Movement ecology of a large teleost pelagic predator, the yellowtail kingfish (<i>Seriola lalandi</i>)

<p dir="ltr">Pelagic fish inhabit a vast, dynamic, three-dimensional environment, and with complex life stages, may use habitats across large spatial and depth ranges. Their highly mobile nature makes the understanding of life history, habitat use and population ecology much more challenging than fully coastal species. Yellowtail kingfish (<i>Seriola lalandi</i>; hereafter referred to as ‘kingfish’) are a large, pelagic marine fish with a circumglobal distribution. Kingfish support important commercial and recreational fisheries throughout Australia and New Zealand (NZ) however their habitat use (vertical, horizontal and thermal) remained largely unresolved at the time of this study, hindering management efforts. The Eastern Australian biological stock spans some ~4 million km², encompassing South Australia, Victoria, Tasmania, New South Wales, Queensland, offshore islands and seamounts within the Tasman Sea, and NZ. Despite recognised genetic homogeneity, spatial connectivity of the stock is not well resolved, with each jurisdiction within Australia and NZ managing the stock independently.</p><p dir="ltr">In this thesis, I first reviewed 1,926 peer-reviewed publications to evaluate how tagging methods have been historically used to fill key knowledge gaps relating to fish life histories, population parameters and spatial distributions, thus providing relevant information for fisheries management. I then used satellite tags, conventional tags and natural tags (otolith composition) to explore kingfish movements, connectivity and habitat use around south-eastern Australia and NZ.</p><p dir="ltr">I analysed half a century of conventional tag-recapture data, 63,432 releases and 4,636 subsequent recaptures, demonstrating the value of large-scale tagging programs in producing ecologically relevant population connectivity insights to inform management. This data revealed extensive cross-jurisdictional connectivity, suggesting management may be enhanced by a multi-jurisdictional approach to coordinate management objectives across jurisdictions.</p><p dir="ltr">I then used satellite telemetry to describe and quantify finer-scale behaviour and habitat use (horizontal, vertical and thermal) of adult kingfish. Forty-three adult kingfish were tracked for a total of 3,020 days and demonstrated considerable connectivity between coastal and offshore environments. Vertical and thermal distributions were highly variable, indicating broad depth and thermal tolerance. The diversity of movements highlights the complexities of kingfish behaviour and population dynamics.</p><p dir="ltr">Finally, I determined the calcium carbonate polymorph structure and composition of kingfish otoliths from both wild and aquaculture environments to both be aragonite, validating the possible use of aquaculture kingfish in otolith microchemistry investigations.</p><p dir="ltr">Using wavelength dispersive spectroscopy, I determined that barium was below detectability in wild kingfish otoliths across all life stages, suggesting kingfish do not extensively use estuarine environments. Extensive use of offshore habitats by adult kingfish, combined with the absence of barium in juvenile otolith annuli, suggests that spawning most likely occurs in offshore environments.</p><p dir="ltr">The cross-jurisdictional connectivity of kingfish, coupled with movement across coastal and offshore environments, and probable offshore habitats for kingfish spawning, indicates a requirement for a co-operative approach to management of the Eastern Australian kingfish stock. Future research relevant to multi-jurisdictional management includes identification of key spawning aggregation hotspots and the sources of juvenile recruits into the stock.</p>