Abundance and trends of harbor seals (Phoca vitulina richardii) in Alaska, 1996-2023

Harbor seals (Phoca vitulina richardii) were the focus of a 28-year study along the coasts of Alaska. These seals were divided into 12 management stocks, each monitored for both their population size and any trends over time. The researchers utilized a two-stage Bayesian hierarchical analysis, combining aerial survey counts with data from satellite-linked bio-loggers to get a more accurate picture of the seals' presence, even when they were not visible to human observers.

The study revealed that harbor seals were consistently abundant, with the population reaching approximately 225,000 individuals in 2015, marking a peak. However, the trend in abundance was not uniform across all regions. In the initial phase of the study, there was primarily a growth phase, but since then, trends have fluctuated. Some stocks, such as those in Bristol Bay, continued to grow, whereas others, like those in Prince William Sound and South Kodiak, experienced declines.

The recent regional declines in seal populations were found to be correlated with climate anomalies, including marine heatwaves and the rapid retreat of tidewater glaciers. These findings underscore the importance of managing this species, as harbor seals are not only a protected under the Marine Mammal Protection Act but also a vital nutritional and cultural resource for Alaska Native communities. Additionally, the seals serve as important indicators of change within the Northeast Pacific marine ecosystem.