A method of observing the spawning behaviour of farmed and wild salmonids in a natural stream habitat
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When farmed fish escape from farms, they may compete with wild individuals for a number of resources, such as food, spawning sites, and mates. As a part of a larger study on genetic impact from farmed fish on wild populations, the spawning behaviour of released farmed brown trout was observed. A 6 by 4 m net enclosure with a steel frame (scaffolding) was placed in the middle of the spawning area of wild brown trout in River Øyreselv, and secured well to resist flows and strong currents. The lower part of the net was kept tight to the river bed by a steel chain and lead weights. A Seametrix underwater video camera with remote-controlled focus and tilt was set up in the middle of the enclosure, about 15 cm above the bottom. The camera was connected to a monitor and the behavioural sequences were recorded on a Thomson U-matic portable videocassette recorder. Five male and five female mature farmed brown trout were released within the enclosure together with wild spawners. Both wild and farmed fish were also studied outside the enclosure. The spawning behaviour of farmed trout is described and compared to the behaviour of wild brown trout. The farmed trout displayed basically normal spawning behaviour, though this was less vigorous than that of wild trout. The different behavioural patterns previously reported for spawning brown trout, such as courting and quivering, testing of the spawning substrate, digging, gaping, and release of milt were observed in the farmed brown trout. Aggressive behaviour and sneaking behaviour by small wild males were also recorded. Finally, viable offspring of farmed trout were collected.