Harbor seals rely on icebergs for daily life, but their environment is changing

Harbor seals in icy regions rely on icebergs shed by glaciers for critical life activities. These floating ice platforms serve as safe spaces for giving birth, caring for young, and molting.

However, as climate change affects glaciers, the size, speed, and number of icebergs change – directly affecting the seals’ habitat.

How harbor seals use icebergs

Mother seals, for example, prefer stable, slower-moving icebergs during the pupping season to ensure safety for their newborns.

In contrast, during the molting season, both mothers and the larger seal population tend to favor faster-moving icebergs near foraging grounds.

This seasonal preference highlights the intricate balance between the seals’ needs and their changing environment.

“Our work provides a direct link between a glacier’s advance and seals’ distribution and behavior,” said Lynn Kaluzienski, a postdoctoral fellow at the University of Alaska Southeast who led the study.

“Interdisciplinary studies like this one, coupled with long-term monitoring campaigns, will be important to understand how climate change will influence tidewater glacier fjord ecosystems in the future.”

Seals and icebergs at Glacier Bay National Park

The study was focused on harbor seals and icebergs in Johns Hopkins Inlet and Glacier, which is part of Glacier Bay National Park in Alaska.

Unlike many glaciers globally, Johns Hopkins Glacier is advancing rather than retreating.

This anomaly is due, in part, to its terminal moraine – a natural barricade of crushed rock and sediment. This barrier shields the glacier from warmer ocean water, thus slowing its melt rate.

However, the terminal moraine also limits the number of icebergs that the glacier sheds into the fjord. Fewer icebergs mean reduced habitat for seals, which fuels the need for research to understand how seals adapt to these changes.

Observing seals and icebergs

Over the past few years, Kaluzienski and her team, in collaboration with the U.S. National Park Service, monitored the fjord using time-lapse cameras and aerial photographic surveys. They documented fine-scale variations in icebergs and seals distribution.

“Icebergs are found throughout the fjord in regions of fast flow, within eddies, and close to the glacier,” Kaluzienski explained.

“We wanted to understand which of these areas seals were using and how this habitat is changing in response to advances at the glacier front and reduction in iceberg numbers.”

The role of freshwater runoff

When an iceberg calves from a glacier, its movement is influenced by wind, ocean currents, and freshwater runoff from the glacier’s base.

This runoff, known as a plume, brings nutrient-rich freshwater to the surface, creating a hotspot for plankton and fish. These areas act as a moving buffet for seals, which use icebergs near the plume to forage.

Using remote sensing data, the researchers pinpointed the plume’s location and compared it to seal and iceberg distributions during the pupping season in June and the molting season in August.

The findings revealed distinct preferences. During the pupping season, seals often rested on slower-moving icebergs, which traveled at speeds of less than 7-8 inches (0.2 meters) per second.

During the molting season, seals increasingly used faster-moving icebergs located in or near the nutrient-rich plumes.

Why the seasonal preferences?

The preference for slower-moving icebergs during the pupping season likely stems from their stability, which provides a safer platform for mother seals and their pups.

Conversely, stability becomes less critical during the molting season, when seals prioritize access to food. Icebergs near the plume offer better foraging opportunities, aligning with the seals’ needs during this period.

Broader impact of glacier dynamics

This study highlights the complex interplay between glacier behavior and wildlife adaptation. Changes in glacier size, speed, and iceberg distribution ripple through the ecosystem, influencing seal behavior and habitat use.

Understanding these dynamics is crucial for protecting harbor seals and the broader tidewater glacier fjord ecosystems they inhabit.

As Kaluzienski emphasized, long-term, interdisciplinary research will play a vital role in assessing the ongoing impact of climate change on these fragile environments.

Kaluzienski presented the findings at AGU’s 2024 Annual Meeting in Washington, D.C. The meeting, scheduled to take place from December 9-13, will host over 30,000 scientists to discuss cutting-edge Earth and space science research.

The research was presented at AGU’s 2024 Annual Meeting.

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