Abstract
Little information exists on the current status of Southern Hemisphere sei whales ( Balaenoptera borealis ). We assessed their distribution and abundance along the west coast of the Falkland Islands (southwest Atlantic) during February and March 2018, using line transect and nonsystematic surveys. Abundance estimates were generated for a single survey stratum using design‐ and model‐based approaches. Sightings of sei whales and unidentified baleen whales (most, if not all, likely to be sei whales) occurred from the coast to the 100 m depth isobath that marked the offshore boundary of the stratum. The modeled distribution predicted highest whale densities in King George Bay and in the waters between Weddell Island and the Passage Islands. Sei whale abundance was estimated as 716 animals ( CV = 0.22; 95% CI [448, 1,144]; density = 0.20 whales/km 2 ) using the design‐based approach, and 707 animals ( CV = 0.11; 95% CI [566, 877]; density = 0.20 whales/km 2 ) using the model‐based approach. For sei whales and unidentified baleen whales combined, the equivalent estimates were 916 animals ( CV = 0.19; 95% CI [606, 1,384]; density = 0.26 whales/km 2 ) and 895 animals ( CV = 0.074; 95% CI [777, 1,032]; density = 0.25 whales/km 2 ). The data indicate that the Falkland Islands inner shelf region may support globally important seasonal feeding aggregations of sei whales, and potentially qualify as a Key Biodiversity Area.