Skills school for fish

To recognise both food and predators, farmed fish need to be given survival tips before release into the wild. But how do you train them? Simon Hadlington reports from class

Dr Culum Brown is a pioneer in teaching life skills. His aim is to take his charges, who have spent their life in a closed, limited environment, and help them adjust to the rigours of life on the outside. To teach them to avoid trouble and fend for themselves. To inject a bit of street wisdom into these institutionalised, naïve creatures.

But these aren't long-term prison inmates or hospital patients. They are fish. Fish that have been born in hatcheries, grown in artificial surroundings and fed a diet of high-protein pellets.

Across the world, around 300 species of fish are farmed – mostly for food. However, there are specialist hatcheries that breed fish to be released into the wild, to build up failing natural stocks. These are genetically compatible with wild stock, and screened for parasites and diseases before release.

For many years, conservationists and fisheries' biologists have tried to use farmed fish to bolster dwindling stocks of their wild counterparts. However, fish reared in captivity have little idea how to survive in the wild. Now, Dr Brown, who is based at the University of Edinburgh, has demonstrated the possibility of training some species of fish to recognise potential items of food in the wild, and avoid natural predators. Intriguingly, it not only appears that individual fish can be given these "skills", but that the fish, once trained, can demonstrate to other fish how it should be done.

"Just about all the commercial species of fish in the world are under threat," says Dr Brown. "Many are reared in hatcheries, and it should be possible to use hatchery fish to help replenish wild stocks."

But there is a problem. "It is thought that out of every hundred juvenile fish you release into the wild from a hatchery, around only three will survive into adulthood," says Dr Brown. "It's assumed that animals are pre-programmed to survive in their natural environment, but this isn't the case. Just as humans learn by experience, animals develop different behaviours, or fine-tune them through learning and memory."

In a typical hatchery, producing salmon or trout, the fish see nothing but thousands of other fish. The temperature and flow of the water remain stable, while the food (pellets) and feeding times are regular.

In the wild, though, a trout or salmon will find a sheltered position in slack water in a stream or river, so it will not have to fight the current, and will be concealed from predators. As a particle of food drifts past, the fish dart out and eat it. Food is variable and consists of different invertebrates.

"Farmed fish, however, are used to food arriving at the surface of the water, so they often take up a position high in the water in the middle of the river," says Dr Brown. "They waste energy fighting the current and leave themselves exposed to predators. Because they recognise pellets as foodstuffs, they largely ignore anything that does not resemble a pellet. Within a few weeks, they will starve or be eaten by a predator."

So Dr Brown, together with Dr Kevin Laland, of the University of St Andrews, and colleagues at the University of Helsinki, started to think about ways to improve the chances of the fish surviving in the wild. "We knew from earlier work with guppies that fish can learn from experience, so we wondered if we could devise some sort of training programme for commercially important fish to prepare them for life outside."

The British researchers chose the Atlantic salmon to work with. The first series of experiments aimed to demonstrate that a young fish could be trained to recognise live prey. Single fish in a tank were presented with live bloodworms, dropped in at the surface of the water. "Some fish became frightened and backed away – not the best response to a food item," says Dr Brown. Other fish simply ignored the bloodworm, even if it touched the fish's head or drifted down in front of its nose. In a small proportion of cases, however, the fish would investigate the object, eventually eating it.

It took 20 or more presentations of bloodworm for some fish to learn that it was food and start eating it consistently. So it is possible to train a fish to recognise a new item of food. But researchers did something more subtle. As the fish were trained to accept the bloodworm as food, another fish was placed in the tank, separated from the first by a transparent partition. It could watch its neighbour learn to take new food.

Then, and in about 80 per cent of cases, when the fish that had only watched its neighbour was presented with a bloodworm, it would take only one trial for it to accept the bloodworm as a food item. "The fish had learned by observation," says Dr Brown.

The scientists also taught fish to recognise that food could be present at the bottom of the tank, rather than just on the surface. Trained fish, or "demonstrators", were put into a tank with naïve fish. Bloodworm were presented at the bottom of the tank. The demonstrator began eating. Within a short time, the rest of the fish began feeding. The naïve fish learnt this in less than half the time than would have been the case if they had been trained individually.

This phenomenon, called social learning, could open the way to training hatchery-reared fish to survive better in the wild, Dr Brown believes. Fish trained to recognise items of prey could be put in with a larger number of naïve fish, and the prey items introduced. This could be done shortly before the fish are released into the wild.

"One could imagine training 20 demonstrators and putting them in with 500 fish for two or three exposures before releasing them," says Dr Brown. "This would be more efficient and cheaper than training the population with live prey, which is expensive."

The Finnish scientists have been working with a number of commercial species, such as the whitefish, to help them recognise predators. A tank was divided into two by an opaque but porous partition. On one side were a number of juvenile whitefish. On the other was a natural predator of the fish, a pike-perch. The partition was removed, allowing the pike-perch to attempt to attack the whitefish, who took rapid evasive action before the pike-perch was removed.

"The idea of the partition being porous was to allow chemical cues from the predator to reach the prey before attack," says Dr Brown. The fish exposed to the predator were placed among naïve counterparts. When a pike-perch was introduced, the demonstrator fish recognised the danger and became visibly cautious – an effect that swiftly spread throughout the school.

"The studies demonstrate that social learning occurs in commercially important fish," he adds. "We would like to scale the experiments up to see if this approach could be used with larger numbers of fish."