A pelagic paradox: the ecology of a top predator in an oceanic desert

Abstract

Sperm whales (Physeter macrocephalus) are widely distributed in all oceans, but principally found in areas of high primary productivity. Historically, they were whaled extensively in the Sargasso Sea and recent surveys have also found large numbers there. However, the Sargasso Sea is an oceanic gyre considered to be low in productivity. This dissertation explores the paradox of a high abundance of large top predators in a body of water once described as an “oceanic desert”. First, I compared the diet of sperm whales in the Sargasso Sea to those off Dominica, in the eastern Caribbean. Results suggested differences in trophic ecology between these two areas, with sperm whales in the Sargasso Sea feeding at a higher trophic level. Second, I examined the spatial and temporal distribution of sperm whales in the Sargasso Sea in relation to environmental variables using acoustic surveys and autonomous recording devices. Sperm whale prevalence around Kelvin seamount, part of the New England Seamount Chain, was higher in the spring compared to the winter. Habitat modeling results suggest that the mesoscale activity associated with the Gulf Stream plays an important role in sperm whale occurrence in this area, likely due to the enhancement of primary productivity in this region. Finally, I estimated the current density of sperm whales in the northwestern Sargasso Sea and compared their present distribution to their distribution during the open-boat whaling era (1775-1921). Sperm whale density in the northern Sargasso Sea is one of the highest found globally, showing that this region remains a hotspot for sperm whales. The area where sperm whale detections per unit effort is presently the highest showed little overlap with areas where whales were hunted historically. Whalers all but ignored this region except when transiting to other whaling grounds, perhaps a result of fixed whaling patterns due to the conservative use of knowledge at that time. My dissertation highlights patterns and processes that help to explain the presence and abundance of sperm whales in the Sargasso Sea and demonstrates the importance of western boundary currents, such as the Gulf Stream, to the distribution of marine top predators.