Harbor Porpoises Benefit from Reduced Winter Sea Ice off West Greenland

The coastal marine environment of West Greenland varies each year depending on the relative strengths of cold-water and warm-water currents that move in from the south. Dominance of the warm water current not only reduces winter sea ice but increases the biomass of fish, such as Atlantic cod (Gadus morhua) and capelin (Mallotus villosus). Abundant stocks of fish attract seasonal predators like the harbor porpoise (Phocoena phocoena). Harbor porpoises are considered one of the top predators in the West Greenland ecosystem when prey are readily available but unlike truly 'Arctic' marine mammals, they do not function well in heavy sea ice.

During September and October of 1995 and 2009, Heide-Jørgensen et al. (2011) purchased intact carcasses of harbor porpoises from local hunters for study (103 porpoises in 1995, 89 in 2009). A number of measurements were taken that reflected body size and condition (including length, mass, maximum circumference and blubber thickness). Teeth were collected for age estimation. Contents from the stomachs of porpoises who had recently eaten (77 in 1995; 88 in 2009) were analyzed to determine the prey species consumed (including fish and squid). Environmental data for the coastal waters of West Greenland were compiled, including average sea ice area (for March, 1980-2010) and sea surface temperature (at June/July, 1950-2010). In addition, statistics were compiled on the spawning stock biomass of Atlantic cod from 1990 to 2009 and the monthly catches of harbor porpoises between 1993 and 2006.

In analyzing the data listed above, Heide-Jørgensen and colleagues report that sea surface temperatures off West Greenland increased significantly during the period 1980 to 2010 (with a peak at 2004 and 2005). Over the same period, winter sea ice coverage declined, leaving coastal areas in the later period ice-free year-round. The biomass of Atlantic cod increased from negligible in 1980 to more than 150,000 tons in 2008 and 2009. Over the thirty year period, the proportion of the annual porpoise catch that was made during winter months (January-June) tripled between the 1990s and the 2004-2008 period.

The authors also report that the harbor porpoises caught in 2009 were in better condition (with higher body mass and thicker blubber ) than those caught in 1995. The proportion of porpoises examined that had Atlantic cod remains in their stomachs went from zero in 1995 to 31% in 2009 and the diversity of prey consumed went from 11 major prey items (distinct species or genera) in 1995 to 23 prey items in 2009. Capelin were the primary prey item in both years (present in 78% of stomachs in 1995 and 95% of stomachs in 2009).

Based on the above findings, Heide-Jørgensen et al. conclude: "the porpoises depend on locating high densities of prey species with high nutritive value and they have apparently responded to the general warming on the banks of West Greenland by longer residence times, increased consumption of Atlantic cod resulting in improved body condition in the form of larger fat deposits in blubber, compared to the situation during a cold period in the 1990s. This is one of the few examples of a measurable effect of climate change on a marine mammal population." It seems that for some marine species that utilize Arctic waters, like Atlantic cod and harbor porpoises, warm water is better than cold water and less ice is better than more ice.