Nocturnal Foraging in the Stone Loach (Barbatula barbatula): Fixed or Environmentally Mediated Behavior?

ABSTRACT

The hypothesis was tested that nocturnal foraging in the stone loach (Barbatula barbatula), a small benthic dweller in temperate rivers and lakes, gradually shifts to daytime foraging when fish are hungry and no acute predation risk is present. Foraging activity in stone loach always remained significantly higher during the night compared to the twilight and the day, independent of food availability, even after the fish had lost more than 20% of their initial body weight. The absolute levels of daytime activity significantly decreased, while nocturnal movements significantly increased. Activity stopped almost completely when a predator was present. Contrary to previous studies on epi-benthic or pelagic species, for which a gradual shift from nocturnal to daytime foraging has been demonstrated, stone loach did not adopt daytime foraging even when there was no acute daytime predation risk and the fish were starving. This indicates that in stone Ioach an anticipated rather than observed predation risk is taken into account when estimating the predation risk at any time of the day. Such a strict behavior might be especially important for benthic dwellers with a low swimming speed, and therefore escape potential, in relation to fast moving daytime predators.