Changing nutritional status causes a shift in the balance of nocturnal to diurnal activity in European Minnows

1. For animals that have access to a predator-free refuge, the daily timing of feeding activity should reflect temporal variation in both the risk of predation while foraging and the rate of food intake: the preferred period of activity should be that which minimizes the predation risk per unit of f ood obtained. 2. However, several species of freshwater fish show flexibility in the dail y pattern of feeding activity. European Minnows Phoxinus phoxinus L. prefer to feed at night at low temperatures, spending the majority of the day hid ing in refuges. Since their feeding efficiency is lower at night, this beha viour is thought to minimize exposure to diurnal predators. The fish become increasingly active in the day as the temperature increases, possibly beca use their food requirements cannot be met by only feeding at night. 3. Here, it is tested experimentally whether nutritional state can affect t he daily activity schedule, by manipulating the energy reserves of over-win tering minnows and recording their relative night- and daytime use of refug es. 4. Well-nourished fish were often observed hiding in refuges, especially du ring the day. As their energetic reserves decreased, they were more likely to be seen out of the refuges both by day and by night. However, the change in activity was greater by day, so that the fish became increasingly diurn al as their nutritional reserves declined. This effect was independent of t emperature. 5. The results support the hypothesis that the fish attempt to minimize the ir exposure to diurnal predators, but must balance this against the need to increase their level of daytime foraging activity when in energy deficit.