Video training to boost fish survival

Most hatchery fish die within two days of their release

Scientists are making videos to train fish reared in hatcheries to cope with life in the wild.

Hatchery fish are reared in a protected environment with a regular supply of food.

But they stand little chance of survival when, at the age of about six months, they are released into oceans and rivers to replenish diminished stocks or provide sport for anglers.

Most of them die within the first two days, and fewer than 5% make it to adulthood, according to studies of reared salmon.

But now scientists are devising training videos for hatchery fish that show fish of their species being devoured by a predator.

Suddenly they are released into the real world, and it is more than just a shock for them

Cullum Brown,
Piscatologist

Piscatologist Cullum Brown, from Cambridge University's department of animal behaviour, told BBC Radio 4's Today programme that hatchery fish were reared in a completely benign environment.

"The tanks are fairly large but there is nothing in them, no cover - weed, rocks or anything," he said.

"They have 12 hours light and 12 hours dark each day, a constant temperature and no currents.

"The fish are fed a commercial pellet food that is nothing like the kind of thing they are exposed to in the wild.

"Suddenly they are released into the real world, and it is more than just a shock for them."

Group training

As well as the instructional video nasties, teaching techniques created by the scientists include group training for fish.

One tactic is to put a clued-up demonstrator fish in a naive shoal and place a predator behind a transparent, porous screen.

The inexperienced fish learn from the reactions of the demonstrator fish that they should flee, and the sight and smell of the predator reinforces that reaction.

Mr Brown told Today: "Even some very short-lived preliminary exposure to predators makes a significant impact on their chances of survival once released."

Shock tactic

But University of Helsinki researcher Sampsa Vilhunen has devised a tactic with even more shock factor, according to the New Scientist.

He fed predator fish in a tank of Arctic charr, and then removed them.

Hatchery-raised Arctic charr were then placed in the water, and the mere odour - apparently from the faeces expelled by the terrified fish as they were hunted down - was enough to teach them to avoid the predator fish in the future.