Prey distribution and dynamics have a strong effect on the foraging behaviour of marine predators. Increasingly sophisticated logging devices allow us to measure the behaviour of these predators while they are foraging. In this chapter we show how these behavioural measures can be used to provide insight into the dispersion and patchiness of krill using Antarctic fur seals and macaroni penguins as example predators. We illustrate how a sound understanding of their ecology and the constraints on their foraging is needed to interpret these data. Examples are provided of how simple measures such as the organization of diving and dive depths have been used to detect differences in the distribution of krill both temporally and geographically.

The food web of the Southern Ocean is dominated by Antarctic krill Euphausia superba. Krill occur in loose aggregations and dense swarms, both of which may extend in size from a few metres to several tens of kilometres across (Miller & Hampton 1989). Krill abundance is also subject to large seasonal and annual fluctuations (Brierley et al. 1999). Despite the high levels of spatial and temporal variability, in some areas krill occur in predictable quantities such that large populations of top predators – including penguins, seals, whales and commercially harvested fish species – rely on it as their main food source.

The highly productive oceanic region around South Georgia supports numerous predators that depend on krill as their principal prey. This includes an estimated 2.75 million macaroni penguins (Trathan et al. 1998) and over 3 million Antarctic fur seals (Barlow et al. 2002).