Experimental hyperbaric chamber trials reveal high resilience to barotrauma in Australian snapper (Chrysophrys auratus)

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A key factor affecting survival of fish which are captured at depth and subsequently released is barotrauma. Barotrauma are a suite of injuries which occur as a result of the decreasing ambient pressure experienced by a fish during capture and are caused by expanding swim bladder gas and formation of gas embolisms. Snapper (Chrysophrys auratus) are of key importance to both commercial and recreational fisheries in southern Australia and are often released after capture suffering from barotrauma. Previous research has yielded inconsistent results for survival of C. auratus after suffering barotrauma, in part due to influences related to capture, ‘cage’ effects and the use of only very short term post-catch and release monitoring. This study therefore used laboratory hyperbaric chambers to simulate the pressure changes associated with capture to investigate whether barotrauma was a direct cause of mortality in C. auratus. The influences of depth and repressurization on mortality, behaviour and symptoms due to barotrauma could therefore be examined in a controlled environment independent of capture and ‘cage’ effects combined with long-term monitoring (220 d) post-simulation. In a pilot experiment C. auratus (n = 12) were depressurized from 8 bar (≈70 m depth), before being repressurized after 2 min at 1 bar (atmospheric pressure) resulting in agitated in-chamber behaviours and abdominal distension as a result of swim bladder perforation, but 100% survival. A second experiment depressurized C. auratus from 4 bar (≈30 m depth); one treatment group (n = 32) were repressurized after 10 min again resulting in behaviour and symptoms indicative of swim bladder perforation and 100% survival, the other group (n = 32) was left at 1 bar and exhibited many additional external and internal barotrauma symptoms and suffered 16% mortality. Despite the death of a single control individual, statistical analyses found a significant effect of treatment on mortality. Mortality occurred within 15 min of depressurization and the cause of death was consistent with the formation of gas embolisms. Staged post-mortems performed on C. auratus that had been depressurized from 4 bar to induce swim bladder perforation indicated that swim bladders were healed sufficiently to hold gas after just 2 d. C. auratus were shown to be a remarkably robust species to the effects of barotrauma, particularly if repressurized quickly (<10 min) after capture and this resilience is likely reflective of their characteristic semi-pelagic physiology. This study also demonstrated the usefulness of laboratory hyperbaric chambers to examine the long term (>200 d) effects of depressurization on deep water fish species independent of capture and ‘cage’ effects.