Population Estimates for Chukchi Sea Polar Bears
Although polar bears and their ice seal prey are in trouble across the Arctic due to retreating sea ice, a polar bear population in Alaska’s Chukchi Sea seems healthy. That’s the word from a new study by university and federal researchers.
The survey used line-transect aerial surveys from icebreakers to count bears and seal tracks. Results from these and other survey methods indicate that the chukchi sea polar bear population has been productive in recent years.
Until recently, reliable population estimates of the chukchi sea (CS) polar bear subpopulation were unavailable due to insufficient recapture rate and movement into and out of the survey area caused by variable ice conditions. Line-transect aerial surveys conducted from icebreakers provide an important tool for monitoring polar bear distribution and abundance in the CS, while enabling demographic parameters to be estimated.
The results of the 2016 aerial surveys suggest that chukchi sea polar bears appear to be healthy and reproducing well, even in the face of substantial sea-ice loss. Abundance estimates were derived from spatially explicit habitat metrics and were similar to, or larger than, a recent estimate of the CS population based on a multi-year integrated population model using capture-recapture and telemetry data (Regehr et al. 2018).
In winter, bears in the Chukchi Sea spend most of their time on sea ice. As the ice retreats in spring, some bears may travel over land on drier coasts such as Russia’s Wrangel Island to search for food or den sites. During this time, they also feed on washed-up whale carcasses, which are abundant in the region.
A new study led by UW scientist Eric Regehr and coauthored by Karyn Rode finds that the chukchi sea population of polar bears is doing well, despite having 30 fewer days to hunt on sea ice than 20 years ago. “It is pretty amazing how resilient these bears are,” says Rode, who leads USGS research in the Foxe Basin.
Regehr and his team used both live-capture data collected during the research period between 2008 and 2016 and GPS satellite tracking to estimate the population size. They also incorporated local and traditional ecological knowledge gathered through surveys of hunters and community members who live in the area year-round.
Scientists have found that polar bears in the Chukchi Sea appear to be surviving, but they face significant long-term threats from climate change. “Fundamentally, polar bears require sea ice to do what they do, which is hunt and kill seals,” Regehr said.
The scientists analyzed satellite radio collar data from 55 polar bears tracked in the Arctic and used an innovative model that integrates scientific and traditional ecological knowledge of polar bears. The new approach considers habitat loss as a threat to polar bears and the ice seal prey they depend on, and adjusts subsistence harvest levels accordingly.
In summer, when sea ice is at its smallest in the region, most of these bears travel to Wrangel Island, where they den and wait on land for the ocean to refreeze. The bears have maintained good body condition and reproductive rates in recent years, despite substantial declines in the extent of summer sea ice associated with climatic warming.
Researchers have used a combination of tracking data, telemetry, mark-recapture and count surveys to identify measurements that relate to female reproductive success and cub survival. They also identified factors such as body size and condition that influence these indicators. They modeled these factors separately to avoid violating assumptions of non-collinearity.
This modeling produced the first empirical estimates of vital rates and abundance for polar bears in the Chukchi Sea subpopulation, and showed that they remain healthy despite a significant loss of their summer sea ice habitat.
The study also found that genetic diversity was relatively high among both the chukchi bears and vagrants reaching Iceland, suggesting that dispersing polar bears could play an important role in maintaining connectivity within the projected fragmented Arctic landscapes of the future, buffering against declines in subpopulation size and genetic diversity. This is one of the first studies to show that genetics may play a key role in how polar bears respond to habitat change.