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).
To answer these difficult questions, researchers in NSW Department of Primary Industries got even more technical and bought their fishing studies into the laboratory. 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). 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. 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%). 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). In contrast to the special case of broadbill, snapper suffer from “run of the mill” barotrauma due to depressurisation.
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). 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. 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.
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. 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. It’s certainly true that deepwater snapper are highly accessible now due to the latest fishing technology. 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.