Size-dependent patterns of diel vertical migration: smaller fish may benefit from faster ascent

Diel vertical migration (DVM) in fish has been studied extensively on the population level, but the individual variability of migration behavior has been widely neglected. However, the individual size of fish modifies the ratio between mortality risk and foraging gain. Therefore, if the rhythmic habitat shifts are considered being adaptive, the average time spent in the nighttime shallow habitat should differ between small and large fish, suggesting that the individual migration trajectories might be size dependent. We studied size-related differences in the vertical migration speed and the onset of DVM in coregonid fishes (Coregonus spp.) in a deep lake. Stationary vertical hydroacoustics was conducted once a month over a 7-month period to track individual fish during the migration periods at dusk and dawn. Quantile regressions revealed that vertical migration speed was higher for smaller than for larger fish. The comparison of detection times of fish migrating upward after sunset revealed an earlier detection of small fish compared with the medium and large fish, thus suggesting that smaller fish ascended earlier and faster at dusk. The reverse trend at dawn was less clearly expressed. Bioenergetics calculations suggest that small fish may profit from a faster and earlier ascent by increasing their daily ration by about 8% relative to fish that would migrate together with the main population. Therefore, plasticity of migration speeds may result in individual competitive advantages and heterogeneous mortality-feeding trade-offs within populations. This variability in individual migration trajectories has to be considered in future studies addressing ultimate causes of DVM.