Center of Expertise in Marine Mammology: Scientific Research Report 2012-2014

Belugas and Ringed Seals: Indicators of Ecosystem Change in the Beaufort Sea

A hunter enjoying freshly cooked muktuk at the whaling camp at Baby Island, Mackenzie Delta, NT (photo: DFO).

A hunter enjoying freshly cooked muktuk at the whaling camp at Baby Island, Mackenzie Delta, NT (photo: DFO).

Lois Harwood

As long-lived and wide-ranging oceanic predators, marine mammals can act as indicators of the state of the ecosystem, providing evidence of changes to the food web and ecosystem structure. They often first respond to ecosystem variation with changes in body condition, a direct link to the year-to-year availability and quality of their prey. Changes in condition can eventually affect reproduction, growth rates and survival of individuals, and impact marine mammal populations.

Belugas

The Beaufort Sea stock of belugas winters in the Bering Sea, and each spring migrates along the north coast of Alaska to summering areas in the Mackenzie Estuary, as well as the offshore Beaufort Sea and Amundsen Gulf. This stock is shared with Alaska and Russia, and is the second largest in Canada. The stock was last assessed by DFO as stable or increasing.

Belugas aggregate in the warm, shallow waters of the Mackenzie River estuary during summer, during which time they are the subject of a sustainable, subsistence harvest by the Inuvialuit of the Western Canadian Arctic. Subsistence-harvested belugas have been measured and sampled since 1980. Our study objectives, using harvest monitor-collected data on the sex, length, age and blubber thickness of landed belugas, were to examine beluga growth rates and blubber thickness, specifically for indications of temporal trends which could ultimately be linked to environmental change.

Our analyses revealed there has been a subtle (0.08% per year), but sustained decline in beluga growth rates, 1.75% over the time series from 1988-2008. Also, of 300+ male belugas landed between 2000 and 2007, there was significant variation in blubber thickness among years, with belugas being thinnest in 2005. This, along with subtle changes in growth of the belugas over the time series, may be a reflection of ecosystem changes that are negatively influencing the availability or quality/quantity or distribution of their prey.

Ringed Seals

Ringed seal (photo: DFO)

Ringed seal (photo: DFO)

We also obtained measurements and samples from ringed seals taken in the subsistence harvests near Ulukhaktok, NT, formerly known as Holman. We worked with our seal monitor at this location, John Alikamik, and his family, since the beginning of the study. The seal harvest in this area is the largest and most predictable in the western Canadian Arctic, and provides the best opportunity to obtain adequate sample sizes on a reliable, long-term basis. We examined the relationship between body condition, reproduction (ovulation rate, percent pups in harvest) and sea ice in a time series from 1992-present, building on work started in this area in the 1970s through similar collaborations among hunters and DFO scientists. A subsistence harvest-based sample of approximately 100 seals per year was obtained annually during 1992-2011, from the family’s traditional hunting camp located on the northwest shore of west Prince Albert Sound, 5 km from east Amundsen Gulf. The results from 2 decades of monitoring revealed two main findings. First, there was a temporal, statistically significant trend of decreasing mean annual body condition of ringed seals (using an index of length, mass, fat depth: adults) beginning in 1994, and detected in adult males, adult females and subadults.

A second and parallel result was that body condition of the seals was negatively correlated with the timing of fast ice clearance in spring, obvious during extreme ice years. Failure to ovulate was striking in the most extreme late ice clearance year in our series, 2005, when only 30.0% of the mature adult females sampled ovulated. This came at a time when seal body condition indices and percent pups in the harvest were among the lowest annual values, and when spring ice clearance in Amundsen Gulf was delayed by more than five weeks compared to the 1992-2011 average. While the seal population in this core habitat appears to recover from natural and extreme-year fluctuations over four decades in this and previous studies, the possible magnified effect of several consecutive extreme ice years, compounded by the simultaneous occurrence of the temporal decline in seal body condition, is of particular concern.

Indications of Ecosystem Change

Concurrent declines in growth, condition and/or reproduction in ringed seals and belugas suggest that changes that are occurring in the Arctic marine ecosystem. Results can be augmented further with published and anecdotal observations from other species (concurrent studies/observations are available for black guillemots, polar bears, Arctic char, bowhead whales) which are indicating environmental change relating to prey shifts and/or changes in distribution. Belugas and ringed seals are excellent indicator species for measuring environmental change. Concurrent changes in belugas and seals, based on 2-decades of consistent, robust monitoring, are particularly informative as this link appears to indicate subtle climatic/oceanographic changes are occurring lower in the trophic pyramid, and this could have cascading and profound ecosystem impacts, and would be difficult or impossible to study by other methods.

Continued Research

To further our understanding of ecosystem change, it is important to continue, among studies of other species, monitoring of beluga growth and ringed seal body condition through long-term, standardized monitored of subsistence-harvested specimens. This, paired with direct (stomach contents) and indirect (isotopes, fatty acids) monitoring of diet, and detailed study of movements and seasonal ranges through telemetry, will refine our understanding of the possible prey shifts and critical habitats used by these marine mammals. Ultimately, this would be coordinated with biophysical and oceanographic sampling, at spatial and temporal scales, and geographic locations, that are relevant to the home range, critical habitats and prey of marine mammals.

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