The Nemuro Strait in Hokkaido, Japan, part of the Shiretoko World Natural Heritage Site, is seasonally covered by sea ice, typically from February to April, and an important habitat for marine mammals. While previous visual surveys and passive acoustic monitoring (PAM) studies have revealed seasonal occurrence patterns of marine mammals in this region, interannual variability in spatiotemporal occurrence patterns and their environmental drivers remain poorly understood. While PAM enables year-round monitoring even in ice-covered conditions, this method generates extensive datasets, necessitating automated methods for efficient data analysis.

In this study, we employed transfer learning using the BirdNET algorithm, originally developed for identifying bird vocalizations, to detect calls of three marine mammal species in PAM data collected between December 2020 and May 2023: ribbon seals (Histriophoca fasciata) downsweeps, killer whales (Orcinus orca) pulsed calls, and Pacific white-sided dolphins (Lagenorhynchus obliquidens) pulsed calls. We analyzed vocalization activity in 30-minute time bins and compared it to satellite-derived sea ice extent in the northern Nemuro Strait to examine how sea ice influences species occurrence patterns.

Our results confirmed consistent seasonal patterns across all years. Ribbon seals were detected during their breeding season (February–March) in periods with extensive sea ice coverage, while killer whales were primarily present during ice-free months (April–August and January–February). Pacific white-sided dolphins were observed in summer and late fall, exclusively in open-water conditions. Notably, all three species were detected from late fall through spring—a period with limited visual observations—highlighting the advantages and importance of PAM for year-round monitoring.

Annual variations were also evident. The number of ribbon seal detections increased during years with more extensive sea ice coverage, reflecting their reliance on sea ice for breeding. Conversely, killer whale activity was highest during periods of low sea ice coverage, while Pacific white-sided dolphins were only detected during periods with no sea ice.

As climate change continues to drive declines in seasonal sea ice extent, there is growing concern over the potential impacts on ice-dependent species, particularly ribbon seals. This study underscores the value of PAM for long-term marine mammal monitoring and conservation, providing critical insights into species-environment relationships in rapidly changing ecosystems.