Post-Release Behaviour and Depth Selection in an Important Recreational Fish Species, Australasian Snapper Chrysophrys auratus, After Application of Barotrauma Relief Methods

Barotrauma, a condition caused by the rapid pressure changes experienced by a fish during capture from depth, is a major contributor to fisheries post-release mortality. Recompression is a practice commonly used by recreational fishers to reduce this mortality by relieving the main effects of barotrauma, excessive buoyancy resulting from hyperinflation or rupture of the swim bladder, and gas embolisms. Two common recompression methods are ‘venting’ and ‘release weights’, however previous results derived from releasing treated fish into cages have provided equivocal support for their use. This study therefore used a novel biotelemetry approach to examine the effects of these relief methods on post-release behaviour and depth selection in a recreationally important fish species, Australasian snapper Chrysophrys auratus. Fish were angled and released after being vented, using a release weight or untreated. The prevalence of barotrauma symptoms increased with capture depth. Regardless of treatment or depth, immediate post-release behaviour of all but one individual (an untreated fish which floated) was to swim towards the bottom. Descent rate was significantly faster using a release weight, but similar for vented and untreated releases. Descent rate was most variable for vented fish. Regardless of treatment or water depth, after returning to depth all individuals remained just above the bottom. This demonstrates that unless the fish is unable to submerge, the post-release depth preference for barotrauma-affected snapper is near the bottom, which is facilitated by both barotrauma relief methods, albeit at different rates. However, the likely impact of increased handling required for both relief methods on post-release behaviour, indicate that untreated release is preferred. If the fish is unable to submerge by itself, a release weight is supported as an appropriate release technique as it mimics natural post-release behaviour and depth preference of the fish, whilst avoiding physical trauma associated with venting.