A pervasive goal of biological oceanography has been to understand the processes that structure pelagic communities. Past research suggests that oceanic species distributions are influence by physical and biological variability at a variety of spatial-temporal scales. The objective of this research is to test the hypothesis that the dispersion and community of top predators vary in concordance with large-scale variability in physical structure and ocean productivity in pelagic ecosystems. To test this hypothesis, an inter-disciplinary survey of marine bird and mammal use of distinct oceanographic domains in the Southern Indian Ocean will be conducted. This Small Grant for Exploratory Research will provide the opportunity to place two U.S. scientists on board a French research vessel near Reunion Island. The French scientists will sample the physical environment and estimate oceanic productivity while the U.S. scientists will survey top predator distributions across a 35 degree latitudinal gradient in physical and biological properties from subtropical to sub Antarctic waters. The primary hypothesis to be addressed is that top predator assemblages are structured by spatial gradients in hydrographic properties and ocean productivity patterns known to influence the distribution and patchiness of their zooplankton, fish and squid prey. It is hypothesized that the overall abundance of marine top predators within a specific oceanic domain is largely determined by ocean productivity. Also, it is hypothesized that the energetic costs of foraging determine which types of marine top predators inhabit specific oceanic domains. Species with high foraging costs are constrained to exploit dense prey aggregations within highly productive areas. Conversely, taxa with low foraging costs are able to inhabit low productivity areas, where they exploit more dispersed prey resources. To test these hypotheses, we will quantify the spatial association of top predator assemblages with specific water masses and the aggregative response of top predators at hydrographic and bathymetric domains. Because top predators respond to oceanographic variability at multiple scales of time and space, we will assess their responses to habitat variability at two specific spatial scales. A variety analytical methods will be used, including compositional analysis of coarse-scale habitat preferences, generalized additive models, recurrent group analysis, ordination of hydrographic data and top predator assemblages and the measurement of top predator aggregation. The research will address spatial variability in pelagic food webs within the context of large (1000s km) and coarse (10s km) scale hydrographic and ocean productivity patterns in the subtropical and sub Antarctic Indian Ocean. This inter-disciplinary perspective will enhance our understanding of the way physical and biological processes structure pelagic communities in the southern Indian Ocean and will provide a model that has broader implications for the oceans as a whole.
