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Catch and release: how deep is too deep?
​The right and wrong ways to do catch-and-release.
By John Eichelsheim

HANDLING FISH

• Poor handling is a major factor in fish mortality after release. Putting fingers in the gills or eyes, squeezing the abdominal area, holding heavy fish by the tail and neglecting to support a fish’s body out of the water can all cause fatal injuries. Lying a fish on a hot, dry deck – even handling it with dry hands – can cause damage to its scales and their protective coating of lime, which often leads to death from fungal infection weeks or months after release.
• If you’re intending to release a fish, it makes sense to handle it in such a way that it has a good chance of survival – otherwise you might as well keep it.
• Where possible use a net, preferably one with soft mesh – maybe one of the many rubber-meshed ‘enviro’ types now available, as these do minimal damage to scales, fins, eyes and the mucus that coats all fish.
• If possible, don’t lift the fish from the water at all, especially if it’s large. Instead, use a tool (pliers or a specialist hook removal tool, but not a gob-stick) to remove the hook while the fish is supported in the water, either in the net or secured by the mouth with pair of fish grips.
• But if you must bring the fish into the boat, handle it with a wet towel and make sure any surface it comes into contact with, including your hands, is cool and wet,. Big fish should be supported around the belly region (not squeezed). Never lift large fish by the tail – you can separate their vertebrae!
• The longer a fish is kept out of water, the fewer its chances of making a good recovery. Keep photo sessions short and be prepared to revive large fish boat-side until they can swim away under their own steam.

I’ve noticed quite a lot of comment in the press and on social media lately about catch-and-release fishing, including speculation about whether or not fish survive the process. It seems there are as many opinions on the advisability of catch-and-release as there are commentators.

I go fishing a lot, and so I release plenty of fish. A couple of snapper is all my small family ever needs for the table, and while I enjoy sometimes sharing my catch with friends and neighbours, I wouldn’t want to catch a limit of fish every time just to give it away.
Releasing fish in good condition is easy enough when they’re taken in fairly shallow water, provided they’re handled with care and respect (see box), but when fish are hooked in deep water, successful release (i.e. where the fish survives) is rather more problematic.
 
Swim bladder
 
Most of the bony fish (teleost) species we catch, such as snapper, tarakihi, hapuku and others, use a swim bladder to maintain neutral buoyancy. Cartilaginous fish, such as sharks and rays, don’t have swim bladders and must keep moving to avoid sinking, or else rest on the bottom.
Certain species of bony fish can withstand sudden changes in depth much better than others. Kingfish show no ill effects after being hooked in 100m of water or more, and jack mackerel used as live baits survive repeated excursions into deep water and back to the surface again. Broadbill swordfish hooked in 500m may swim straight to the surface and breach with no ill-effects, while tuna and marlin often dive deep and then return to the surface within a matter of minutes.
Other fishes, however, are not so lucky. While many can move freely from deep water into more shallow water and vice versa, to maintain equilibrium they must do so relatively slowly, regulating the gas volume in their swim bladders as they go. Expanding gas in the swim bladder is re-absorbed into the bloodstream via a network of capillaries as the water shallows and when they swim deeper again, it has to transfer back into the swim bladder from the blood.
 
Floaters
 
Many popular fish species ‘blow up’ when pulled to the surface from deep water, as gas in their swim bladder expands faster than it can be reabsorbed into the bloodstream. Some fish species are unaffected by the sudden pressure changes caused by changes in depth, but hapuku, snapper and many others do suffer the effects of sudden decompression when drawn to the surface. Barotrauma, as it’s called, has a range of physical effects on fish, from mild bloating which prevents them swimming back down to a comfortable depth, to massive internal injuries causing death.
Most of us have seen ‘floaters’ – fish that struggle on the surface when released because of gas in the swim bladder and/or abdominal cavity – and deep water fishers will be familiar with hapuku, bass and other species surfacing with stomachs protruding from their mouths, eyes popping from their sockets and, in severe cases, the scales standing away from the skin.
These, along with broken blood vessels around the anus, ruptured swim bladders, intestines forced out of the vent and, in severe cases, fish that actually fizz with gas escaping though the skin are all physiological manifestations of barotrauma caused by sudden decompression.
 
