Food availability and the nocturnal vs. diurnal foraging trade‐off in juvenile salmon

Abstract

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.

Partitioning of Time as an Ecological Resource

“…Species that can be either nocturnal or diurnal can be used to test whether diel activity patterns respond to variations in predation pressure (Metcalfe et al 1999). Many fishes can change their activity pattern from nocturnal to diurnal and vice versa, usually on a seasonal basis (Sanchez-Vazquez et al 1996, Yokota & Oishi 1992.…”

“…As temperatures drop they increasingly seek refuge during the day in crevices and emerge at night (Metcalfe et al 1999). Higher temperatures imply greater energetic requirements, hence the diurnal activity at higher temperatures and more temperate zones.…”

“…Higher temperatures imply greater energetic requirements, hence the diurnal activity at higher temperatures and more temperate zones. Increased food levels enabled juvenile salmon to minimize their exposure to predators by preferentially reducing daytime (in contrast to nighttime) activity (Metcalfe et al 1999).…”

“…Salmon preparing to migrate to the sea, which would experience size-dependent mortality during migration, were more diurnal than fish of the same age and size that were delaying migration for a year (Metcalfe et al 1998). Dominant brown trout (Salmo trutta) preferred to forage during hours that are thought to minimize the predation risk incurred per unit of food obtained (Metcalfe et al 1999). Thus risk of predation governs not only the activity patterns of dominant individuals, but also those of lower-ranking individuals that they displace (Alanara et al 2001).…”

Nature and Functions of Cover for Riverine Fish.

“…Daytime predation risk from diurnally endothermic predators (birds and mammals) has been suggested as the major factor governing diurnal concealment (VALDIMARSSON and METCALFE, 1998;METCALFE et al, 1999). It also reduces downstream displacement and physical injury during freshets or ice formation (BUSTARD and NARVER, 1975;TSCHAPLINSKI and HARTMAN, 1983).…”