Poorer body condition than 30 years ago
Harp seals are iconic top predators in the Arctic. They are good swimmers and undertake long migrations in the northern most parts of the Atlantic but prefer to be near sea-ice all year round. During summer, many of them gather at the ice edge north of Svalbard to hunt for food.
The sea-ice in this region has gradually retreated northwards in the last decades and is now often located over much deeper waters than previously.
To investigate how the retreat of the ice edge has affected the harp seals foraging conditions, researchers have compared data from a survey in September 2016 with comparable data from the early 1990’s.
The study shows that young seals (body length < 145 cm) now are slimmer than 30 years ago, suggesting that the overall body condition of young seals has declined.
This finding is supported by earlier observations, but the experts struggle to give a definite explanation for the poorer body condition.
Potentially, the currently large stock of Atlantic cod in the area is a strong competitor for prey, hence decreasing the amount of prey obtainable by young seals.
Harp seal on sea ice. Photo: Andrea Taurisano, NPI .
Themisto libellula. Photo: Nansen Legacy/ Fredrik Broms
Polar cod. Photo: Nansen Legacy/ Fredrik Broms
Harp seals’ main prey has not changed
In contrast to the body condition, the main prey of harp seals has not changed. Both now and 30 years ago, the seals mostly prey on polar cod and the pelagic amphipod Themisto libellula, an up to 6 cm big shrimp-like crustacean.
However, changes were detected in the seals’ additional prey. While the seals preyed on bottom-associated prey, such as prawns and bottom-dwelling fish, 30 years ago, the seals now feed on pelagic fish, such as Atlantic cod and blue whiting.
This shift may relate to the displacement of the ice edge from areas of 100 to 350 meter water depth 30 years ago, to areas of more than 500 meters of water depth today.
The harp seals’ favourite prey remains polar cod
The study also sheds light on the food preference of harp seals. Though the researchers observed a lot of krill north of Svalbard in 2016, they found none in the seals’ stomach. This suggests that either harp seals do not like krill during this time of the year, or that they prefer not to hunt below 100 m where most of the krill was.
Amphipods, on the other hand, were found in similar percentage in the water column and in the seals’ diet, suggesting that amphipods are a welcome prey but not harp seals’ favourite.
The seals’ favourite prey remains polar cod.
The study illustrates the complexity in evaluating the ecological consequences of the displacement of the ice edge from the shallow shelf to the deep central Arctic Ocean.
While some aspects of the foraging behaviour may stay the same, changes in prey availability and competition may lead to an overall new ecological setting. The consequences of such changes for single species and species communities are not yet understood.
Haug T, Biuw M, Gjøsæter H, Knutsen T, Lindstrøm U, MacKenzie KM, Meier S, Nilssen KT (2021) Harp seal body condition and trophic interactions with prey in Norwegian high Arctic waters in early autumn. Progress in Oceanography 191: 102498. doi.org/10.1016/j.pocean.2020.102498