Abundance and trends of Type B killer whales (Orcinus orca) around the western Antarctic Peninsula

The western Antarctic Peninsula (WAP) is warming rapidly and we need to understand the impact of these physical changes on the marine ecosystem. The WAP is surrounded by a complex marine food web involving a number of predator and prey populations, and empirical data on abundance and trends is required to understand these trophic dynamics. The apex predator inWAP coastal waters is the killer whale (Orcinus orca), represented in this area by three phenotypically, genetically and culturally distinct ecotypes (Types B1, B2 and A). Here we report on the movement patterns and abundance of both forms of Type B killer whales: a larger mammal-eating form (B1) that apparently specializes on hunting ice seals on pack ice floes, and a smaller, more gregarious form (B2) that has been observed to feed on pygoscelid penguins. We integrated satellite telemetry to describe movement (B1: n = 8 tags, median duration = 20 d; B2: n = 22 tags, median duration = 66 d) and photo-identifications (6 y; B1 = 8,925 photographs, B2 = 13,621 photographs) to estimate abundance in the coastal waters of the WAP during austral summers from 2008/09 to 2013/14. Both forms typically occurred close to the WAP coastline in the austral summer, with periodic long-distance migrations to sub-tropical waters (up to 4151 km from tagging site) for hypothesized physiological maintenance migrations. Both types were mostly sympatric in their distributions, but B1s typically occurred further south (extending as far as 69⁰S), whereas all tracks and encounters of B2s occurred north of 68⁰S. There were notable hot spots for B1 killer whales between Adelaide Island and the WAP mainland, and for B2s in the Gerlache Strait between the WAP and Anvers Island; both types were regularly encountered in the Weddell Sea around the northern end of the WAP. B1 killer whales had a higher re-identification rate (61% photographed in more than one year) within a highly connected social network compared to B2s (25% re-identification rate). A mark-recapture model allowing for temporary emigration from the study area indicated that B1s were stable in abundance, ranging from 39 to 53 whales using the study area per year from a small parent population averaging 50 whales (75% Probability Interval 46-57). In contrast, there was a high probability (p=1) that B2s were increasing in abundance with annual estimates ranging from 181-299, from a larger parent population averaging 502 (75% PI = 434-662).