Diel feeding periodicity, daily ration and prey selectivity in juvenile brown trout in a subarctic river

Feeding of age-1 brown trout Salmo trutta in a third-order river in norther n Finland was usually highest in the twilight hours and lowest around midda y. Diel periodicity in food intake was less distinct and rarely significant for age-0 trout. Daily rations declined seasonally, being lowest in Octobe r, and highest in June (age-1 trout) or early August (age-0 trout). Prey se lection did not differ between day and night, but differences between age c lasses and sampling dates were distinct. Age-0 trout preferred Ephemerella nymphs in summer and Micrasema larvae later in the season. Age-1 trout fed selectively on caddis larvae on all sample dates. Aerial insects and Baetis nymphs were avoided by both age classes on most occasions. These patterns of preference are probably explained by a trend towards epibenthic feeding, which may be a predominant foraging mode in some trout populations. Nevert heless, crepuscular peaks in feeding periodicity suggest that trout were ab le to capitalize on the increased availability of drifting prey in the twil ight, especially in the early summer months. (C) 1999 The Fisheries Society of the British Isles.