Fronts and Fine-Scale Distribution of Three Cetacean Species within the Dynamic Mid-Atlantic Bight Shelf Break System

Erin LaBrecque1,2, Gareth L Lawson3 and Patrick N Halpin1,2, (1)DUML, Beaufort, NC, United States, (2)Duke University, Durham, NC, United States, (3)Woods Hole Oceanographic Institution, Department of Biology, Woods Hole, MA, United States
The Mid-Atlantic Bight (MAB) shelf break region is a highly dynamic and productive area that provides a wide range of habitat to many marine species over every trophic level. At least 23 cetacean species occur in the MAB shelf break region and their distributions are thought to be influenced by the MAB shelf break front. This research characterizes the spatial distribution of common dolphins (Delphinus delphis), Risso’s dolphins (Grampus griseus), and sperm whales (Physeter macrocephalus) through multi-dimensional scaling (MDS), classification tree analysis and random forest analysis of marine mammal line-transect survey data, multi-frequency active acoustic data and fine-scale in situ hydrographic data. Multi-frequency active acoustic data were broadly classified into proxies of middle trophic level groups through frequency response methods. Surface temperature fronts were observed in all sections of the shelf break region. The strongest surface fronts were within 15 km of the shelf break (~150 meter isobath) on 10 of the 19 cross shelf transects. MDS presented clear environmental distinction between common dolphins and Risso’s dolphins and sperm whales. Environmental separation between Risso’s dolphins and sperm whales was evident but less distinct. In both the classification tree and random forest analyzes, the common dolphin models had the least error (0.33 and 0.28 respectively). Depths less than 145 meters and area within 10 km shelf-side of the shelf break were the primary variables that described common dolphin habitat. Risso's dolphin habitat was selected as the area between 20 km shelf-side to 20 km offshore of the strongest surface thermal gradient. Offshore salinity and distances greater than 26 km to density fronts were the primary variables selected to describe sperm whale habitat. When mapped back into geographic space, these three cetacean species occupy different fine-scale habitats within the dynamic Mid-Atlantic Bight shelf break system.