A 26-Year Record of Seasonality and Interannual Variability in Marine Mammal Sightings From Northern Marguerite Bay, Antarctica.

Whales and seals are an important and often conspicuous component of the Southern Ocean pelagic fauna. Some species have been studied extensively, but almost exclusively in the austral summer, and winter data for any species are sparse. Here we report the results from a 26-year record (1998-2023) of year-round weekly observations of marine mammals in Ryder Bay, an inlet of northern Marguerite Bay on the western Antarctic Peninsula. Humpback Whales are summer visitors and can be present any time between December and May, though a few individuals may remain over winter. Minke Whales and Killer Whales are present year-round, as are the three ice-obligate phocid seals, Crabeater, Weddell and Leopard Seals. A few (1-12) Weddell Seal pups are born in most years. Southern Elephant Seals are seasonal visitors, with variable numbers hauled out to moult from November to June; a small number (1-6) of pups are born occasionally. Antarctic Fur Seals are present from February to June, often in large numbers, typically leaving once fast ice starts to form. All species were seen in pack-ice concentrations ranging from open water to close pack, but none were seen once extensive fast ice developed. In those species that were present year-round, sighting frequency was lower in winter than summer, suggesting a seasonal movement away from nearshore waters. Sighting frequency of Humpback Whales has increased over the study period, with whales arriving earlier by ~18 days per decade, and staying longer. Over the same period, sightings of Minke Whales have become less frequent, and those of Killer Whales more frequent. These changes are likely linked to shifts in ice dynamics driven by regional climate change along the western Antarctic Peninsula.