimilar and/or closely related species. The marked tendency for such habitat partitioning, allied with partitioning of food resources, would reduce the potential for competition for both spatial and prey resources.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science
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
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