Physical and ecological factors explain the distribution of Ross Sea Weddell seals during the breeding season

Weddell seal Leptonychotes weddellii populations can potentially serve as indicators of change in Southern Ocean food web structure, but tracking populations at regional to continental scales has so far been impossible. Here, we combined citizen science with remote sensing to learn about environmental and biological factors that explain fine-scale distribution of Weddell seal haul-outs in the Ross Sea, Antarctica. We employed the crowd-sourcing platform Tomnod (DigitalGlobe) to host high-resolution (similar to 0.5-0.6 m) satellite imagery of the Antarctic fast ice during November in 2010 and 2011 and asked volunteers to identify seals on images. We created a 5 km x 5 km grid of seal presence per year, and modeled habitat suitability for seals using a generalized linear model. The top Ross Sea-wide model that best explained seal presence included proximity to fast-ice cracks, deep water, and emperor penguin Aptenodytes forsteri colonies. This model also revealed that seal presence decreased with proximity to Adelie penguin Pygoscelis adeliae colonies and size of the nearest emperor penguin colony, suggesting the potential for trophic competitive exclusion by large penguin colonies. With respect to 3 sub-regions within the Ross Sea (North and South Victoria Land in the western Ross Sea, and Marie Byrd Land in the east), we found that 3 habitat variables differed in their effects among sub-regions: proximity to emperor penguin colonies, proximity to deep water, and relative ice width. Our results represent a step toward effectively monitoring Weddell seal population trends, and disentangling biological and environmental factors influencing locations of Weddell seal haul-outs around Antarctica.