Development of the marlin fishery off the East Coast of Australia

<p dir="ltr">Poor understanding of long-term changes in recreational fisheries limits our capacity to predict and manage them into the future. Compared to their commercial counterparts, reporting is seldom mandatory in recreational fisheries, as such, we lack the data required to study long-term changes. However, non-conventional approaches, such as using citizen science data sets or surveying recreational anglers, can be used to fill these data gaps. The Australian southeast coast is home to one of the world’s oldest recreational marlin fisheries, catching three species of marlin: black (<i>Istiompax indica</i>), blue (<i>Makaira </i><i>nigricans</i>) and striped (<i>Kajikia audax</i>) marlin. Being an old fishery suspected to have changed over its life and home to several experienced anglers and long-term non conventional datasets, it provides a unique opportunity to study change in recreational fisheries. Assessing long-term change also requires considering the unique ecologies of the three marlin species. Black marlin are typically the smallest in this fishery (avg. ~ 40kg) and occur closest to the coast, striped marlin on the continental shelf (avg. ~ 70kg), and blue marlin, the largest (avg. ~110kg) fish and the one inhabiting furthest from the coast, typically on the edge of or past the continental shelf. However, the region lacks up to date information on trophic ecology, which is essential to understand the methods, lures and baits anglers use to target them. </p><p dir="ltr">In this thesis, we used recreational datasets to identify changes in catch composition, then assessed the diets of the three marlin species. We surveyed anglers to understand how social changes and changes to fishing practices drove observed changes in catch composition. Analysis of the historic datasets showed an abrupt shift away from a black marlin dominated catch to more comparable catches of all three species in the 1980s. Dietary analyses showed each marlin species had different prey preferences, with black marlin primarily consuming coastal teleosts, blue marlin predominantly consuming scombrids but also a range of other teleost and cephalopod species, and striped marlin consuming a range of schooling teleosts and cephalopods. We developed and tested a method that suggested that a range of the prey items found in blue marlin stomachs may not have been consumed by the blue marlin itself, but instead consumed by the prey of the marlin and remained in the stomach after the prey was digested (termed secondary predation). Finally, the surveyed anglers suggested that improvements in boat technology and learning new fishing methods drove an offshore shift in the fishery in the 1980s and allowed anglers to exploit the unique ecologies of each marlin species, resulting in the observed increase in the catch of blue and striped marlin. As the Australian east coast marlin fishery continues to advance, it is likely anglers will continue to increase catch rates which needs to be considered in any catch or stock assessment using recreational data. These findings provide a rare example of long-term restructuring of a recreational fishery due to improved technology and angler knowledge. More research into change in recreational fisheries is essential to accurately incorporate them into management frameworks and ensure they can be enjoyed by many recreational anglers for years to come.</p>