Fish Facts: Air Exposure

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Dr Diggles practicing the “keepemwet” technique with a Spanish mackerel. Large, high performance fish like mackerel benefit greatly from in-water releases, but watch out for those sharks!
Dr Diggles practicing the “keepemwet” technique with a Spanish mackerel. Large, high performance fish like mackerel benefit greatly from in-water releases, but watch out for those sharks!

WHILE air time might be good for TV personalities and radio shock jocks, as a general rule its not very good for fish! Anyone who has smiled at the antics of a mudskipper, marveled at flying fish in full flight or tried to work out the logic of jumping mullet would agree that some fish regularly go the aerial route as part of their day to day existence. But is removing fish from the water during a catch and release process a good idea?

If you want to maximize the survival chances of the fish you’re releasing, scientific evidence is accumulating to suggest as a general rule, its not. “Best Practice” dictates those fish which are to be released should ideally remain in the water while being photographed and de-hooked, and in other parts of the world there are growing sportfishing movements which aim to spread this message as broadly as possible through initiatives such as the “Keepemwet” campaign ( But as with many things in life, its not all black and white.

Most fish can certainly survive out of the water for several minutes – certainly much longer than an untrained person can survive underwater. This has a lot to do with the relatively enormous availability of free oxygen in air compared to underwater. Air is 21 per cent oxygen by volume (23 per cent by weight), but in contrast the maximum levels of oxygen dissolved in water are relatively miniscule, around seven to eight parts per million at full saturation (and often less depending on water temperature, salinity and other variables).

This means that measured by weight, air has around 28,000 times more oxygen content than water, which explains how simple diffusion through the skin and mouth can provide air adapted fish like mud skippers with supplemental oxygen to extend their time limit out of water. Indeed, it’s thought that 250-300 million years ago at the time mudskippers were evolving to life out of the water, atmospheric oxygen content may have been in the vicinity of 30-35 per cent, and that this oxygen nirvana may have been one of the factors driving some aquatic dwellers to evolve into land animals. But I digress...

For many fish species, brief periods of air exposure are a common, if not mandatory, experience in the normal day to day existence experienced, for example, by small baitfish fleeing larger predators. But, these natural air exposures are very brief and the fish haven’t been previously subjected to exhaustive exercise like an angling event. Research into the effects of air exposure on the survival of fishes released by anglers has found that various factors influence how detrimental air exposure is to different fish species. At the completion of a long fight with a large fish, for example, the problems with subsequent air exposure become threefold.

This is due to the fact that:

  1. Large fish have longer fight times, leading to greater physiological changes such as oxygen debt, lactic acid build up and so on;
  2. The skeletal elements of large fish are not supported by the water, opening the way for traction injuries to the axial skeleton or damage to the jaw mechanisms, slime coat etc. during handling; and,
  3. Air exposure is stressful and results in physical changes such as collapse of gill lamellae, impairing blood flow through the gills and vastly reducing the available surface area for oxygen uptake.

This triple whammy means the end result of air exposure after a long fight is, physiologically, the equivalent of the fish having to hold its breath, just as humans would have to being underwater.

Given its well established that keeping fish in the water is the “gold standard” best practice for them physiologically, in some situations removing the fish from the water is still required, often for reasons of angler safety. For example, while rock fishing, or if in a boat, sharks and crocs can represent a risk to anglers hands, or indeed, whole anglers, particularly in the event of unforeseen issues such as difficulties in de-hooking the fish. Of course, a quick grip and grin photo of an exceptional fish is also a valid reason why anglers might want to remove a fish from the water. In these situations the critical question about air exposure then becomes, how long is too long?

A review of recent scientific studies concluded that there may be a need to provide species specific guidelines for “maximum air exposure”. For example, studies of bonefish showed that fish air exposed for 3 minutes took over 10 times longer to recover equilibrium following an angling encounter compared to fish exposed to air for 1 minute. For a high performance fish like bonefish, more than 1 minute air exposure effected post release survival chances due to greater metabolic imbalances, and/or more predation in areas where sharks were abundant.

Similarly, recent tagging studies of white marlin in the Atlantic Ocean found that mortality rates of released fish increased from...

  • 1.7% in fish that were not removed from the water,
  • to 17% for fish that were out of the water for one minute,
  • to 40% for fish out of the water for three minutes,
  • and 57% for fish out of the water for 5 minutes.

So for a large high performance fish like white marlin, even one minute air exposure greatly increased post release mortality compared to keeping em wet. On the other hand, up to seven minutes air exposure caused no change in post release mortality rates of smaller, less acrobatic species like flatfish and walleye. In an attempt to cover all scenarios, the authors of the review recommended that air exposure, if necessary at all, should be restricted to 10 seconds or less in order not to reduce the chances of post release survival of a fish.

So, the science is in and the upshot is, anything that anglers can do to release their fish in the water, or minimise the surface interval out of the water to 10 seconds or less is considered best practice. While this may seem a bit extreme, 10 seconds is often enough to get a good photo if you have a competent assistant with camera at the ready. Fortunately, for the vast majority of fish species, a minute or so air exposure should be OK and will lead to no lasting long term problems. So next time you release a fish, try to remember that photos of the fish in the water are more interesting anyway, and try to see if you too can “keep em wet”.

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