Slowly does it?
 
Like many fishers, I always thought I could lessen the effects of decompression on fish by bringing them to the surface slowly. But it would appear that in a fishing context it makes no difference, because even the slowest retrieve is still much too fast. Experiments in the US with rock fish showed it took three days for fish pulled from 30m of water to regain neutral buoyancy. Many did not survive.
At a depth of 10 metres water pressure is twice as great as it is at the surface; at 30 metres it’s four times as great. That means the gas in a fish’s swim bladder expands to four times its volume when the fish is pulled to the surface from 30m of water, and many of the fish we catch are unable to deal with this expanding gas in time to prevent barotrauma.
What this means for recreational fishers is that there is a practical depth limit to catch-and-release fishing.
I’m most familiar with snapper. In my experience, fish caught in more than 20m of water become difficult to release successfully. Many of them will show signs of barotrauma, and even when they don’t, they may have difficulty swimming away when released. Large specimens seem the worst affected.
Much of my snapper fishing takes place in water 10m deep or less and snapper taken from these depths and handled with care generally release well. By-catch species such as pigfish, caught from the same depths, are often too badly traumatised to release, which just demonstrates that every species is different.
Fish hooked in 15m of water can be more difficult. I’ve sometimes been forced to keep large snapper I’d normally release because, despite showing no outward signs of barotrauma, they were unable to swim back down even after protracted efforts to revive them at the surface.
Research from Australia shows a direct correlation between the depth in which snapper are caught and their survival rate when released
(http://www.dpi.nsw.gov.au/fisheries/recreational/saltwater/catch-and-release/research). 600 snapper were caught from a variety of depths up to 75m before being returned to the water, where they were then monitored in holding cages for several days. All fish hauled from 20m of water showed external signs of barotrauma, but although when anaesthetized and examined there was evidence of internal damage such as bleeding and displaced organs, at the end of the experiment all of them seemed to have recovered well. Fish hooked in deeper water showed more physical evidence of barotrauma and had lower post-release survival rates but a good proportion of even those snapper taken from 70m-plus survived release.
 
Blowing bubbles
 
Most snapper taken from 40-50m of water show obvious signs of decompression at the surface. Smaller fish sometimes swim down again when released, but many fish of all sizes float on the surface to become food for the birds.
When snapper are drawn to the surface from 40 or 50m of water their swim bladders expand so much and so fast they usually rupture; this is indicated by a burst of bubbles and often occurs at or just under the surface as the fish is drawn towards the boat. Ironically, once the air bladder has burst, snapper can often successfully swim away from the surface when released.
Many anglers wishing to release fish that are obviously blown ‘vent’ them using a sharp knife or a hollow needle. This deflates the swim bladder or, if the bladder has already burst, releases gas trapped in the abdominal cavity, which may allow the fish to swim back down to a depth where it can recompress. (Although note: the ‘swim bladder’ protruding from a fish’s mouth is actually its everted – turned inside-out – stomach.)  Regardless, while it can seem effective in the short term, puncturing a fish’s swim bladder or stomach is invasive and may cause infection and damage that will later prove fatal.
Fish need their swim bladders to maintain their equilibrium and control buoyancy, so can they survive a ruptured swim bladder and any organ damage caused by its uncontrolled expansion?
Once again, overseas research, this time using Pacific cod, showed remarkable survival rates and extremely quick recovery from a ruptured swim bladder. Most of the surviving cod had normally-functioning swim bladders within a day or two of being returned to the depth at which they were caught.
For me the jury remains out, though. Handled carefully, I think the majority of released snapper survive, but I try and avoid releasing snapper taken from water more than 20m deep.