Oceanographic and behavioral monitoring inferred from seal CTD tagging in the Ross Sea, Antarctica

Monitoring changes in the marine environment is important for both oceanography and ecology study as it helps us understand the process by which oceanic conditions influence the entire ecosystem. In Antarctica, marine mammals encounter substantial changes due to seasonal variation of water mass composition and the complicated submarine topography. However, unbroken observations on ocean conditions are highly restricted to the summer season only when research vessels are available. In this study, we explored how seasonal variations in oceanic conditions affect the foraging behaviors of Weddell seals (Leptonychotes weddellii) in the Ross Sea, Antarctica using miniaturized CTD tags. Over the course of three consecutive years, from 2021 to 2023, 64 adult individuals were instrumented to collect data on water temperature and salinity as well as head acceleration. From the head movement, we found that seals foraged more often in modified shelf water and ice shelf water compared to Antarctic surface water. Additionally, as the lower boundary of Antarctic surface water descends from March to July, the seal dived to greater depths. Additionally, CTD profiles from CTD tags were classified using machine learning methods. Temperature and salinity from each profile were linearly interpolated across depths from 1 to 600 meters. Principal Component Analysis was then applied to extract three principal components for each profile. These components were subsequently used as input for a Gaussian Mixture Model, which classified the profiles into four distinct clusters. Each cluster had distinct temperature and salinity profiles and showed spatial and temporal separation (Warm surface Cluster: Distributed near Terra Nova Bay, predominant in summer (February); Intermediate surface Cluster: Distributed near Terra Nova Bay, predominant in fall (March); Cold Cluster: Distributed near Terra Nova Bay, predominant in winter (May, June, and July); Warm subsurface Cluster: Distributed near the shelf break, predominant in April and May). Prey capture attempts of dives were highest in Warm surface Cluster (3.16) and lowest in Intermediate surface Cluster (2.91). This study reveals the spatial and temporal shifts of foraging behavior with the surrounding oceanographic conditions, and it further emphasize that these oceanic factors should be considered for estimating their foraging activities.