Wild Sea Trout

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An Atlantic sea trout at night, in the shallow headwaters of its natal river.


Although the vast majority of trout spawn in the gravel of river beds, the sea-going gene may well be present in all trout varieties. It can indeed be argued that all trout are sea trout by ancestry because, over the past 10,000 years or so, most of our rivers and lakes were colonised by trout from the sea. Thus, the argument goes, those trout that live their entire lives today in freshwater are just sea trout that do, not go to sea, as did their ancestors. Furthermore, the genes controlling the sea-going habit can still be activated in some lake and river trout. When brown trout from Loch Leven were put. into inland waters of New Zealand and the Falkland Islands, they soon produced sea-going progeny that subsequently colonised other lakes and rivers.

The process continues. today, as artificial stocking grows more common. In parts of northern. Europe, river rainbow trout have already produced the sea-going steelhead: as recently, as 1988 a 14 Ib steelhead was caught in a stream in northern England. This fish had, presumably, developed from an introduced rainbow trout which had run to sea and then, after two years of sea-feeding, made a spawning run back into fresh-water. Similarly, in the northeastern USA and southeast Canada, introduced brown trout have spawned a strain of the Atlantic sea trout. Thus it is quite possible that, when an angler catches a large steelhead on the Columbia River or Fraser River, or a big Atlantic sea trout in Norway or Scotland, the parents of that fish were not sea-going, but freshwater trout.

Once the adaptation is made, however, it is almost impossible to detect the origins of an individual, for sea trout are remarkably consistent in shape and colour. The steelhead, sea-run cutthroat and Atlantic sea trout vary in size, depending upon the duration of sea-feeding and the richness of their feeding grounds; but they do not have the variety found in lake and river forms. Steelheads are similar from Alaska to the Gulf of Mexico; Atlantic sea trout vary in little but size from Western Ireland to Scandinavia. The distinctions once made between the eastern' sea trout of England, the 'Cambrian sea trout' of Wales and the 'western' sea trout of Ireland and western Scotland reflected differences of size and 'build': the consequences of diet rather than genetic variation.

For a special strain or subspecies of trout to evolve, the population must be isolated from all others over a long period, so that selective pressures can mould them into a distinct form adapted to their own particular water. At sea, no such isolation of a separate trout population can occur: the steelheads or sea-run cutthroats born in one river will mix freely with those from adjacent rivers. And although most sea trout eventually return to spawn in-the river of their birth, some do not. Thus there is a small but continuous interchange of genes from one population of sea trout to the next.

However, all sea trout are clearly separated from the freshwater forms of their own species by their life history. Steelheads spawn in late winter and spring, but for cutthroats and Atlantic sea trout the breeding season is usually during early winter, in October, November and December, when the adult sea trout pair up in small gravel-bedded streams. After a brief courtship the female cuts into the gravel with scything slashes of her tail to form a redd deep enough to hold the eggs. With the cock fish lying close alongside her, she sheds a stream of round pink eggs into the redd; the male then moves forward and ejects a stream of milt, which contains the sperm that fertilizes the eggs. The female then flicks loose gravel over the eggs.

Sometimes a female may cut two or more redds, so that if one is predated, or the eggs are lost because a second female sea trout or salmon cuts into the same gravel, or the redd becomes iced up, some eggs may still survive. After completing the act of reproduction the spent pair, now known as kelts, usually move off downstream into deeper water where they may rest for some time before heading back to the sea.

Some streams, or even parts of streams, are used by many pairs of sea trout, while other streams are ignored by all save, possibly, an occasional pair. On one major river system in northern England, just three becks hold the vast majority of the breeding sea trout, while scores of apparently similar streams have few. On one stream up to 1,400 pairs have spawned along a quarter of a mile reach. Likewise in the steelhead and sea-run cutthroat trout rivers of Oregon, Washington, British Columbia and Alaska huge numbers of sea trout spawn in particular river tributaries, and avoid others. Some sea trout share their spawning grounds with salmon, but in many rivers, the salmon choose different spawning areas from the trout.

Where there are spawning streams for trout and salmon close to large cities or towns poachers may take fish from the redds before they have spawned. Poachers can make a very quick profit from such streams and, in a short time, almost wipe out the fishery. So in many regions bailiffs must camp by the water to protect the fish when they are most vulnerable. Looking at the problem through human eyes, it would seem better for the fish to scatter throughout the river system. But presumably they have chosen these nursery areas over the years because there is an advantage in doing so: possibly the waters provide better conditions for the early stages of the sea trout life than other parts of the river.

It is difficult to estimate what proportion of sea trout kelts survive the long journey back to sea. But survival rates seem to be significantly higher than for the salmon, where mortality among cock kelts is go par cent or more, and among females at least 70 per cent for the Atlantic salmon and 100 per cent in the Pacific sockeye. Huge shoals of sea trout kelts are sometimes found in February and March resting in deep slow pools in the river's lower reaches; where there is a large lake on the river system, the kelts will often lie there as they recover from the trauma of spawning. In spring they move off to sea where they feed, restore wasted tissues and form more eggs or sperm before making another spawning run into freshwater. But there is evidence that fish that have once spawned never regain the condition they had when they first ran the river. Sea trout caught as they are moving upstream on their second spawning run are generally 10-20 per cent lighter in weight than fish of the same length making the journey for the first time, and the quality of their flesh is often very poor.