The Factors Conditioning the Rising of Trout (Salmo trutta) in a Small Fresh-Water Lake

THE problem of the has long occupied the attention of observant anglers. It is obvious vthat a proper understanding of the factors controlling the characteristic behaviour of trout, which at one minute may be quite quiescent and the next minute be rising eagerly at flies which have been on the surface for some time, would be of considerable practical value. It is the sudden changes of behaviour which take place with no obvious reference to external climatic conditions that have presented the problem. The greater part of the food of trout (Salmo trutta L.) is obtained below the surface of the water, in the form of insect larvae, crustaceans and snails, the nature of the food depending on the habitat of the trout and the season of the year (Pentelow, 1932; Slack, 1934). Surface food, mainly insect imagos, forms only a very small percentage of the total food consumed, being as Allan (1936) aptly puts it, more of a savoury, the underwater food being the roast beef. Rising of trout therefore involves perhaps more than a change of diet; it may be a change of behaviour, the temporary forsaking of one habit for another. Behaviour in the lower vertebrates appears largely to be conditioned by external environmental factors, hence a study of the physical and chemical conditions prevailing before, after, and during the rise may give some idea of the state of environment most favourable for the exhibition of this type of behaviour. The various factors which have been held responsible for the rising of trout have been summarized by Allan (1936). These have been suggested as