Thirty-five years of cetaceans in a canyon: patterns, shifts, and surprises in the Gully marine protected area

The Gully, a large submarine canyon on the edge of the Scotian Shelf, was made a marine protected area (MPA) in 2004. Together with 2 neighbouring smaller canyons, it has a particularly high density and diversity of cetaceans. We compiled sightings made from vessels during summer months between 1988 and 2023 to describe temporal and spatial trends of occurrence for 9 commonly sighted cetacean species in the Gully MPA and nearby waters. We looked for correlations between species as well as potential drivers of trends. Sighting rates increased for Sowerby’s beaked whales Mesoplodon biden and long-finned pilot whales Globicephala melas ; decreased for humpback whales Megaptera novaeangliae , Atlantic white-sided dolphins Leucopleurus acutus , and striped dolphins Stenella coeruleoalba ; while northern bottlenose Hyperoodon ampullatus and fin whales Balaenoptera physalus showed U-shaped trajectories. White-sided dolphins were more abundant in June, when water temperatures are cooler, whereas most of the other species showed a preference for the warmer months. Blue whales Balaenoptera musculus were most abundant with sea temperatures of ~18°C. The northern bottlenose whale is exceptional in that a large portion of the population is in the Gully at any time, there was no seasonal variation in sighting rates, and the inter-annual U-shaped trend can be linked with some confidence to reduced anthropogenic impact following the introduction of the MPA. For the other species, drivers are more speculative but include changes in relative resource abundance due to oceanographic and ecological processes, climate change, and variation in anthropogenic impacts.