Though populations of harbor seals – the captivating species seen in almost every zoo – are stable in other areas of the world, they are seeing declines in southeastern Alaska. These particular seals use icebergs calved from nearby glaciers as a place to rest and breed, but changes in ice availability are affecting these behaviors, crucial to their survival and reproduction.
Two separate studies, one by the National Park Service (NPS) and one by the Alaska Department of Fish and Game (ADFG), have independently found that seals may be changing their distribution and behavior to match the shifting locations of ice, as glaciers retreat.
Jamie Womble, leading the NPS research in Glacier Bay, is providing a new way of relating glacier ice extent and harbor seal territory, both in location and seasonality. Womble and her team aim to find the exact distribution and movements of these Alaskan harbor seals. Aerial tracking– flying above the ice and counting the seals–is a method that works effectively in the region. They also glue GPS transmitters to the seals, and track their movements on land-based monitors. These transmitters come off safely during the next summer’s molt, so they present only minimal risk to the animals.
Womble and her team found that “[d]espite extensive migration and movements of seals away from Glacier Bay during the post-breeding season, there was a high degree of inter-annual site fidelity (return rate) of seals to Glacier Bay the following pupping/breeding season.”
In addition to studying the distances which the seals traveled, Womble and her group also examined the patterns of seal movement in relation to the glacial ice. The team studied the ice distribution within John Hopkins Inlet, which they coordinated with aerial tracking data to examine the relationship between the ice extent and the harbor seals.
John Hopkins Inlet, the main area of research for Womble, is home to Johns Hopkins Glacier and Gilman Glacier which are among the few advancing glaciers in this region. Seals were found to congregate in areas with the highest percentage of ice.
“Tidewater glacier fjords in Alaska host some of the largest seasonal aggregations of harbor seals in Alaska,” Womble told GlacierHub in an interview. Many of these tidewater glaciers – glaciers that run into the sea and calve frequent icebergs – are thinning, and a few have begun retreating.
In particular, rapid retreat on the east side of Glacier Bay is leading to decreased seal pupping. During this critical season when the pups are newborn, mother seals and the weaning baby seals use flat icebergs to rest. “By 2008, no seals were pupping in Muir Inlet, and fewer than 200 seals were counted in McBride Inlet near the terminus of the McBride Glacier, the only remaining tidewater glacier in the East Arm of Glacier Bay,“ the NPS team stated in a recent report.
In a report, ADFG emphasizes the importance of studying “…why, how, and when harbor seals use glacial habitat, and whether the rapid thinning and retreat of Alaskan glaciers associated with climate change could negatively affect harbor seals…” Their research documented similar instances of glacier thinning and retreat and they are also monitoring seal movement, as well as other topics, including seal diet, seal weight and bodily composition and disturbances by tour vessels. Though ADFG began their work in Glacier Bay, the same site as the other team, they moved their research to Tracy Arm Ford’s Terror Wilderness Area – more than 200 miles to the southeast.
The ADFG team has attached transmitters such as SPOT to track the seals. These beam data on location, heart rate and other biological indicators up to satellites. To gather data, the researchers depend on the seals surfacing to breathe or rest, since the satellites cannot receive signals that are released underwater. The tracking for both research projects was most important during winter months, since researchers were interested in monitoring movement and feeding after the summer breeding season. (More tracking information, here)
ADFG also saw regular return rates for the sea populations which they studied. They hypothesized that they may travel to find food in the winter, but still return to Glacier Bay in the summer for the safety that icebergs provide from land-based predators. Icebergs are also important sites for the animals to haul out, since many beaches are entirely covered during high tides.
The ongoing research conducted both by Womble’s group and by the Alaska Department of Fish and Game show how recent changes in glaciers have already had large effects on the seal life cycle, specifically pupping. Continued monitoring of seal reproduction and movement in the context of glacier retreat will allow for predictions of the future of this important species in a critical section of its range.