The Ferox Trout

To some naturalists a ferox or great lake trout is simply a large brown trout that feeds primarily on fish. To others it is a species of trout in its own right, the Salmo ferox. But all agree on the main characteristics of the ferox: a big wild trout that is found in natural lakes in Europe; with a big head and grotesquely long, broad powerful jaws that bear some resemblance to the pike's; a fairly plain silvery or dull brown coloration, possibly with a greenish or olive hue; a fish-eater. To these physical features we must add one other ferox characteristic: great longevity.

Most of the British and Irish lakes that appear to hold ferox trout tend to be fairly unproductive: lakes like Lochs Morar, Ness, Arkaig, Awe and Quoich in Scotland, Ullswater and Bassenthwaite in Cumbria, Loughs Eask and Melvin. in Ireland and Llyns Padarn and Peris set amidst the mountains of Snowdonia. In these waters a typical brown trout caught on rod and line will be less than a pound in weight and between three and five years of age. But very occasionally the angling press will announce the capture of a trout weighing up to ten pounds or more.

There is considerable evidence that the number of big ferox trout has declined during this century. In the 1800s they were not uncommon, but a wild ten-pounder is now a rarity. This may be related to the decline of the Atlantic salmon and sea trout, on which the ferox feed. Most ferox lakes have adults of these species passing through them to spawn in the lakes' feeder streams. After spawning, the much-weakened kelts drop back to the lakes; and two or three years later the sea-going smolts pass through the lake on their way to the sea. These are the ferox's prey; and the great reduction in numbers of these rich sources of food has presumably contributed to the scarceness of big ferox.

In a study of Lough Melvin, biologists from the Queen's University, Belfast showed that the ferox trout is genetically different from the other forms of trout that inhabit that lake. They spawn with other ferox trout in separate streams from the other trout that live there. Furthermore, Andrew Ferguson and his co-workers showed that it is possible to identify, by genetic finger-printing, tiny ferox trout that may superficially resemble ordinary brown trout. On these grounds Ferguson argued, in the Went Memorial Lecture given to the Royal Dublin Society on 20 November 1985, that the ferox in Melvin were a full species in their own right that had evolved from a single colonisation of the lough follow'mg the last Ice Age.

As yet, we have no similar studies from other ferox lakes, so the biological identity of the ferox remains a puzzle. If ferox are merely overgrown carnivorous trout, why do some trout become fish-eaters and grow to great age and size while the vast majority do not? But if ferox are members of a separate species, why are there no records in the angling literature of small ferox trout?

Only the most tentative conclusions can be drawn at present. Briefly, within the brown trout stocks of some lakes are genes which confer on their possessors the ability to grow to great age and to great size, and a behavioural trait to turn almost exclusively to a diet of fish once they have reached a certain size. Other trout, the majority, do not have these genes; they have a much shorter lifespan and although like all trout they will feed keenly on very small fish fry at times, they will not change their feeding pattern to concentrate on hunting fish.