Multiannual co-occurrences between Saimaa ringed seal individuals suggest social associations during the moulting season

While social network analysis was originally developed for human sociology studies, it is also a valuable tool for characterizing behavioural aspects in many animal populations. For instance, social network analysis can be used for assessing the transmission of diseases or information within groups. Here, we describe co-occurrences and potential social associations of ringed seals, Pusa hispida saimensis , in Lake Saimaa, Finland, from 2016 to 2022, based on photoidentification data. Throughout seven successive moulting seasons (April to June), we identified 425 individuals, of which 178 had pairwise co-occurrences in altogether 185 dyads. These seals had a variable number of associates (up to 10), and co-occurrences were observed in 1–7 years. In general, observations of pairs or larger groups of seals (up to five individuals) were rare, and mainly occurred in the two most densely populated basins of the lake. Most of the group observations occurred on ice rather than on rocks. Overall, the fragmented nature of the lake system, site fidelity and moulting habitat availability are key factors that probably affect the observed co-occurrences. Most of the dyads were observed during only one year, which may indicate social tolerance while using the same moulting site. However, 21 dyads were observed during multiple moulting seasons, with some at different sites separated by up to 3 km in different years, suggesting the existence of actual long-lasting associations. Our study represents the first description of co-occurrences and potential social associations in the endangered Saimaa ringed seal. These results can be used for designing and implementing mitigation strategies in case of unforeseen and sudden changes in the environment, such as a disease outbreak. • Saimaa ringed seals mainly moult solitarily, but co-occurrences are also observed. • Over 40% of the identified individuals co-occurred with others. • Habitat availability and site fidelity shape seal co-occurrences. • Intersexual co-occurrences were more frequent, possibly indicating mating pairs. • Long movements of multiannual dyads suggest existence of long-lasting associations.