XII.—The Reproduction and Fecundity of the Powan, Coregonus clupeoides Lacépède, in Loch Lomond, Scotland

SynopsisAdult powan, Coregonus clupeoides Lacépède, in Loch Lomond show regular diurnal and seasonal migrations, connected normally with either feeding or reproduction. As the breeding season approaches, most of the adult population gather in deep water (usually in the vicinity of the areas of wave-washed gravel and small stones in shallow water, which form the main spawning grounds). The male fish start to move on to these grounds nightly towards the end of December each year; at the end of this month, but mainly during January, the females ripen and join them for spawning. This is completed by the beginning of February and the population then gradually disperses over the rest of the loch. The present fecundity estimates, carried out for female powan during two seasons, are in general agreement with those found for Coregonus elsewhere in the Palearctic region. They give an average of 5750 eggs per adult fish, the mean fork length of which was just over 27 cm. Though there is a positive correlation between the numbers of eggs produced and the size of the females, it is not a strong one. After being laid, the eggs immediately undergo a heavy mortality and are eaten in considerable numbers by trout, Salmo trutta L., powan, roach, Rutilus rutilus (L.) Phryganea larvae, and various other animals. The adult powan too are preyed upon extensively at this time in certain parts of Loch Lomond by pike, Esox luciusL. The reproductive ecology of the powan shows several basic similarities to that of other Salmonidae in the British Isles, the main features fitting well with what has been postulated about the origin and distribution of the family in this country.