Wild Trout in Lakes
An immense variety of lakes are inhabited by wild trout. On the cols of the highest mountains they live in small rocky tarns, or deep pools filling the ice gouged floors of high level corries. Lower down the slopes are valley lakes, long, narrow, deep and - as the valley meanders - sinuous. These were carved out by the ice sheets that once covered much of the wild trout's geographical range. When the glaciers retreated, great piles of rubble known as morraine were left blocking the entrances to these valleys: natural dams behind which water was retained.
Where there are flat peaty moorlands, as over much of Canada, the northwestern United States, Lapland and northwestern Britain, lakes have formed in hollows set in rock outcrops and depressions in the peat. In such countryside, a complex of lakes may occupy a greater area than the dry land. Close to Achmore on the Isle of Lewis in the Outer Hebrides you can see thirteen such pools at once, all within a mile or so, without moving your head, and you can catch wild brown trout in each of them. Close to the Finnish town of Rovaniemi the lakes and pools are so numerous that it would be-impossible to visit them all in one summer month.
By contrast, the lakes found in the Irish plain are set on major river systems, in shallow rock and peat hollows. These lakes are nowhere of great depth, but may be large in area and broken up by inlets and bays and a scattering of islands and skerries. Lough Corrib, in County Galway, is reputed to have 365 such islands-one for every day of the year. But if two people count them independently the figures they arrive at are always different.
With so much variety among lakes, it is not surprising that the fish themselves vary tremendously. This is the result of several interacting factors: the quality of the lake water and its plant nutrients, which determine the nature and quantity of the trout food; the presence or otherwise of food species such as large crustaceans and small forage fish; the ecological niches within the lake and the behaviour of the trout in these niches; and the genetic composition of the trout. In broad terms, variation among lake trout is determined by the available food supplies and the genes of the trout themselves.
Unlike river trout, which obtain the bulk of their food from the flow at or close to the water surface, lake trout must swim in search of their food. Only a small proportion is gathered at the surface. The vast majority of lake trout seek out the more productive shallow water at the lake margins or amongst skerries and islands in the lake. Here they are likely to find the most plant growth and the water animals that feed off the plants. Some lakes produce large numbers of crustaceans, such as freshwater shrimps, hog-lice and crayfish. Such a diet provides a high level of carotenoids-the orange-red pigment found in crustaceans-and promotes the brightest coloured trout, with vivid red or pink markings on the fins and scales and a 'salmon-pink' flesh colour. Where there is little in the way of crustacean food, the diet is dominated by insect-life or small forage fishes and the trout tend to be drabber: reds and pinks are fainter and the flesh is usually white or pale grey.
In larger lakes it is possible to find trout in several-different niches, often with different coloration. Some of the trout might patrol areas of weedbed or shallow rocky shores, where aquatic snails and crustaceans are abundant. These fish will tend to be brighter in colour than trout that lie in the mouth of inflowing streams and feed mainly on aquatic insects: other trout in the same lake may feed almost entirely on other fish. These will tend to have the plainest coloration of all.
So, although it may be possible to identify a lake trout purely on the-basis of colour, an individual gillaroo, Kamloops rainbow or Yellowstone cutthroat may well be different ingeneral coloration from others in the same lake. Some gillaroos are much less red than others; some Kamloops trout have much brighter pink scales and fins than others; some Yellowstone cutthroats are a much darker brown than others. Yet despite this variation it is usually possible to recognize a particular variety because the underlying features are always present.
The behaviour of trout in lakes is directly linked to feeding. Some trout, including lake rainbow trout and sonaghens, feed predominantly on zooplankton in the open deep water, especially in summer. Since these zooplankton often occur in localized high concentrations, the lake trout tend to shoal, since a large group of trout hunting together are more likely to find such food than an individual fish.' By contrast some of the predatory, fish-eating trout tend to be solitary; a shoal of small forage-fish are more easily stalked by an individual than by a shoal. The majority of trout concentrate on taking a wide range of invertebrate foods from the shallow lake margins. They form loose associations or shoals and cruise over vast areas in search of their insect, mollusc and crustacean foods. However, some margin-feeders, notably the gillaroo are more specialized: they occupy small areas of the lake bed, which they scour for food rather like a river trout in its lie.
