The Lake Brown Trout

A bag of a dozen brown trout from one lake - whether it be a peaty lochan in the Hebrides, a deep rocky loch from the Scottish highlands, a Cumbrian lake such as Windermere, an Irish lough or a Swedish tarn - will often exhibit an exhilarating range of colour. Some may be dark, almost black, with hardly any red or black spots. Others appear as if fresh-run from the sea: silver with small black spots. Between the two extremes is the characteristic, brownie with dark brown back, golden yellow belly, red adipose fin, bright red spots and black speckles; one of the most beautiful of fish.

Where such variation occurs in one lake the fish have possibly been taken from different niches within the lake. The darkest, almost black, trout are usually found in holes close to the bank: perhaps where inflowing streams enter the lake through deep-cut peaty beds. Those that inhabit sandy bottomed areas of shallow water often exhibit a pale sandy brown coloration. Those with the more typical brown trout colour are usually taken from the weedy lake margins. And the lighter silvery trout are often caught in deep water, where they feed on animal plankton.

The fish are camouflaged in their niches, and their coloration can change if the trout settle into a different background. It appears that trout can alter their pigmentation through a mechanism that works through the nervous and endocrine systems, triggered by light playing on the eyes. Trout that live in underground streams where there is no light at all, as in the limestone caves and potholes of the Yorkshire Dales and Alps, are usually colourless. However, blind fish are always dark in colour and this probably accounts for the mention, in many angling texts, of horrid, old black trout that refused to rise to the surface fly.

Diet as well as habitat influences the trout's coloration, especially in determining the extent and intensity of the colours red and orange. Fish caught in deep water, with bold red spotting and pink flesh, are presumably feeding on tiny planktonic crustaceans like Daphnia. If caught in the shallow margins they are probably feeding on freshwater shrimps. Such crustaceans, which have massive amounts of the orange pigment carotein in their tissues, provide much of the red pigment in wild brown trout. Trout that never eat crustaceans have subdued red spots and pale flesh.

There are almost certainly more wild brown trout living in lakes than in rivers, where they often face competition from introduced trout. But nearly all brown trout spend some of their lives in rivers, for most lake trout spawn in the rivers entering or draining the lake. They begin their move towards the spawning streams in September, when the autumn rainfall swells the river flow. At this time quite huge concentrations can be found near the mouths of inflowing streams. The shoals Move upstream to spawn, usually during November or December. The lake trout and river trout often share the same spawning areas, but it is not known whether lake and river trout interbreed. If they do, then we must consider that the river and lake brown trout are of the same stock.

The adults return downstream, but it is unusual to find brown trout parr in a lake, which suggests that the young trout do not enter the lake until they are at least two years old. This is borne out from observations on the smallest of lakes, where the feeder streams are mere trickles. These tiny feeder streams may be crammed with small brown trout parr, but where the water deepens as the stream enters the lake the parr are absent. Possibly the deep stillwater of the lake is too dangerous a place for small trout parr. There are often predators there: divers (loons) and grebes, goosanders and mergansers, perch and pike, even large trout. In the deep open water of a lake the parr may be far more vulnerable.

So as fry and parr, lake trout live aide by side with river trout. We do not know why some parr eventually move to the lake while others remain in the river. Is it by conditioning or by genetics, by chance or by choice? Do some brown trout become lake trout onne year and river trout the next? Although scientists have been studying wild brown trout for a century and more, and anglers have watched their habits for hundreds of years, many questions still remain unanswered.