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The ecology of Octopus aff. O. tetricus and prevalence of anisakid nematodes in near-coastal waters of North Fremantle, Western Australia

Claybrook, Jorja (2020) The ecology of Octopus aff. O. tetricus and prevalence of anisakid nematodes in near-coastal waters of North Fremantle, Western Australia. Honours thesis, Murdoch University.

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Abstract

In contrast to most fished groups, global landings for cephalopod have been increasing over the last two decades. This and their fast-growth rate and short lifecycles, responsiveness to environmental change, and their function as mesopredators within the marine environment have led to an increased focus on their ecology. Within Western Australia, the common octopus, known currently as Octopus aff. O. tetricus is the target species of the commercial fishery which operates in Cockburn Sound. This Thesis focused on the population of O. tetricus inhabiting near-coastal waters of North Fremantle, ~7 km north of the current commercial operations and assessed any inter and intra-population variability between the two sites. Samples of octopus were collected by commercial fishers setting four lines of shelter pots, two in shallow (≤10 m) and two in deeper water (15-17 m) on seven occasions between March and June 2020 in North Fremantle. One sample of octopus was collected from Cockburn Sound on 1st April. These data were used to examine if the distribution, abundance, and sex ratio differed across the sampling period and between depths.

A total of 701 octopus were caught ranging in size from 35 to 218 mm mantle length (ML) and 5.13 to 691.62 g mantle weight. Over the duration of the study, the mean number of octopuses caught decreased. Males were more abundant in March and May, while females were more prevalent in April. The mean size of female octopus fluctuated in the first four hauls the trap lines and then declined from late-April to June, indicating a possible migration out of the area by larger females. Females caught in Cockburn Sound over a wider range of habitats were larger than those from North Fremantle at the same time. The mean size of male octopus increased gradually over the duration of the study with the highest mean ML recorded in June. Male octopus matured during the study with the number of immature males decreasing from March to June, in both depth regions.

Initially, this Thesis aimed to include a detailed analysis of the dietary composition of Octopus aff. O. tetricus, however due to time constraints only a preliminary analysis is presented in this dissertation. The contents of the gastric tract (crop and stomach) of 701 octopus were examined. A total of 12 taxa were identified in the gastric contents, with crustaceans identified as the most common prey, followed by teleosts and non-cephalopod molluscs. Cannibalism was observed in 51 octopus (10.8% of gastric contents with dietary items).

During the dissection of octopus mantles, parasites, tentatively identified as anisakid nematodes, were recorded on the internal organs and this became an additional focus of the research. The incidence of parasites was nearly 60% in North Fremantle and when present, always occurred in the gastric tract but were also found in the mantle muscles, digestive gland, posterior salivary glands and gonads. Binomial logistic models with a logit link function showed that the probability of parasite incidence was nearly ten times higher in the North Fremantle population compared with Cockburn Sound. The probability of parasite occurrence was also significantly greater on medium (80-160 mm ML) and large octopus (>160 mm ML) than small octopus.

The results from this study show that the nearshore waters of North Fremantle are likely a nursery habitat for juvenile and maturing male octopus’ but females probably migrate to deeper waters to mate and brood their eggs. The prevalence of parasites and location suggests that nematodes are being ingested within North Fremantle at a much higher rate than in Cockburn Sound.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): Environmental and Conservation Sciences
Supervisor(s): Loneragan, Neil and Hart, A.
URI: http://researchrepository.murdoch.edu.au/id/eprint/59571
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