Most lake trout, however, will, change their behaviour during the year, or even from day to day. In early spring, when zooplankton is scarce, all the trout may be concentrated around the lake margins. As the zooplankton populations build up in late spring many trout will leave the margins for the deep water plankton clouds. Then, as autumn rains cool the lake and the zooplankton stocks decline, the deep-water trout move back either to the margins or to the mouths of feeder streams. Even in mid-summer a shoal of trout that are feeding on zooplankton, many feet below the surface, may change behaviour and diet rapidly. An evening fall of insects on to the water surface will cause them to rise; or torrential rain may result in a spate of the feeder streams, so the trout will move there to gather food carried by the floodwater and, as they do, they take up lies like river trout.
The Size of trout varies enormously from one lake to another. This is clearly seen in the table below, which gives measurements of fish from lakes in the British Isles. The fish were measured from tip of the snout to the fork in the tail (using scale readings):
Such measurements give a clear indication of the productivity of each lake: it is clear that Red Tarn, a high level corrie in the English Lake District, is very unproductive, while Loch Lanlish and Lough Derravaragh are highly productive. But why should some lakes produce fast-growing wild trout?
Latitude, which is sometimes held to be an influence, evidently has little effect. The highly productive lochs of Sutherland are in the far north of Scotland; while Malham Tarn, set in the Yorkshire Moors, is one of the most southerly natural trout lakes in Britain and also produces sizeable fish. In North America, some Alaskan lakes produce larger trout than lakes in the central Rockies; and in continental Europe some Alpine lakes produce smaller trout than lakes in the subarctic regions of Norway and Sweden.
Another common theory is that the fewer trout a lake contains, the larger on average they will be: the inference being that fewer trout means more food per fish and therefore more growth. So strongly has this belief been held that anglers have introduced predatory pike into some trout lakes in an attempt to thin out the tiny fish so that those that remain have more food and will grow bigger. Elsewhere trout have been netted out in the hope that those that remain will grow larger.
These efforts have largely proved futile, because in reality the trout that are left cannot take all the food that would have been taken by those that were weeded out. In such lakes the food is so sparse that the energy expended in seeking the extra food offsets the gain: the trout may eat more, but they do not convert the extra to flesh. They were on a subsistence diet before the cull; after the cull the remainder still do not have enough food readily available for rapid growth. In any case, as soon as the remaining fish breed, the stock is replaced by more small fish. The problem is simply that the lake does not produce enough food for fast growth, no matter how many trout are in the lake.
Others have argued that the trout in some lakes are genetically adapted for more rapid growth. There is some evidence for this: the Pyramid Lake variety of cutthroat was large; but other strains introduced into Pyramid Lake never attained such size, perhaps because they lacked a gene for rapid growth. Elsewhere, the results have often been different. Tiny wild brown trout have been netted from some lakes where the food production is low and the trout correspondingly small, and put into nearby lakes where the productivity is high. The result: the tiny stunted trout suddenly begin to grow rapidly. So the growth rate, and thus size, of wild trout in a lake probably depends mainly on the amount of food produced by the lake.
To understand why one lake produces more food and bigger trout than another, we must examine the food chain, or food web, in those waters. At the base of the chain, as with all food production, are the green plants which feed the herbivores, which in turn are eaten by the carnivores in the lake. The trout itself is a carnivore, feeding mainly on lesser herbivorous animals in the water such as midge larvae, water snails, mayfly nymphs and many other creatures. The more herbivores in the water, the faster the growth of the trout. But the herbivores in turn rely on the plant life of the lake for their food. Thus the more plant life in a lake, the more food for the herbivores and the more herbivores for the trout. The key to a productive trout lake is its ability to produce lush plant life.