Contrasting responses to climate change by two Arctic marine mammal species

Research notes

Ringed seals and white whales live year-round in Svalbard’s coastal waters. One of these species seems to be adapting to use Atlantic water (and associated fish species), which are both increasing in this region. The other continues to rely on Arctic habitats and fish species that are in decline.

By: Charmain D Hamilton, Jade Vacquié-Garcia, Kit M Kovacs, Jack Kohler, Christian Lydersen
// Norwegian Polar Institute and Rolf A Ims // UiT The Arctic University of Norway

1.

The environment around Svalbard is in a state of rapid change. The sea-ice extent is declining rapidly, especially in west coast fjords where only limited amounts of land-fast ice (i.e. ice connected to shorelines) now form in the winter. Atlantic Water is increasingly intruding into Svalbard’s coastal areas, bringing with it Atlantic fish species such as capelin and Atlantic herring. Glaciers are also shrinking and the number of tidewater glaciers (glaciers that terminate in the ocean) has decreased over the last few decades.

Ringed seals and white whales are endemic Arctic marine mammals that are found in coastal areas of Svalbard throughout the year. They predominantly eat ice-associated prey and have historically spent a lot of their time near tidewater glaciers. Due to their long lifespans and the rapid pace of climate change, they will not be able to adapt genetically to the drastic changes that are taking place in their habitats. If these species are to thrive in the Arctic of the future, they will need to adjust to a new ecological reality by changing their behaviour and diet (i.e. by exhibiting behavioural plasticity).

However, their capacity to respond to new conditions through behavioural adjustments is currently unknown: will they begin to use the new type of environment and prey types or will they seek out Arctic refugial areas where their “traditional” prey remain?

Our natural experiment

To address this question, we used data from biotelemetry tags. These tags are attached to individuals from each species and send information via satellite systems about where animals, are, as well as aspects of their behaviour (e.g. dive depths, dive durations, time spent resting on sea ice). We compared data from animals tagged when conditions were historically “normal” (1995-2003) and data from animals tagged after the environmental changes began to occur in earnest (2010-2016) in areas that are influenced by Atlantic Water (west coast of Spitsbergen and Storfjorden).

We analysed how much time each species spent in areas near tidewater glaciers in each time period, and if their use of different glacier fronts depended on the length of the glacier’s calving front or the water depth at these sites. We focused on changes during the summer and autumn, as these seasons are important foraging periods for both species.

2. Atlantification: Impact on top predators

We found that ringed seals and white whales had opposite responses to the large environmental changes that have occurred in Svalbard’s coastal regions. When conditions were historically “normal” (1995-2003), both species spent about half of their time in areas near tidewater glaciers and their diets were dominated by polar cod, an Arctic fish species that live in cold water, often in cracks in sea ice when young and at various water depths depending on age. However, after the conditions had changed (2010-2016), ringed seals spent 93% of their time near tidewater glaciers, significantly more time than in the past, while white whales spent significantly less time (36%) near tidewater glaciers. One behaviour that had not changed was that both species preferentially foraged near the largest tidewater glaciers in the region. These glaciers generally have long calving fronts that lie adjacent to deep water.

The frontal areas of tidewater glaciers are important foraging areas for many marine mammal and seabird species in Svalbard.

Due to meltwater discharge from the glacier and ocean circulation patterns within fjords, prey species are concentrated near tidewater glaciers.

In addition, these areas serve as refugia, retaining Arctic water masses and Arctic fish species, such as polar cod. Calved pieces of glacier ice also provide resting platforms for seals and various species of seabirds. Ringed seals are more tightly coupled to these areas now than in the past and have smaller home ranges (i.e. areas where an individual seal spends most of its time) than a few decades ago.

In contrast to ringed seals, white whales are not retracting into Arctic glacial refuge areas. They currently have larger home ranges than a few decades ago and are spending more time in the central areas of fjords, where they have been observed milling at the surface (i.e. indicative of foraging activity) in recent years. In these mid-fjord areas, they are presumably feeding on Atlantic fish species that are transported into the Arctic with the increased volume (and temperature) of Atlantic Water reaching high latitudes. Differences in the dietary flexibility between these two species likely underpin their differing responses to climate change. Research from other areas of the Arctic has found that white whales are more flexible in their dietary choices than ringed seals.

Consequences for the future

The behavioural and dietary flexibility exhibited by white whales bodes well for their chances of adjusting to the new environmental conditions in Svalbard. However, the ringed seals’ continued reliance on shrinking Arctic habitats and declining Arctic prey is a serious concern. Species that lack the behavioural flexibility required to respond to changes occurring in their habitats are almost certain to decline as the climate continues to warm.

This study highlights that Arctic marine mammals are being impacted differently by climate change. Monitoring and research on individual species are needed to support management and conservation efforts and help ensure the continued existence of top predator species in a time of rapid change.