Early learning

Abstract

The growth in the technology and experience of rearing farming fish of several species has prompted thoughts that hatchery-reared fish might be used to restock over-fished and thus heavily depleted natural populations. Most attention so far has focused on salmon and trout but results have generally been very disappointing and the practice has drawn anger from many ecologists, concerned about their impact on local wild populations. Initial attempts have revealed two key problems. Hatchlings that have been reared in featureless tanks with pelleted food readily available, appear to be ill-prepared for the transition to patchy live food back in the wild. And some reintroduction projects have paid little heed to local genetics and used parent stock often from widely varying sources. Not surprisingly, the results of many such reintroductions have been dismal. Fish naturally suffer high mortality in their larval and juvenile stages but mark–recapture experiments suggest that as little as two per cent of hatchery-released fish survive for any substantial time. A report of new work (Proc. R. Soc. series B., published online) holds out hope that this situation can be improved. Victoria Braithwaite at the Institute of Evolutionary Biology at the University of Edinburgh, and Anne Salvanes of the University of Bergen, have studied the behaviour of young cod reared in four different hatchery environments. One of these represented the standard homogeneous rearing conditions but the others either just varied the feeding regime or the physical environment in the tanks to include rocks, gravel and kelp, to greater match the normal nursery grounds of developing cod. The fourth regime included both feeding and physical environment variability. After a period under these conditions the researchers tested the fish’s response to a number of behavioural tests, including the ability to find live prey and response to a simulated predator. The fish reared under the fourth regime clearly showed greater behavioural flexibility, finding live prey and recovering from potential threats more quickly. The researchers believe that fish reared under the relatively simply enriched environmental conditions used in the fourth regime of their experiment may prove to have a much better chance of survival in the wild than those under standard conditions. And they may soon have a clue: large numbers of marked salmon reared from locally caught parents under either standard hatchery conditions or with both physical and food variability were released in the west of Scotland last spring. Although the life cycle of the fish out at sea is very variable, the first returnees may be expected this spring. All eyes will be on the appearance of the marked fish to see whether a little environmental enrichment can help fish get a head start in the tough conditions found in the wild.