Spatiotemporal patterns of grey seal-white shark co-occurrences in Atlantic Canadian waters

Abstract Ecosystems are networks of organisms connected through biotic and abiotic interactions shaping food-web structures. These interactions change over time, leading to variable effects on ecosystem functioning. In Atlantic Canadian waters (ACW), white shark presence has recently increased, potentially intensifying predation pressure on grey seals—an abundant generalist mesopredator with broad and complex trophic relationships across the ecosystem. Yet, spatiotemporal dynamics of seal-shark co-occurrence remain poorly understood. To address this gap, we quantified relative probabilities of grey seal and white shark co-occurrence in ACW using mobile acoustic telemetry and satellite biologging, and compared them to simulation-based null distributions of random shark movement that accounted for the spatiotemporal distribution of seal acoustic monitoring effort. Our findings indicate that seal-shark co-occurrences are more likely in coastal regions with important grey seal haul-out aggregations in the Gulf of St. Lawrence, such as in the Magdalen Islands, than in other coastal regions, offshore areas, and the Sable Island region, which hosts the largest grey seal colony in ACW. Co-occurrences began in June, increased through September, and declined through November. Seals co-occurred with female and male white sharks with the same probability, although our dataset had limited adult females. Co-occurrence probability did not vary with male shark body size, yet exhibited a weak non-linear relationship with female body size. These results revealed substantial spatiotemporal variability in seal–shark co-occurrence in ACW. Depending on the severity of associated lethal and sub-lethal effects, this heterogeneous seascape for predation risk and competitive interactions may shape seal behavior, distribution, and demography. Such shifts could in turn cascade through the ecosystem, changing trophic dynamics and community structure in the years to come.