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The biology of Samson Fish Seriola hippos with emphasis on the sportfishery in Western Australia

Rowland, Andrew Jay (2009) The biology of Samson Fish Seriola hippos with emphasis on the sportfishery in Western Australia. PhD thesis, Murdoch University.

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This thesis had two overriding aims. The first was to describe the biology of Samson Fish Seriola hippos and therefore extend the knowledge and understanding of the genus Seriola. The second was to uses these data to develop strategies to better manage the fishery and, if appropriate, develop catch-and-release protocols for the S. hippos sportfishery.

Trends exhibited by marginal increment analysis in the opaque zones of sectioned S. hippos otoliths, together with an otolith of a recaptured calcein injected fish, demonstrated that these opaque zones represent annual features. Thus, as with some other members of the genus, the number of opaque zones in sectioned otoliths of S. hippos are appropriate for determining age and growth parameters of this species. Seriola hippos displayed similar growth trajectories to other members of the genus. Early growth in S. hippos is rapid with this species reaching minimum legal length for retention (MML) of 600mm TL within the second year of life. After the first 5 years of life growth rates of each sex differ, with females growing faster and reaching a larger size at age than males. Thus, by 10, 15 and 20 years of age, the predicted fork lengths (and weights) for females were 1088 (17 kg), 1221 (24 kg) and 1311 mm (30 kg), respectively, compared with 1035 (15 kg), 1124 (19 kg) and 1167 mm (21 kg), respectively for males. Despite these differences, female and male S. hippos attained similar maximum age, i.e. 29 (1470 mm FL) and 28 years (1280 mm FL), respectively. The maximum age determined for S. hippos is greater than that recorded for any other Seriola spp. The largest female and male S. hippos recorded during this study were encountered during the tagging component and had fork lengths of 1600 mm and 1380 mm, respectively.

Seriola hippos has a protracted spawning period, ca four months, which starts in late spring and continues through summer into early autumn during which time many individuals engage in large spawning aggregations on the lower west coast of Australia. The length at which 50 % of the females in the population reached maturity was 831 mm FL (888 mm TL) and approximately 4 years of age, whilst all females over 950 mm FL were mature. Whilst aggregated for spawning S. hippos ceases feeding, however, during the non-spawning period this species can best be described as an opportunistic carnivore which feeds on a variety of pelagic and demersal prey.

This study has greatly increased our understanding of S. hippos movement on the west and south coasts of Australia and has documented, for the first time, the migratory behaviour of a carangid in these waters. Many S. hippos individuals undertake long distance migrations to join spawning aggregation sites near Rottnest Island. Individuals tagged at these aggregation sites where recaptured throughout this species distribution along the south coast of Australia, some after travelling distances of over 2400 km. Many S. hippos individuals displayed strong temporal and spatial spawning ground fidelity as numerous fish released at the spawning aggregations were recaptured at the exact same spawning site at similar times in subsequent years. Tagging data suggest that on the completion of spawning S. hippos individuals return to a resident location and remain in that general vicinity over the winter months.

This study has developed a hypothesis describing larval dispersal associated with the S. hippos spawning behaviour exhibited near Rottnest Island. It is proposed that variations in the prevailing ocean currents, at this important spawning location throughout the protracted spawning period, leads to high intra and inter-annual variation in larval distribution and survival. The affect of this variation on the evolution of the spawning and migratory behaviour displayed by S. hippos is discussed.

A recent increase in the popularity of S. hippos as a catch-and-release sportfish has led to concerns by some anglers about post release survival of this species, particularly due to the depth of capture. Short term mortality of S. hippos capture at the sportfishing sites was assessed by monitoring fish held within an enclosure near the site of capture for up to 31 hours post release. The total hooking mortality of S. hippos subjected to catch-and-release angling within the Rottnest Island sportfishery is approximately 8%. Most of this observed mortality is delayed and occurs sometime after release. Although best handling practises require ongoing development, the current level of mortality associated with this catch-and-release fishery is considered acceptable. Furthermore, this mortality is likely to have little effect on the S. hippos population due to the high abundance of this species and the fact that even the highest fishing effort yields a relatively low catch. Seriola hippos exhibits a typical teleost neuroendocrine stress response associated with catch-and-release. The physiological dysfunction associated with the stress of capture in this species does not appear to cause any post release mortality. Instead, most mortality was attributable to barotrauma, however, although mortality in S. hippos increases with capture depth, this species is much less susceptible to depth induced mortality than other commonly targeted species in which barotrauma has been observed.

This study developed key handling protocols for fishers who catch-and-release S. hippos at the Rottnest Island aggregation sites. These protocols cover aspects of catch-and-release fishing such as hook type, water depth, time at surface, release method and shark predation.

Almost all S. hippos observed during capture from deep water released large quantities of gas from the opercular region, particularly during the last 10 to 20 m before reaching the surface. This phenomenon has also been witnessed by divers and fishers to occur under natural conditions. Investigations into this release of gas revealed this physoclistous species to exhibit unique swim bladder characteristics. Seriola hippos possess a membranous tube that connects the posterior-dorsal surface of the swim bladder internally to a region under each operculum externally. This connection, termed the swim bladder vent, allows the escape of expanding swim bladder gases on rapid ascent. The presence of the swim bladder vent provides an explanation as to why the incidence of external barotrauma symptoms in S. hippos captured from the deepwater was low. The ability to expel excess swim bladder gases during rapid ascent whilst retaining full swim bladder function is likely to offer this semi-pelagic species considerable advantages when hunting prey, avoiding predators and engaging in spawning activities.

Preliminary estimates of total mortality indicated that S. hippos is not currently subjected to a high level of fishing pressure. However, managers must remain mindful of the fact that the size at which females reach sexual maturity, i.e. 888 mm TL, is greater than the current minimum legal length, i.e. 600 mm, and thus fishers are currently allowed to harvest sexually immature fish. Furthermore, the effectiveness of future conservation measures must consider the large scale migration and spawning strategy undertaken by this species in order to ensure its protection.

The collaborative research approach undertaken during this study demonstrated that a high level of community engagement produced a large amount of research interest, increased stakeholder satisfaction from project input, improved understanding of research outcomes, and increased research uptake, all of which has led to increased stewardship and conservation of the S. hippos fishery and fisheries resources in general. Indeed, projects of this nature would not be possible without this type of approach.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Howard, Gill and Mike, Mackie
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