The migratory sea trout

The migratory sea trout

One of the great arguments among fish scientists over the past 150 years has been the relationship between the various kinds of trout in Europe. From a situation where up to a dozen types of trout were recognized in Britain, the pendulum swung to the opinion that there was only one species which varied considerably in response to local environmental conditions.

Echoes of this argument can still be heard: it has recently been claimed that Lough Melvin in County Sligo, Republic of Ireland contains three species of trout distinct from the trout in the rest of the British Isles!

As any angler who fishes for wild trout can confirm, the fish are remarkably different in appearance depending on whether they come from a Scottish loch, a Devonshire moorland stream or the west coast of Ireland.

The sea trout is not a separate species, but a migratory form of brown trout. As such it can be confused with both the brown trout and the salmon, having the body shape of the former and the colouring of the latter.

The sea trout has a thick wrist to the tail fin and the long upper jaw bone of the brown trout, contrasting with the slender wrist and shorter jaw bone of the salmon. Brilliantly silver in colour, the sea trout has more black spots than the salmon, particularly on its cheek and gill covers.

First dig your hole

Having entered fresh water during the summer, sea trout spawn between October and January — the eggs are laid in fine gravel well upstream. The spawning redd is dug by the female who bores into the surface and then dislodges the small stones with powerful sweeps of her tail. In contrast to salmon, which usually die after spawning, sea trout spawn year after year.

Hatching takes place after six to eight weeks, but the young trout stay buried in the gravel for a further month or so before emerging and starting to feed.

At this stage they are indistinguishable from young brown trout, but a year or two later they drop downriver towards the estuary, where they become silvery smolts. Why they do this – when ordinary brown trout stay where they are – is a mystery.

Seaward bound

Some of these young fish stay in the vicinity of the estuary and return upriver after only a few months – they are known locally as whitling, finnock, herling or slob trout -but very few of them spawn. Most, however, swim out to sea to feed on shellfish and other creatures for at least a year. Sea trout do not make the same long journey to the high seas as salmon, but stay in the coastal waters in search of rich feeding grounds. For example, the large numbers of sea trout off Norfolk have mostly come from Yorkshire and other north-east coast rivers to feed on the schools of sprats and sandeels.

Bloodhounds of the sea

Like salmon, sea trout are strongly imprinted with the scent of the river in which they were born, and most return to their home stream to spawn. However, the number of wanderers is greater with sea trout than with salmon. This accounts for the occasional appearance of sea trout in rivers in which it does not breed – the Thames and Medway for example.

Migratory species have suffered greatly from pollution in the lower reaches of rivers and from the construction of dams, weirs and locks. Recently populations of sea trout seem to have declined in areas where they were at one time extensive — a worrying development.