Multi-year occurrence of sei whale calls in North Atlantic polar waters

The sei whale is a large baleen whale, in the same genus as the blue whale, found in most of the world's oceans outside the Arctic. From 2009 to 2014, we searched acoustically for sei whales off the northeast coast of Greenland on the edge of the Arctic Ocean. We did this by deploying hydrophones (underwater microphones) and recording ambient sound, then searching the resulting recordings for calls of sei whales. We heard many sei whale calls every summer in regions 600 km (400 mi) north of where they were known to routinely occur. We do not know if these detections surprisingly far north are due to changes in conditions there, as the Arctic is warming rapidly, or to the use of acoustic techniques. Past surveys mostly used visual methods, which may not work as well for a hard-to-see whale in the rough seas of the area. We also heard sei whales at our more easterly recording sites, near Svalbard (Spitsbergen), where warmer and saltier water moves north from the Atlantic Ocean toward the Arctic Ocean.