Feeding behaviour of Atlantic salmon, Salmo salar L, and brown trout, Salmo trutta L, during the emergence related habitat change in semi-natural conditions

First feeding captures of Atlantic salmon and brown trout sac fry were analysed by stomach contents. Yolk sac fry samples of both species were obtained from wire mesh baskets containing 50 eggs stocked in gravel of four artificial outdoor parallel troughs fed by a natural stream. In two larger baskets with downstream trap, 100 eggs of each species were stocked for development monitoring. Simultaneous analysis of emergence timing, degree of yolk sac absorption, fry growth and stomach contents, showed that feeding took place after emergence but began with weight loss, before complete yolk sac absorption in the two species. Feeding behaviour occurred earlier in trout. In salmon, prey capture was preceded by a period of plant debris ingestion, which was associated with the position of the fry on the substratum.