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 404 days where they conducted extensive offshore movements and spent an average of 90% of their time over deep waters (> 1000 m) in the North Atlantic. When entering deep water, they increased their daily travel rate significantly from 22.5 to 36.7 km d−1. Five of the 15 porpoises provided information on dive depth which suggested that dive depths > 100 m 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.
Mikkel V. Jensen is thanked for modifying the tags to instrument harbour porpoise, and the hunters Svend and Knud Heilmann are thanked for helping with capture and tagging of the porpoises. Mikkel Sinding and Bolethe Skifte Egede are thanked for helping with tagging of the porpoises. Michael Quaade from the Danish Society of Astronomy provided almanac data. The project was supported by the Greenland Ministry of Education, Church, Culture and Gender Equality, the Danish Cooperation for the Environment in the Arctic (Danish Ministry of the Environment), and the Greenland Institute of Natural Resources. Two anonymous reviewers are thanked for providing helpful comments on the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no actual or potential conflicts of interest in relation to this work.
All work presented here complies with the current laws of the country in which they were performed (Greenland).
Research involving animals
The tagging of harbour porpoises in this study was performed with permission from the Government of Greenland, permit no. 2012-069733, Doc. 1265044.
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