Breeding (spawning)

By late summer and early autumn the adult fish have lost their sleek and silvery appearance and begun to take on their spawning colours. The females are drab brown and grey and are swollen with eggs. In contrast, the males become brown and red in colour and develop a characteristic hook ('kype') on their lower jaw.

Spawning normally begins in mid-October and continues through to early January. The timing and duration of spawning is a river and stock-specific character. The eggs are buried in structures called 'redds' that are constructed in the gravelly shallows of streams and loch margins. The female uses her tail to construct a shallow pit in the gravel. The male fertilises the eggs as they are discharged into the excavation. The female then covers the fertilised eggs with gravel.

Egg number and size increase with female size, as does the depth at which they are buried. The eggs hatch after 100-150 days incubation. Nourished by their yolk sacs, the resulting alevins remain in the gravel for a further 35-50 days until they emerge as free-swimming fry. Fry emergence usually takes place during the early spring.

Following spawning, the spent adults are called 'kelts'. Many sea trout kelts are taken by predators (e.g. otters and mink) or die due to other natural causes. Those that survive the rigours of spawning quickly regain their silvery coloration and return to the sea to resume feeding. These fish may then go on to make several further spawning migrations.

Once spawning has taken place it continues annually for as long as the fish lives. Sea trout from upland rivers tend to have a longer life span than their lowland cousins and may make several spawning visits. Sea trout have been known to spawn up to thirteen times.

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