Freshets and Fish

Abstract

The action of freshets on the runs of fishes in streams has been long known to man, but has never been scientifically studied. Angling for Atlantic salmon in the Margaree River, Cape Breton, was found to be dependent upon entrance of the fish from the sea which required heavy freshets. These freshets in 1935 made an abrupt transformation in fish present and in angling. In comparison with the neighbouring Cheticamp River, the tendency of the fish to enter chiefly late in the season, months after they had appeared on the coast, has been related to the difficulty for the salmon to enter through the strongly tidal estuary mouth until heavy rains come in the fall. Experiments with sharp, but not large artificial freshets in the Moser River, Nova Scotia, gave double the expected number of both salmon and brook trout entering from the sea and gave good angling when the temperature was not too high. There is evidence that at all stages salmon respond more or less to freshets by ascent of streams. Ascent occurs chiefly as the freshet is subsiding. Descent of salmon at all stages occurs with freshets but chiefly at the height of the freshet. The phenomenon is a general one and is probably not confined to fishes. In essence, it is the response of the organisms to displacement over the solid substratum by the fluid medium. In the fish, it is part of the rheotactic response, effected through sight, contact with the bottom and possibly the action of turbulences on the lateral line organs. Freshets effect migration of fish through such a stimulus to ascent, through carrying the fish down stream, and through breaking up the “homes” of the individual fish.

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