Including krill consumption by humpback whales in winter in the Spatial Overlap Analysis in Subarea 48.1

The Spatial Overlap Analysis (SOA, previously named Risk Assessment) provides a means of distributing the catch limit for Antarctic Krill (Euphausia superba) both temporally and spatially. One of the caveats of the SOA is that it is not able to account for missing data. Whilst it is generally agreed that cetaceans remain in Subarea 48.1 into the winter months, abundance estimates are not available during this period and consequently we have been unable to develop a data layer for the consumption of krill by baleen whales during winter to be included in the SOA. We use tracking data from humpback whales tagged in the Gerlache Strait to model their distribution at the Antarctic Peninsula from April to July and use date of departure to estimate monthly abundance as a proportion of their summer abundance. We also implement a sensitivity analysis whereby we assume 100%, 50% and 25% of summer abundance is present from April to July. We show that including krill consumption by humpback whales in winter has only a marginal effect on the spatial and temporal distribution of krill catch, although baseline risk increases. In contrast, we show that the layer describing the distribution and density of krill in winter is highly influential to the outcome of the SOA. We recommend that both the summer and the winter krill layers are updated to include data collected across a larger proportion of the study area if such data are available. We also reiterate that although the SOA can be useful in its ability to subdivide the krill catch limit, many caveats remain in its structure and implementation.