The Bull Trout

A variety of brown trout that was almost wiped out by pollution, the bull or 'slob' trout was classified as Salmo eriox, a species in its own right, by the early 19th century taxonomist William Yarrell in A History of British Fish (1841). Yarrell identified it on the basis of quite small anatomical features in the gill cover and tail structure. When other naturalists obtained specimens, the arguments began. Some remarked that the bull trout they had examined were ordinary sea trout; others that they were river brown trout. In retrospect it seems that the published descriptions of the bull trout - though precise enough in terms of body structure - missed one vital point: the most distinctive feature of the fish- was its behaviour.

In the 19th and early 20th century, the bull trout was frequently seen in three rivers of northeast England and southeast Scotland: the Tweed, Tyne and Coquet. Writing in 1885, Scottish game angler Lord Home wrote: 'The bull trout has increased in numbers in the Tweed prodigiously within the last forty years, and to that increase I attribute the decrease of salmon trout.... The bull trout take the river at two seasons. The first shoal come up about the end of April and May. They are then small, weighing from two to four or five pounds. The second, and by far the most numerous shoal, come late in November. They then come up in thousands, and are not only in fine condition, but of much larger size, weighing from six to twenty pounds.'

Close observation finally revealed that the bull trout is a river brown trout that migrates downstream after spawning to live in the tidal estuary. Unlike sea trout, the bull trout does not go through a smolt stage, because it can cope with the brackish waters of the estuary without making the physiological adaptations needed for survival in sea water. Bull trout normally take on a sandy coloration which blends with the colour of the estuary water and substrate, just as river and lake brown trout become camouflaged to the waters that they inhabit.

Where the river is very clean from source to estuary, bull trout parrs will move down to the estuary to feed in the brackish water just as other brown trout move to the lower freshwater reaches. Clean river estuaries are often more productive than the higher freshwater reaches, with shoals of sea fish fry, shrimps, shore crabs, molluscs and marine worms. So trout that do move down to the estuary grow much larger and attain a more portly bull-like shape than those that remain in freshwater.

Where an estuary becomes polluted, no trout can stay there through the spring and summer months, even though sea-going trout might be able to run quickly to and from the open sea. So few rivers in Europe have estuarine bull trout today. Some, like the Tyne, lost virtually all their bull trout when the estuaries became polluted early in the 20th century. But where pollution has been cleaned away, bull trout are gradually returning to some estuaries. Today they are occasionally seen not only in the Tweed, Tyne and Coquet, but in the clean sandy rivers of north and west Scotland, Wales, northwest England, Ireland and Norway.