Post release survival of swordfish (Xiphias gladius) caught by a recreational fishery in temperate waters

Abstract

Swordfish (Xiphias gladius) are large, economically and ecologically important predatory fish with a wide circumglobal distribution. While swordfish are coveted by anglers, development of recreational fisheries has been limited historically, due in part to the species’ tendency to migrate into the mesopelagic zone during the day, effectively out of reach of typical game fishing methods. Recently, however, the adoption of ‘deep-dropping’, targeting swordfish during daytime with baits at > 300 m depth, has expanded access to the fish and led to the emergence of new regional fisheries. In 2014 a deep-dropping recreational swordfish fishery emerged in temperate southeast Australia and attracted international attention after yielding several swordfish line-class weight records. However, information is needed to guide best practices for emerging deep-dropping swordfish fisheries. Here, we present the first assessment of capture-related morbidity and post-release survival of swordfish caught by deep-dropping in southeast Australia. Among swordfish assessed to be in suitable condition for release, affixed pop-up satellite archival tags indicated 85.6% (57.8 – 95.7%; n = 13) survived after release. Including swordfish assessed as moribund after attempted resuscitation as mortalities, the survival rate for landed swordfish was 44% (95% CI 25.1 – 64.8%; n = 25). Severe abdominal distension (a notably protruding abdomen presumably due to swim bladder overinflation) and observed internal hooking injury (in the gut or gill area) were strong predictors of reduced survival (odds ratios 0.008 and 0.015 respectively), while angling duration and fish weight did not have a discernible effect on mortality. While the swordfish fishery is superficially like more common istiophorid billfish game fishing, the unique physiology and behaviour of swordfish and depths at which they are targeted present a unique challenge for stewardship as the typical catch-and-release billfish game fishing ethos may not be appropriate. The results of this preliminary study suggest swordfish caught deep-dropping are a poor candidate for purely catch-and-release angling. Predictors of post-release mortality are readily observable, so fishers should be prepared to humanely dispatch fish exhibiting symptoms of injury from gill or gut hooking, or abdominal distension severe enough to prevent the fish from righting itself beyond a brief resuscitation period.