The science of catch and release: Fish physiology dictates careful handling

By Robert Koopmans

On the grand scale, there is no doubt that catch and release is good for fish. Sport anglers who release portions of their catch keep populations of vulnerable fish species from being quickly depleted.

Catch and release allows ever increasing numbers of anglers to continue chasing fish, in places where the harvest of fish cannot be supported.

But catch and release is not without risk to fish, says Brian Heise, an associate professor of aquatic ecology at the University College of the Cariboo in Kamloops.

There is no guarantee a sport-caught fish will survive, despite the best intentions of those holding the rod. Even in ideal conditions, biologists believe five per cent of released rainbow trout die — out of sight and long after being put back in the water.

The mortality rate increases dramatically if anglers don't handle and release fish properly, Heise says. The rules of catch and release — play the fish fast, use a proper landing net, keep the fish in the water and release it quickly, and use barbless hooks — are important.

To understand why requires a closer look at the physiology of rainbow trout.

Play it fast

Rainbow trout are finely tuned predators designed to find and catch food efficiently. Everything about a trout — from the composition of its muscle fibres to the chemical balances in its body — is designed to accomplish that task.

It's a delicate balance, one that can be easily upset. And once a fish's chemical and physical systems are out of whack, its very survival is threatened.

Rainbow trout muscle is comprised of two different kinds of fibres — red and white, Heise says. The distinctions between the two types of fibre are complex and not visible to the eye.

Red muscle is aerobic and burns oxygen pulled from the water by a fish's gills. Red muscle is used for long-distance cruising, for getting around — the human equivalent of walking, or light jogging.

White muscle fibres are used for “burst movement” — activities like chasing quickly moving prey, or escaping predators like loons, ospreys and bears.  White muscle is anaerobic, burning glycogen stored in the muscles instead of oxygen. When the supply of glycogen is used up, white muscle fibres stop working. Glycogen is not replenished quickly.

Heise says the intense fight of a sport-caught rainbow trout is powered mostly by white muscle fibres. When the fish burn up all their white muscle fuel, they are physically exhausted and stop trying to escape.

The byproduct of that intense glycogen burning is lactic acid, Heise says, which builds in great quantities in a fish's muscles and blood.

Humans have similar anaerobic muscle fibres that allow for fast sprinting, or other intense activities like heavy weight lifting. Human muscles also create lactic acid when heavily worked. Lactic acid causes that intense burning that precedes a muscle's complete exhaustion.

But unlike humans, fish cannot clear their muscles of lactic acid quickly. While the legs of a fit human sprinter may be ready to go again in as little as 45 minutes, rainbow trout may not be able to effectively use their white muscle fibres for as long as 12 hours.

The more exhausted the fish, the more lactic acid and the longer the recovery period. In some cases, it can take up to 72 hours for complete recovery to occur. A fish that is recovering from being caught cannot chase down food, or escape easily from predators. Both scenarios can prove dangerous to fish.

Bringing a fish quickly to the net with fight still in it means it will recover faster, increasing its chances of survival. An utterly exhausted fish may not be able to restore proper chemical balances in its body, because its heart cannot move sufficient quantities of oxygenated blood through its body quickly enough.

There is another benefit to releasing fish with some kick still in them, Heise says.  Studies have shown that rainbow trout that perform some slow, steady swimming immediately after release will clear lactic acid build-ups in their muscles in as little as two hours.  A fish that is not played to complete exhaustion is better able to swim away and cruise for a while.

ON THE WATER — Use heavier gear when necessary, says Kamloops fisheries biologist Brian Chan. Stronger line, leaders and tippet will allow anglers to bring bigger fish quicker to the net.

It's a factor that becomes more crucial as water temperatures increase, he says. In late summer, oxygen levels in the Interior's shallow eutrophic lakes are often lower and fish are even less able to recover from exhaustion. It's important to get them off the hook quickly.

Use a proper landing net

A trout is covered by a fine coating, a slime that serves as a barrier against bacteria, viruses and fungal infections, says Heise. As well as serving as a protectant, the slime acts like a lubricant, increasing a fish's hydrodynamic ability.

Rough handling, or contact with a beach, rocks or the bottom of a boat, can remove portions of the slime, leaving fish vulnerable to disease.

There are many naturally occurring pathogens in lakes and rivers that target fish, he says.

Some diseases are lethal, while others cause disability.

It's crucial a fish's slime be protected, by careful handling.

ON THE WATER — A good quality landing net is made of tightly woven soft nylon or cotton, Chan says. There are no knots in the weave to abrade a fish's skin or remove the slime. The gaps in a net's weave should be small, so that fins, noses and mouths do not get caught.

Always wet the net first before using it to trap a fish. 

Keep it in the water and release it quickly

Fish are built to live in water, where the effect of gravity is virtually nonexistent. As a result, their organs and skeletal systems are not able to support their weight out of water.

A fish grabbed from underneath and lifted out of the water by a well-meaning angler can quickly suffer lethal internal injury, says Heise. The potential for injury increases as fish increase in size.

As well, fish that are removed from the water cannot breath. A temporary lack of oxygen isn't fatal, Heise says — just like humans, fish can survive short times without fresh oxygen.  But when people go swimming, they do not fill their lungs with water and expel it later, he says. That's what being pulled into the air is like for a fish.

The delicate gills are not properly supported when they are out of water and often cling or stick together. Prolonged periods out of water can damage gills permanently. 

And fish are most in need of oxygen at the end of a fight. Fish kept briefly out of water may not die, but could suffer brain damage as as result of lack of oxygen, he says.

ON THE WATER — Fish that are to be released should be netted, and the net kept in the water, says Chan. Remove the hook and pull away the net to free the fish. 

If a fish is showing signs of exhaustion, it should be revived, by moving it lengthwise back and forth in the water, he says.

The motion forces water across the gills, increasing the transfer of oxygen. With a big fish caught in warm shallow water, it may be necessary to spend two or three minutes giving resuscitation, Chan says. A fish should never be released unless it can swim away under its own power.

If you want to take a picture of a fish, be ready, Chan says. The person with the camera shouldn't spend long times focusing and getting set as the fish waits out of water.

The fish should be well supported, quickly lifted, then returned to the water within two or three seconds. Ideally, the picture can be taken with the fish mostly in the water.

Use barbless hooks

Comprehensive research shows the risk of injury to fish is reduced with the use of barbless hooks, Heise says. A barb disrupts more flesh going in as well as coming out. The open wound is an entry point for pathogens.

Hooks sometimes cause injury in places other than a fish's mouth. A hook buried in a fin or the fish's muscle can better be removed if there is no barb.

Even a fish hooked in the eye can survive if hooks are barbless. The eye will be lost, but studies have shown trout can function with only one eye. As long as structures and veins around the eye are not damaged, the fish can live.

But hook injury that causes substantial bleeding in any area but the mouth indicates the fish is badly wounded and likely will not survive, Heise says. Hooks in the gills are always fatal.

ON THE WATER — Pinch the barbs, particularly on smaller hooks that are more easily ingested, or hooks constructed so that they penetrate deeply, Chan says.

Any hook buried in a part of the fish that could cause significant injury if removed should be left.

A hook in a throat will not hinder the fish and will rust away or be expelled in a short time. Pulling a deeply embedded hook free will almost certainly kill the fish, even if the hook is barbless, he says.

In lakes, steams and rivers where retention of fish is not allowed, anglers need to take extra care to ensure hooks do not cause lethal injury.