Though commonly encountered in continental shelf waters in the North Atlantic little is known about harbour porpoise movement patterns outside coastal waters. However, a recent study of Greenland harbour porpoises reveals long-range oceanic movements and deep diving as well as strongly seasonal coastal site fidelity.

Nynne Nielsen of the Greenland Institute of Natural Resources and colleagues actively caught 30 harbour porpoises in West Greenland and instrumented them with satellite transmitters.  Their study is the first to document harbour porpoises leaving shelf areas to conduct large-scale wintering movements over deep waters in the North Atlantic, with some animals moving more than 2000 km from the tagging area.  The area of use by the porpoises was 7.5 times larger than that used by porpoises from the North Sea.

Minimum convex polygons (MCP) of the total area covered by 30 harbour porpoises tagged in West Greenland (Gl, green line), and 71 porpoises tagged in Denmark (Dk, blue line). Small dots represent all recorded satellite positions of porpoises tagged in West Greenland (green) and Denmark (blue) for the duration of the tags. Larger dots highlight the positions of porpoises tagged in West Greenland in May (green), June (yellow), July (pink) and August (red), thus illustrating the return of porpoises to the East or West Greenland shelf over this period. The dashed line indicates the limit of the North Atlantic Ocean defined in this study.

West Greenland harbour porpoises

The scale of movements and seasonal selection of offshore habitats are very different from what is typical of tracked porpoises in the North Sea or the Northwest Atlantic.  However, surveys for harbour porpoises in the North Atlantic have always been conducted during summer, when porpoises are located in shelf waters, as seen in the current study also.

The porpoises in the Greenland study were instrumented using stainless steel nuts which allowed for long-term monitoring, including inter-annual movements (up to 1047 days). Such longevity of the tags provided novel insights into porpoise movements over several years.  During winter/spring tagged porpoises from West Greenland moved off-shore into areas with significantly greater water depth than where they spend their summer/autumn and two of the tagged female porpoises performed dives to nearly twice the maximum depth previously reported for the species (390 and 410 m, respectively).

Despite having wide-ranging dispersal capability, harbour porpoises with transmitters lasting >1 yr exhibited site fidelity, returning to specific summer feeding grounds in West Greenland. Five of the 6 porpoises that returned to West Greenland and to the very same place where they were tagged, were presumed to be sexually mature, suggesting the importance of the site as a feeding, breeding and mating ground.

The authors suggest that harbour porpoises from West Greenland are specialised in using a different niche than other harbour porpoise populations. The authors go as far to suggest that the dietary, migratory and diving behaviour of the West Greenland porpoises are suggestive of a unique oceanic ecotype.

We have briefly summarised here what is a really interesting read.  Check out the full publication for yourselves:

Nynne H. Nielsen, Jonas Teilmann, Signe Sveegaard, Rikke G. Hansen, Mikkel-Holger S. Sinding, Rune Dietz, Mads Peter Heide-Jørgensen (2018). Oceanic movements, site fidelity and deep diving in harbour porpoises from Greenland show limited similarities to animals from the North Sea. Marine Ecology Progress Series, 597:259-272.