First insights into the oceanographic characteristics of a blue whale feeding ground in northern Patagonia, Chile

Blue whales Balaenoptera musculus select highly productive feeding habitats where dense and abundant euphausiid aggregations dependably arise. This study seeks to understand the oceanographic characteristics that provide suitable foraging conditions for endangered blue whales in northern Chilean Patagonia (NCP). We present the first in situ oceanographic data (temperature, salinity and oxygen profiles, nutrients and chl a, mesozooplankton abundance, euphausiid species and size structure) associated with sightings of foraging blue whales collected during 3 austral summer campaigns and 1 winter campaign (2011 to 2013) at 2 feeding sites in the southeast Corcovado Gulf (similar to 43 degrees 53' S, 73 degrees 19' W). Satellite data (MODIS) were also examined. 92% of all animals (n = 51) displayed foraging behavior, and sighting rates were higher in summer. The mesozooplankton was dominated by the euphausiid Euphausia vallentini. During summer 2012, significantly lower whale sightings coincided with significantly lower euphausiid abundance, chl a and silicate and observations of large localized salp blooms, compared to summer 2013. These conditions could be explained by changes in the inflow of oceanic and continental waters into the Corcovado Gulf leading up to summer 2012. A generalized linear model revealed that sightings of foraging blue whales during summer 2013 were significantly associated with a drop in chl a, most likely reflecting a synoptic succession pattern (days to weeks) of phytoplankton blooms-euphausiid aggregation-arrival of blue whales at a feeding site. Spatial variation suggests the important role of bathymetry in concentrating planktonic organisms. These findings are the first insights into the oceanographic factors that influence feeding conditions for endangered blue whales in Patagonia.