Environmental Context of Eurasian Otter Sprainting Sites Across a Human Disturbance Gradient During Winter in Northeastern China

Winter imposes severe ecological constraints on Eurasian otters (Lutra lutra), yet the environmental characteristics of their winter marking sites remain poorly understood. We characterized winter sprainting sites along a human disturbance gradient in Changbai Mountain, northeastern China. In January 2025, 55 sites were recorded along three rivers and assigned to urban or wild river-reach contexts based on the broader spatial and management context of each reach. We measured topographic, hydrological, riparian, fish-diversity, and anthropogenic variables. Broad environmental gradients were summarized using Factor Analysis of Mixed Data (FAMD), and five local variables were compared between river-reach contexts using exploratory tests with Holm adjustment. Of the 55 sites, 54 were beside flowing, unfrozen water. Intermediate-to-deep water, natural banks and forested riparian backgrounds were common. The first two FAMD dimensions explained 52.5% of environmental variation, representing a spatially coupled gradient involving anthropogenic remoteness and river-section fish resource background, and a topographic and channel-structure contrast. Urban and wild sites differed in water depth, bank type, and vegetation cover, while sprainting sites occurred in both. These findings show that winter marking activity was concentrated within accessible river sections embedded in contrasting landscape contexts.