Multidecadal shifts in hooded seal dive behaviour in a changing ocean

The high northern latitudes are warming rapidly, reducing sea ice coverage and altering salinity regimes, with cascading effects on marine ecosystems. Although horizontal range shifts of marine animals under climate change are well documented, long-term changes in vertical habitat use by marine predators remain poorly understood. We analyzed a three-decade biologging dataset (1992-2019) from 45 hooded seals (Cystophora cristata) breeding in the Gulf of St. Lawrence to investigate how this population has altered its diving behaviour in response to climate change. Using multinomial generalized additive mixed models, we quantified dive duration and depth across sex, functional oceanographic domains, decade, and season. Sexual dimorphism in diving behaviour persisted despite climate-induced change, with females making shorter and shallower dives more frequently than males. However, all individuals exhibited marked shifts in dive duration, from predominantly 5-15 minute dives in the 1990s to 15-25 minute dives in the 2000s and 2010s. These changes were spatially structured, reflecting basin-scale thermohaline reorganization, and temporally coordinated with migration. Shallow dives dominated early and late migration, whereas deeper, longer dives peaked mid-migration. Our results demonstrate behavioural plasticity in response to changing vertical habitat conditions.