Intra-season variations in distribution and abundance of humpback whales in the West Antarctic Peninsula using cruise vessels as opportunistic platforms

Fine-scale knowledge of spatiotemporal dynamics in cetacean presence and abundance throughout the Antarctic Peninsula (AP) is lacking yet essential for effective ecosystem-based management (EBM). We used cruise vessels as platforms of opportunity to investigate an important area both for migratory humpback whales (Megaptera novaeangliae) and Antarctic krill (Euphausia superba) fisheries to assess potential spatiotemporal interactions, for future use in EBM. Whale observations were collected using tourist cruise ships as platforms of opportunity during the austral summer of 2019/2020. Data were analyzed using both traditional design-based line transect methodology and spatial density surface hurdle models to estimate the abundance and distribution of whales in the area, and to describe their temporal dynamics. The latter were fitted using a set of physical environmental covariates (sea surface temperature, ocean filaments, bathymetry and their derivatives). Our results indicate that few humpback whales are present in the Bransfield and Gerlache Straits in early December, however, their abundance increase rapidly to late December in the northern Gerlache Strait, reaching a stable abundance by mid-January. The distribution of humpback whales appeared to change from a patchier distribution in the Gerlache Strait to a significantly concentrated presence in the northern Gerlache and southern Bransfield Straits, followed by a subsequent dispersion throughout the area.  Depending on the modelled abundance, estimates agreed well with previous literature, increasing from approximately 7000 individuals in 2000 to a peak of 19 107 in 2020. Based on these estimates, we project a total krill consumption of between 2.0 and 5.2 million tons based on traditional and contemporary literature, respectively. Based on our results and catch data in the study area, we conclude that there is minimal spatiotemporal overlap between humpback whales and krill fishery activity during our study period of November – January. However, there is potential for significant interaction between the two later in the feeding season, but cetacean survey efforts need to be extended into late season in order to fully characterize this potential overlap.