Diel shifts in community composition and feeding of juvenile fishes in the pelagic area of a large shallow lake

Diel cycles of changing ambient illumination have been shown to have strong influence on fish community composition in freshwater systems, mainly due to diel habitat shifts of fish between sheltered and more exposed, but profitable sites. Low information is available, however, from diel patterns of fish community composition and feeding in the central areas of large shallow lakes where lake depth and diameter may hamper diel migrations. We studied whether there are diel differences in species abundance and shifts in feeding modes of the juvenile pelagic fish species over an 8 months sampling period in a shallow lake. The strong decline of perch numbers towards the night suggests low nocturnal activity for this species. In contrast, the nearly constant numbers of roach over the full diel cycle points to pronounced activity under changing light intensities. Increased ruffe numbers at night reflect the good adaptation of this species to feed efficiently at low light intensities. Niche segregation of fish species along the diet axis was low. There was low evidence that diel shifts in the fish community composition were attributed to diel horizontal migrations of species into or out of the pelagic zone. Because availability of preferred zooplanktonic prey was high, differential diel activity patterns of species reflect their genetically fixed, albeit varying adaptations to feed under low-light conditions, instead of being an active avoidance to reduce competition.