Temporal scales of foraging in a marine predator

Ecology, March, 1996 by I.L. Boyd

INTRODUCTION

The way in which predators search their environment for prey and the rules they use to determine how long they should spend in a specific location have received considerable attention in the last two decades (for reviews see Stephens and Krebs 1986, Mangel and Clark 1988). Many predictions rely on the assumption that predation of patchily distributed resources is a rate-maximizing process, i.e., that predators will adjust the time they spend within a patch according to their expectation of locating a new and richer patch when they move on. Although the exact responses of a particular predator to changes in the spatial distribution of prey may not be fully understood, temporal variation in predator behavior is likely to provide an insight into the spatial distribution of a highly dynamic prey source that may be difficult to track in other ways (Mangel and Adler 1994). This has particular relevance in pelagic marine systems because, unlike most other systems, it is difficult to measure key variables by direct observation. This presents a challenge, which can be met using instrumentation and new statistical approaches to measure behavior and to use this to interpret food distribution. In general, we have a poor understanding of the spatial behavior of top predators in pelagic marine systems, and this approach is essential if progress is to be made in understanding the foraging behavior of many open-ocean consumers.

In the past, attempts have been made to model the interactions between trophic levels in Southern Ocean food chains based largely upon the assumption of short, homogeneous food chains (e.g., May 1979, May et al. 1979), but more recent approaches have begun to consider the influence that the temporal and spatial dynamics of this marine ecosystem can have upon the pathways of energy flow (Croxall et al. 1988, Murphy et al. 1988, Priddle et al. 1988). Antarctic fur seals (Arctocephalus gazella) are top predators within this system and, at South Georgia where they are most abundant (Boyd 1993a), they feed mainly on krill (Euphausia superba) (Croxall and Pilcher 1984, Doidge and Croxall 1985; Reid, in press) that occur in dense swarms in the seas over the continental shelf (Priddle et al. 1988). Periodic reductions in the prey available to fur seals at South Georgia appear to occur every 47 yr (Croxall et al. 1988, Lunn et al. 1993). These conditions result in increased pup mortality, reduced pup growth rates, reduced birth rates, and low weaning masses of pups (Croxall et al. 1988, Lunn et al. 1993). Such reductions in the rate of acquisition of prey also result in an increased foraging trip duration by females (Croxall et al. 1988), showing that individuals adjust their foraging behavior to the changing conditions. However, this index of prey availability provides no information about the causes of the decline in the rate of prey acquisition during those years of poor reproductive performance. One possibility is that large-scale (hundreds of kilometres) changes in oceanographic conditions result in fewer krill being present within the foraging range of the fur seals (Heywood et al. 1985, Priddle et al. 1988), but an alternative hypothesis is that the spatial dynamics of krill at both meso- and fine-scales (hundreds of metres to tens of kilometres and tens of metres to up to hundreds of metres, respectively) make them more difficult for fur seals to exploit. Such responses may be the result of changes in hydrographic conditions in the oceanic mixed layer or variation in the demographic structure of the krill populations.

Although female fur seals can dive to [greater than]100 m to feed, they can probably only exploit krill efficiently in the upper 30 m of the water column (Boyd and Croxall 1992; Boyd et al., (1995). In addition, the dispersion of krill within swarms is also likely to affect the foraging efficiency of the fur seals. Therefore, the response of fur seals in terms of the frequency with which they encounter patches of prey, the time they spend within patches, and the time they spend traveling between patches, should reflect the dispersion of prey. For example, when prey patches are more dispersed we would expect fur seals to spend longer traveling between patches. In these circumstances fur seals would be expected to remain longer within patches if they are attempting to maximize the average rate of energy return during foraging (Krebs et al. 1974, Charnov 1976). Alternatively, if the dispersion of prey patches remains constant but there is a decline in patch quality, then individuals would also be expected to remain longer in patches, but there would be no change in travel time between patches.

During lactation, female fur seals alternate periods of 3-5 d at sea when feeding occurs, with periods of 1-2 d ashore when pups are fed (Boyd et al. 1991). Precise records of the diving activity of these females during their foraging trips to sea (Boyd and Croxall 1992, Boyd et al. 1994) can be obtained using electronic time-depth recorders. In species like fur seals, diving is thought accurately to reflect foraging activity because these species remain at the surface for much of their time at sea and dives are, therefore, excursions from the surface to forage. Consequently records of diving provide a measure of foraging activities. This contrasts with species such as elephant seals (LeBoeuf et al. 1988), which appear only to surface to replenish their oxygen supply and in which many activities in addition to feeding occur while they are submerged.