Fish Facts: Snapper barotrauma

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Image: NSW DPI
A snapper reease weight. Image: NSW DPI

Ever wondered whether that undersized snapper (Chrysophrys auratus) you just caught from deep water survived when you released it back into the depths? I certainly have, and this month I’ll explore some recently published research where scientists determined just that. It’s certainly true that deepwater snapper are highly accessible now due to the latest fishing technology. They are more accessible than ever to GPS guided anglers with reliable and efficient modern boats, who can target even the smallest of reefs way offshore using thin no stretch lines, and realistic plastic lures complete with extremely effective scents. This increased fishing pressure on snapper, together with their relatively slow growth rates and longevity has resulted in a need to manage their stocks by increasing size limits and reducing bag limits, all of which results in more snapper being released.

Of course we all now know (or should know) that catching bottom dwelling fish offshore in water deeper than around 15-20 meters can result in depressurization damage (correctly called barotrauma) to captured fish akin to “the bends” experienced by divers who ascend to the surface too quickly. In most cases barotrauma presents as the inability of the fish to swim back down due to expansion of the gases in their swimbladder (swimbladder hyperinflation), as well as expansion of gases in other spaces in their body and bloodstream. These gas expansion problems are typical barotrauma, though you may remember last month I discussed how swordfish can also exhibit apparent barotrauma symptoms (swim bladder hyperinflation), but in their special case the problems may actually be a physiological issue due to stimulation of the swim bladder gas gland (due to lactic acid buildup in the blood). But I digress.

In contrast to the special case of broadbill, snapper suffer from “run of the mill” barotrauma due to depressurisation. Research conducted in Queensland around 5 or 6 years ago found that snapper caught and released from waters between 37 and 180 meters deep had surprisingly low short term (3 day) mortality rates, with 90% of snapper caught from “shallow” water (37-50 meters) surviving, reducing to 86% for those caught from “moderate” depths (51-100 meters), and 85% for fish caught from deep water (101-180 meters). Overall, the survival rate of all snapper studied during that experiment was a surprisingly high (given the depths being fished) 88%, with fish that were vented by piercing the swimbladder through the everted stomach having the highest survival rate (95%), compared to control fish that were not vented (87%). Another interesting finding from that particular study was the researchers recorded rapid healing of perforations in both the swimbladder and stomach of the fish they sampled, with 64% of stomachs and 45-55% of the swimbladders of the fish examined healing within 3 days of capture.

Of course scientist being scientists, their quest for information is never ending, and the next question they asked was what are the longer term survival chances of snapper caught from depth ? The Queensland study referred to above kept captured fish inside large vertical “sock” enclosures out in the ocean for 3 days, but what about longer term ? Also, another tricky question is what percentage of mortalities were related to the depressurisation, and what was due to hooking damage (survival was only 20% in gut hooked fish, suggesting hooking location was a significant predictor of mortality).

To answer these difficult questions, researchers in NSW Department of Primary Industries got even more technical and bought their fishing studies into the laboratory. They used a hyperbaric chamber to pressurise snapper under controlled conditions that simulated various water depths (10 meters of water is equal to an additional 1 atmosphere of gauge pressure = 1 bar which is 14.7 psi or 100 kpa above atmospheric pressure). In this latest study snapper were pressurized to 8 bar (= approximately 70m depth) then depressurised to sealevel (1 bar) for 2 minutes to simulate an angling event, then repressurised to 8 bar. Survival after this was 100% after 220 days, showing that the pressure change alone from being caught from 70 meters and returned to that depth was not fatal to snapper. When the same experiment was repeated from the equivalent of 30 meters, the same result was obtained, i.e. all snapper survived 220 days provided they were repressurised to the capture depth within 10 minutes, however some fish were not repressurised - i.e. they were left at sealevel pressure (which is equivalent to floating on the surface) and for these a mortality rate of 16% was recorded with the fish dying after 15 minutes on the surface. All fish that died showed classical barotrauma signs, including distended abdomen, swim bladder perforation, corneal haemorrhage, bloodshot cloacas, viscera displacement and/or haemorrhage including liver trauma (due to hydraulic rupture of blood vessels by the expanded swimbladder), stomach eversion, hepatic vein damage and an enlarged spleen.

This relatively high survival rate (84% survival over 220 days) from this latest highly technical study thus supported the results from the earlier Queensland field study which found 88% survival over 3 days, and indicated that earlier studies which found lower survivals (30-45%) for snapper captured from depths of 60-65 meters may have been confounded by other factors such as mortalities related to holding fish in sea cages, rather than the pressure damage alone. Of course, in the real world, hooking damage is a real factor that does come into play, and we know that regardless of these encouraging studies, dragging undersized snapper from depths of 80 to 100 meters is still not ideal for their survival. Consequently, you should not be considering snapper (or any other bottom fish) as catch and release sportfishing targets when taken from deep waters (say over 20 meters deep). The usual mantra of taking only what you need is recommended, use circle hooks when using bait, or use lures to prevent mortalities due to deep hooking, carry a release weight with you at all times to recompress floating fish, and always be prepared to move to a new spot when large numbers of undersized snapper are encountered. If you master all of these, you can enjoy your snapper session content in the knowledge you are following scientifically proven best practice!

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