Abundance and trends of harbor seals (Phoca vitulina richardii) in Alaska, 1996-2023
A 28-year Bayesian analysis of Pacific harbor seals in Alaska reveals an overall abundant population of over 200,000 individuals in 2023, though trends have shifted from initial growth to recent variability and regional declines linked to climate anomalies and glacial retreat.
Original paper dedicated to the public domain under CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Pacific harbor seals are the most common marine mammals along the coastlines of Alaska, ranging from the icy waters of the Bering Sea down to the temperate fjords of Southeast Alaska. These animals spend much of their time in the water hunting for fish, but they must leave the ocean to rest, give birth, and shed their old fur. Because they are so visible when they are out of the water, scientists have long used aerial surveys—planes flying over the coast to count them—as the primary way to track their numbers. However, counting these seals is tricky. They are not always where the planes are looking; they dive underwater, they move between different resting spots, and they are often hidden by the weather or the time of day. To get an accurate picture of how many seals exist, researchers must account for the ones they cannot see. This challenge of counting animals that are only partially visible is central to understanding the health of the species and the ecosystem they inhabit, especially as the climate changes and the environment shifts around them.
A comprehensive new study has now provided the most detailed look yet at these populations, analyzing data collected over twenty-eight years, from 1996 to 2023. The research team, a large group of scientists from government agencies and universities, did not simply count the seals they saw from the air. Instead, they built a sophisticated statistical model that combined those aerial counts with data from tiny electronic tags attached to individual seals. These tags, known as bio-loggers, recorded exactly when each tagged seal was on land and when it was in the water. By understanding the daily and seasonal habits of these tagged animals, the scientists could estimate how many untagged seals were likely hiding underwater at the exact moment the planes flew overhead. This method allowed them to correct the raw counts and produce a reliable estimate of the total population for twelve distinct groups, or "stocks," of harbor seals across Alaska.
The results show a complex story of recovery followed by recent instability. In the first half of the study period, the overall population of harbor seals in Alaska grew, reaching a peak of approximately 225,000 individuals in 2015. This growth was a positive sign after decades of decline observed in earlier decades. However, the trend has shifted since that peak. While the total number of seals remains high, with an estimated 201,122 individuals in 2023, the population is no longer growing uniformly. The study reveals that different groups of seals are moving in different directions. Some stocks, particularly those in Bristol Bay, have continued to grow. Others, such as the populations in Prince William Sound and South Kodiak, have declined significantly, with some of these groups showing a drop of nearly three percent per year over the last eight years.
The researchers found that these recent declines in specific areas coincide with major environmental changes. The years following the 2015 peak saw a series of marine heatwaves—periods of unusually warm ocean water—and a rapid retreat of the tidewater glaciers that many seals rely on for resting. In some of the glacial fjords, the ice has melted or grounded, removing the floating platforms where seals traditionally gather to molt and give birth. The study suggests that the loss of this ice habitat, combined with the stress of warmer ocean conditions, may be driving the population down in these specific regions. Conversely, in areas where the ice remains or where the seals rely on rocky shores rather than ice, the populations have remained stable or continued to grow.
This work is more than just a headcount; it provides a critical tool for managing the species. Because the study accounts for the uncertainty in the data, it gives managers a clear picture of which populations are healthy and which are at risk. The findings confirm that while the harbor seal is not currently in danger of disappearing from Alaska as a whole, the species is facing new pressures. The study explicitly moves away from the idea that a single number can describe the health of all seals in the state. Instead, it highlights that the fate of these animals is local and specific, tied closely to the changing conditions of their immediate surroundings. The research confirms that the harbor seal remains a vital part of the Alaskan ecosystem, serving as a key indicator of the health of the coastal waters and a crucial resource for the Indigenous communities that have relied on them for generations. By understanding the specific trends of each stock, scientists and managers can now make more informed decisions to protect these animals as the Arctic and sub-Arctic environments continue to change.
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