Movement patterns and bycatch of pelagic sharks in Australian waters

Author: Matthew Heard

Heard, Matthew, 2018 Movement patterns and bycatch of pelagic sharks in Australian waters, Flinders University, College of Science and Engineering

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With the increase in global fishing effort and expansion of fishing fleets into the open ocean, large predatory fishes are under increasing threat of extinction. Understanding the impacts of open ocean fisheries on top predators, such as sharks, is critical to the management of these fisheries, particularly for species caught as bycatch. In the past decade, several species of pelagic sharks have been recognised as vulnerable migratory species and included on Appendix II of the List of Migratory Species on the Convention on the Conservation of Migratory Species of Wild Animals (CMS). As a signatory to the CMS, Australia is committed to progressing actions that will improve the management and conservation of these species.

The major objectives of this study were to investigate the levels of commercial and recreational catches of migratory pelagic shark species and to determine the factors that influence these catches in Australian waters. I used a permutational analysis of pop-up satellite telemetry data to define the vertical movement patterns of five blue sharks (Prionace glauca) and one common thresher shark (Alopias vulpinus). I conducted a survey of recreational game fishers to determine the level of catch of shortfin mako (Isurus oxyrinchus) and A. vulpinus within this fishery and to investigate fishers’ behaviours and attitudes towards sharks. Generalised and distance based linear modelling techniques were used to investigate commercial catches of A. vulpinus in the Southern and Eastern Scalefish and Shark Fishery and P. glauca, I. oxyrinchus, Lamna nasus and Alopias spp. in the Eastern and Western Tuna and Billfish Fisheries.

The minimum horizontal displacement of satellite tagged sharks was variable, with the largest displacement exceeding 5 000 km and pop-up locations indicating the importance of highly productive upwelling areas. Movement patterns included surface-oriented, reverse diel vertical, and normal diel vertical movements, with the latter being the most common pattern identified for both P. glauca and A. vulpinus. Catch and release of pelagic sharks was practised by over half of the recreational anglers that were surveyed at game fishing tournaments. The majority of anglers asserted that they attempt to release sharks in good condition, but there was a relatively low use of circle hooks, that have been shown to increase post-release survival. Season and depth were the most important explanatory variables for catch rates of A. vulpinus in gillnet fisheries operating in South-eastern Australia. Catch rates were higher in summer and there was an inverse relationship between catch rate and depth. In pelagic longline fisheries, sea surface temperature was the most important environmental variable that influenced catches of P. glauca, I. oxyrinchus, and L. nasus, while depth was the most important variable for Alopias spp.

It is important that measures implemented by managers are based on evidence of the level of threat that fisheries pose to these highly migratory pelagic shark species. For recreational anglers, an increased emphasis on tagging competitions at tournaments and promotion of catch and release, and associated best practices should improve the sustainability of tournament angling. The importance of diel movements, depth preference, season, and temperature are highlighted by the satellite telemetry and analysis of gillnet and longline data. A better understanding of these parameters provides critical information for assessing the encounterability and susceptibility of pelagic sharks to different gear types within Australian waters.

Keywords: Alopias vulpinus, Prionace glauca, satellite telemetry, vertical movement, recreational fisheries, commercial fisheries

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Charlie Huveneers