When certain types of fish are brought to the surface after being hooked or trapped at depth, they can sustain injury (barotrauma) from the change in pressure. I witnessed this myself a long time ago fishing for rockfish off of the California coast - the fish would come to the surface bloated by the air that had expanded as it ascended. Even if the fish would have been released, it would have died.
Our most recent trip to the Dry Tortugas included some researchers who wanted to tag Mutton Snapper with an acoustic tag to be able to track their movement. In previous years, they tried capturing the fish at depth, bringing them to the surface, and doing the surgery to insert the tag on the surface. The problem with this approach is what I mentioned above - the fish are stressed by the change in pressure and are likely to die from the procedure or be eaten by a predator in their weakened state. So, on this trip, the challenge was to do the surgeries underwater, at depths that the Mutton Snapper call home. I was lucky enough to get to participate in some of the surgeries as well as video and photograph them. I did a very impromptu interview with Mike Feeley, the Principal Investigator on the project, and included some snippets of that conversation in the video. Unfortunately, I still have very limited internet access so I have to upload only a low-res copy of the video for now.
Despite some of the challenges associated with the procedure (like capturing only the target species, keeping an eye out for any sharks or goliath groupers who might want an easy Mutton Snapper meal, and performing the whole procedure at depths ranging from 95 to 123 feet) we were able to successfully tag 8 Muttons and 3 Groupers. I'm going to be very interested to see the data from the receivers to see how our little surgical patients are faring.
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