Abstract
The spatio-temporal variability in marine resources influences the foraging behaviour and success of top marine predators. However, little is known about the links between these animals and ocean productivity, specifically, how plankton density influences their foraging behaviour. Southern elephant seals (Mirounga leonina) have two annual at-sea foraging trips: a two month post-breeding foraging trip (Nov – Jan) that coincides with elevated summer productivity; and an eight month post-moulting foraging trip (Feb – Oct) over winter, when productivity is low. Physical parameters are often used to describe seal habitat, whereas information about important biological parameters is lacking. We used electronic tags deployed on elephant seals during both trips to determine their movement and foraging behaviour. The tags also recorded light, which measured the bio-optical properties of the water column, the bulk of which is presumably influenced by phytoplankton. We investigated the relationship between plankton density and seal foraging behaviour; comparing trends between summer and winter trips. We found a positive relationship between plankton density and foraging behaviour, which did not vary seasonally. We propose that profitable concentrations of seal prey are more likely to coincide with planktonic aggregations, but we also acknowledge that trophic dynamics may shift in response to seasonal trends in productivity. Seal prey (mid-trophic level) and plankton (lower-trophic level) are expected to overlap in space and time during summer trips when peak phytoplankton blooms occur. In contrast, aggregated patches of lower trophic levels are likely to be more dispersed during winter trips when plankton density is considerably lower and heterogeneous. These results show that southern elephant seals are able to exploit prey resources in different ways throughout the year as demonstrated by the variation observed between seal foraging behaviour and trophic dynamics.
Highlights
The Southern Ocean (SO) is one of the world’s most productive oceans, supporting a highly dynamic and heterogeneous marine ecosystem where food resources are patchy in both time and Seasonal resource influence on predator foraging space (Arrigo et al, 2008)
A seasonal interaction term was retained in the dive effort model (Table 1), suggesting that dive effort response to plankton density was influenced by the time of year
In the past it has been difficult to investigate the linkages between lower trophic levels and the foraging behavior of deep diving predators because concurrent data were often lacking, in the polar regions
Summary
The Southern Ocean (SO) is one of the world’s most productive oceans, supporting a highly dynamic and heterogeneous marine ecosystem where food resources are patchy in both time and Seasonal resource influence on predator foraging space (Arrigo et al, 2008). Many marine predators have evolved migratory patterns that allow them to adjust their foraging behavior in relation to seasonal variability (Costa et al, 2012). Physical parameters used to identify meso-scale eddies (potential sites of elevated productivity) have been related to southern elephant seal (Mirounga leonina) foraging activity (Bailleul et al, 2010). These studies often do not directly assess the links existing between these physical structures, the biological activity associated with them (e.g., plankton aggregations) and the foraging strategies of top predators, due to the paucity of data on lower and mid-trophic levels in the SO. Biological information would prove invaluable for fitting such habitat models, but these data are difficult to obtain
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