Seals as Sentinels: Harbor Seals (Phoca vitulina) as Climate Indicators in the Gulf of Maine

Abstract

The Gulf of Maine is warming three times faster than the rest of the ocean, making it an ideal system to study climate change. However, underwater ecosystems are uniquely hard to monitor, so it can be useful to choose a sentinel species to serve as a proxy for environmental health. I evaluated whether harbor seals could serve as this sentinel species in the Gulf. I used 12 years of photo surveys from Shoals Marine Laboratory to track changes in molting phenology, as well as a bottom trawl dataset from Northeast Fisheries Science Center, to create a generalized additive model to determine the best environmental predictors of molting timing. Additionally, I used three years of scat data to assess hormonal and isotopic markers for physiological health in the seals. I found that seal molting is moving earlier and that spring surface salinity and the availability of silver hake in the prior fall are the best predictors of this phenology change in a given year, suggesting that body condition plays a role in molting timing. The scat dataset showed year-to-year differences in stress and metabolic hormones, but additional years of data are needed to tie these to environmental changes. Additionally, the δ¹³C showed significant differences between 2023 and 2024. Across all data, 2024 showed a unique trend, likely caused by the sudden increased influence of the Labrador Current, which was indicated by low salinity and δ¹³C ratio. This work is the first of its kind to document long-term change in seal molting phenology, and it indicates that seal molting is influenced by changing environmental parameters and can therefore be used as a visible metric to assess ecosystem changes. Additionally, both hormonal and isotopic metrics from seal scat showed year-to-year differences, strengthening the case for the harbor seal's usefulness as a sentinel species.