Sea ice conditions are an important factor in the health of many arctic mammals use the sea ice to breed and to carry out many social behaviors. The harp seal is one of those species, using the sea ice as a substrate for pupping and nursing its young. Johnston et al. (2012) examined the impact of decreased sea ice on harp seal mortality. The scientists used Northern Atlantic Oscillation (NAO) data to represent climate change; NAO is the dominant pattern in climate variability in the North Atlantic. The breeding regions that Johnston et al. used were located in the Northeastern US, the Gulf of St. Lawrence, and the White Sea near Norway. Using the NAO data, sea ice measurements, and recorded mortality rates of deal harp seal pups, the authors established a relationship between decreased sea ice ice/poor NAO conditions and increased mortality in young harp seals. They also established a link between NAO conditions and sea ice cover. Johnston et al. suggest that the harp seal is stable at the moment, but could be harmed by the cumulative effect of human influences and rapid climate change. –Connor O’Boyle
Johnston, D., Bowers, M., Friedlaender, A., Lavigne, D., 2012. The effects of climate change on harp seals (Pagophilus groenlandicus). PLoS One 7, e29158.
Johnston et al.studied the effect of climate change on harp seals in three ways. They examined the differing sea ice levels in the Gulf of St. Lawrence with neonatal mortality rates in harp seals in the Northeastern US. The authors linked NAO conditions to differing sea-ice levels and did a retrospective cross-correlation analysis of NAO conditions and sea ice in two breeding regions of harp seals. Lastly they showed the relationship between NAO conditions and sea ice by doing linear multiple regression models that accounted for short-term variation in ice driven by the NAO. In order to link reductions in sea ice cover and seal mortality the scientists used mortality data for dead harp seals and compared that with sea ice cover data from the Gulf of St. Lawrence during the same time period. For the retrospective analysis of NAO conditions and sea ice the authors looked at two breeding regions of harp seals, the Gulf of St. Lawrence and White Sea region. The authors compared these two areas within a retrospective assessment of published harp seal neonatal mortality data. They conducted a wider investigation using two addition breeding regions from Newfoundland and the Greenland Sea. This second investigation was used to assess longer-term trends in sea ice cover across the entire North Atlantic. Johnston et al. used sea ice data obtained from the National Sea Ice Data Centre and NAO data from the National Center for Atmospheric Research.
The results showed that sea ice and seal mortality were significantly correlated. Lighter ice conditions were linked with increased numbers of stranded dead seals. The regression model between NAO data and seal mortality showed a similar relationship. Breeding regions in the White Sea and the Gulf of St. Lawrence showed significant differences in sea ice cover and NAO data. In the White Sea heavier ice coverage was seen during negative NAO periods and lighter ice coverage was seen during positive NAO periods. The western North Atlantic ice conditions were opposite with heavier ice coverage during positive NAO periods and lighter ice coverage during negative NAO periods. The results from their mixed effects models revealed a statistically significant annual decline of sea ice cover in all four breeding regions, regardless of variation in NAO conditions.
The negative relationship between sea ice coverage and seal mortality rates shows how climate change is having an impact on seal populations. The scientist’s regression revealed that an increase in first year mortality occurred in years with lighter sea ice coverage and lower NAO index values. This also shows how the NAO determines sea ice dynamics in harp seal breeding regions. The retrospective analysis of NAO conditions revealed that the NAO was consistently negative in light ice conditions in the Northwest Atlantic, and in years with less ice coverage the harp seal populations decreased significantly. The Northeast Atlantic breeding regions are out of phase with the NAO and the NAO was positive in times of decreased sea ice, years with decreased seal populations correlated to positive NAO indices and lower sea ice levels. The fluctuations of harp seal populations over time corresponds to increases and decreases in sea ice as well as the NAO indices, showing how climate change directly impacts sea ice levels, which then disrupt harp seal reproduction. The authors found that the ice cover in the breeding habitats for harp seals has been declining since 1979, and along with this the harp seal’s yearly mortality rate has gone up since 1979. Johnston et al. conclude their paper by stating that harp seals could be a vulnerable species in the future. The authors state that the harp seals are well suited to deal with natural shifts in climate however the cumulative effects of human influences such as hunting and global warming could put them at a higher risk. Other artic seal species could be at risk as well sharing many of the characteristics and breeding regions of the harp seal.