1. Much attention has been devoted to explaining the spatial distribution of foraging animals, but rather little to their temporal distribution (i.e. whether they are diurnal, nocturnal or crepuscular). Many animals face predictable diel cycles of food availability or predation risk, and so the approach of measuring the relative ratio of mortality risk to food gained (the ‘minimize μ/f’ rule) can be applied equally as well to different time periods of the day as to alternative food patches or habitats. 2. This method is used here to investigate the diel activity patterns of juvenile Atlantic salmon, which have previously been shown to become increasingly biased towards nocturnal activity in winter, hiding for much of the day in streambed refuges. Calculations based on published data show that nocturnal foraging in winter is far safer per unit of food obtained than is diurnal, despite greatly reduced food capture efficiency at night‐time light levels. 3. Using an automated activity monitoring system based on passive integrated transponder (PIT) tags, this study shows that winter diel activity patterns in salmon are dependent on food availability. A change in food density led to a parallel change in time spent in the refuge, but (as predicted by the μ/f rule) the effect was greatest at the time of day with the least favourable ratio of predation cost to feeding benefit. Thus an experimental increase in food availability led to a 16% reduction in time spent in nocturnal foraging but a 98% reduction in time spent foraging by day, with fish spending only 0·6% of the daylight hours out of the refuge at the highest food density. 4. However, brief daytime foraging bouts had a major impact on growth rates (presumably because feeding efficiency was much greater in daylight), especially when food was scarce. Daytime feeding was thus profitable in terms of rapid food acquisition but normally suboptimal in terms of risk of predation. 5. Daily activity patterns are therefore suggested to be the result of a complex trade‐off between growth and survival, which takes account of diel fluctuations in food availability, food capture efficiency and predation risk; individual variation in the extent of diurnal feeding in salmon may result from state‐dependent differences in the benefits of rapid feeding and growth.

Summary1. Animals can reduce the competition for a limiting resource by temporal segregation, whereby individuals exploit the resource at different times. However, the pay-offs may vary predictably over time, and it can be predicted that (a) more dominant competitors should gain access to resources at the preferred times and (b) the degree of temporal segregation will vary with the intensity of competition. 2. Here we show experimentally that individual brown trout Salmo trutta (L.) made sequential use of foraging areas, with dominant individuals feeding mainly at the most beneficial times of dusk and the early part of the night while more subordinate fish fed at other times. 3. However, the degree of overlap in foraging times between high-ranking fish was dependent on energetic demands. At low temperatures (when requirements were low) the temporal activity patterns of top-ranking fish were synchronized, with foraging concentrated at the preferred times. In contrast, when temperature was raised to increase energetic requirements, activity patterns showed strong temporal segregation: the most dominant fish remained predominantly nocturnal, whereas second-ranking fish became increasingly diurnal. 4. This is the first experimental demonstration of shifts in the daily pattern of activity caused by varying intensity of intraspecific competition.

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Animal species have usually evolved to be active at a speci¢c 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 Atlantic 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 detect food more easily by day than by night, but the risk of predation is greater. Nocturnal foraging should generally be preferred, but the greater the need for growth, the greater should be the shift towards diurnal activity. In line with this prediction, all ¢sh were predominantly nocturnal, but salmon preparing to migrate to the sea, which would experience size-dependent mortality during the forthcoming migration, were more diurnal than ¢sh of the same age and size that were delaying migration for a further year. Moreover, the proportion of activity by day was negatively correlated with body size within the intending migrants. It has previously been shown that overwinter survival in ¢sh delaying migration is maximized not by growth but by minimizing exposure to predators. As predicted, daytime activity in these ¢sh was correlated with the prior rate of weight loss, ¢sh being more diurnal when their risk of starvation was greater. To our knowledge, these are the ¢rst quantitative demonstrations of state-dependent variation in the timing of daily activity.

We measured the breeding performance, body condition, time budgets and foraging ranges of Kittiwakes Rissa tridactyla at Sumburgh Head, Shetland, in two years of contrasting food availability. Kittiwakes in Shetland generally feed their young almost entirely on sandeels, and fisheries data indicated that stocks of sandeels in Shetland waters were at least ten times higher in 1991 than in 1990. Fledging success of Kittiwakes was nil in 1990 and 68% of eggs laid in 1991, although clutch-size and hatching success were no different between years. Post-hatching foraging trips in 1991 were of comparable duration to those recorded at other colonies in conditions of good food supply (2-3 h), while trips recorded during incubation or post-hatching in 1990 were approximately three times longer on average than at corresponding stages of the breeding season in 1991. Radio-tracking data indicated that adults generally stayed within 5 km of the colony in 1991 but flew more than 40 km from the colony on each trip in 1990. Eggs were apparently not left unattended in either year, despite the fact that this required adults to incubate for periods in excess of 4. 4 h in 1990. The extent to which adults were able to increase trip durations, foraging ranges and incubation shift lengths between years, while maintaining hatching success, indicates the degree to which Kittiwakes are normally buffered against adverse feeding conditions during incubation. Reduced nest attendance and lower body-condition of adults post-hatching in 1990. in conjunction with complete post-hatching breeding failure, indicate that adults were beyond the limits of their buffering capacity during chick-rearing in 1990.

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Passive integrated transponder (PIT) tags potentially allow the automated recording of the movements of individual animals. Here we describe a system designed to log the activity patterns of fish that use a discrete refuge when they are not feeding. The system was demonstrated with juvenile salmonids, which in winter seek shelter during the day in crevices under stones in the streambed but emerge to feed at night. The PIT tag antennae monitored a tube connecting a refuge to a standard hatchery tank and recorded each movement of a fish in or out of the refuge. This method allowed the quantification of individual time budgets for a group of fish sharing the same tank. The efficiency of the system was high, but occasionally the direction of movement could only be ascertained by referring to the previous position of the fish. The technique revealed very synchronized movement by Atlantic salmon Salmo salar out of the refuge at dusk and into it at dawn and intermittent brief excursions from the refuge during daylight hours. The system could be adapted for any species that is large enough to be given a PIT tag and that uses a refuge or burrow (e.g., for breeding or predator avoidance).