Indications of mesopelagic foraging by a small odontocete

The mesopelagic layer is represented in all oceans and is of crucial importance to the pelagic communities, and in this paper it is hypothesised that the Greenlandic harbour porpoise (Phocoena phocoena) is seasonally dependent on mesopelagic prey when abandoning the ice-covered continental shelf areas and remains in offshore areas. Data from 15 harbour porpoises instrumented with satellite-linked transmitters in West Greenland were analysed with regard to foraging that may target mesopelagic prey. Contact with the porpoises was maintained for an average of 404days where they conducted extensive offshore movements and spent an average of 90% of their time over deep waters (>1000m) in the North Atlantic. When entering deep water, they increased their daily travel rate significantly from 22.5 to 36.7kmd(-1). Five of the 15 porpoises provided information on dive depth which suggested that dive depths>100m are important for these porpoises both day and night; however, the porpoises dove significantly more at nighttime compared to daytime. Harbour porpoises from West Greenland probably target vertically migrating species from the mesopelagic layer that are accessible at shallower depths at night and at lower energetic cost than during the day.