Nb. Metcalfe et al., STATE-DEPENDENT SHIFTS BETWEEN NOCTURNAL AND DIURNAL ACTIVITY IN SALMON, Proceedings - Royal Society. Biological Sciences, 265(1405), 1998, pp. 1503-1507
Animal species have usually evolved to be active at a specific time of
the daily cycle, and so are either diurnal, nocturnal or crepuscular.
However, we show here that the daily timing of activity in juvenile A
tlantic salmon is related to the life-history strategy that they have
adopted (i.e. the age at which they will migrate to the sea) and their
current state (body size/relative nutritional state). Salmon can dete
ct food more easily by day than by night, but the risk of predation is
greater. Nocturnal foraging should generally be preferred, but the gr
eater the need for growth, the greater should be the shift towards diu
rnal activity. In line with this prediction, all fish were predominant
ly nocturnal, but salmon preparing to migrate to the sea, which would
experience size-dependent mortality during the forthcoming migration,
were more diurnal than fish of the same age and size that were delayin
g migration for a further year. Moreover, the proportion of activity b
y day was negatively correlated with body size within the intending mi
grants. It has previously been shown that overwinter survival in fish
delaying migration is maximized not by growth but by minimizing exposu
re to predators. As predicted, daytime activity in these fish was corr
elated with the prior rate of weight loss, fish being more diurnal whe
n their risk of starvation was greater. To our knowledge, these are th
e first quantitative demonstrations of state-dependent variation in th
e timing of daily activity.