The Atlantic Sea Trout

atlantic sea trout

An Atlantic sea trout. The bronze sheen reveals that this hen fish has spent  a week or two in freshwater; while at sea, the scales would be bright silver.
Specimaen study/watercolour

For the angler and naturalist, the Atlantic sea trout is quite different from the river or lake brown trout. When it returns from the sea it is a bright bar of silver, dripping with tide lice. Bigger specimens can never be confused with the freshwater trout: in fact, to the uninitiated, they more resemble the Atlantic salmon. Many an angler has shown off a 5 lb. salmon, only to learn that the square tail, long jaws, and structure of the anal fin show it to be a sea trout.

Naturalists of the 19th century described several 'species' of Atlantic sea trout. As well as Salmo trutta, the sea trout of Scotland and Ireland, they identified Salmo cambricus, the sewen or sewin of Wales, southern England and Scandinavia; Salmo gallivensis, the Galway sea trout; Salmo brachypoma, the eastern sea trout of the British rivers Tweed, Forth and Ouse; and Salmo argeneus, the silvery salmon of the Atlantic rivers of France.

These 'species' had little in the way of scientific merit, being identified. and described from very few specimens and on fairly tenuous grounds. The Reverend Houghton himself failed to obtain a specimen of the eastern sea trout despite much trying, and was forced to borrow GŁnter's pickled specimen from the Natural History Museum. Eventually all Atlantic sea trout were given the name Salmo fario; but modern fish biologists have considered that brown trout and Atlantic sea trout are one and the same species; so today they are officially clumped together as Salmo trutta.

Yet if, as seems likely, Atlantic sea trout are merely a migratory form of brown trout, the question remains: why do some become sea trout and not others? Obviously this is not a matter of a small trout deciding to visit the coast.Those trout parr that will become sea trout go through an extra stage of development that lake or river trout omit from their life cycle: the smolt stage. They develop saltexcreting glands and the characteristic silver coat of the smolt. This extra stage must be genetically induced. Genes from sperm and egg decide that, at two or three years of age, the individual will develop into a smolt and become a sea trout.

Furthermore, sea trout produce more sea trout. The evidence comes from hatchery stations, where returning sea trout are stripped of their ova and milt, and their progeny are raised in controlled conditions and later released to boost the output of natural spawning. These sea trout off-spring develop the smolt stage and head off to sea. Hatchery-raised brown trout do not: they grow from parr into river or lake trout without the smolt stage.

So it would seem that within some varieties of Freshwater brown trout, including the trout of the River Thames and Loch Leven, remain the latent genes of seagoing trout. Such genes could be inherited from the original stock of trout that colonised these waters from the sea, thousands of years ago. If so, these genes may require a trigger before they confer the sea-going trait.

Hunger could be enough. The greatest Atlantic sea trout rivers produce little food, and their resident brown trout tend to be tiny: rivers like the Welsh Dovey and Conwy, the Cumbrian Esk, the Border Esk, Scotland's Spey and Loch -Maree, some of the barren Irish and Hebridean lochs, and the glacier-fed rivers of western and northern Norway. Possibly it is scarcity of food, a near-starvation diet, that triggers the genes which turn the brown trout into a seagoing fish.