Social Distancing Seals

• Main Author: Hoekendijk, Jeroen
• Other Authors: Jeroen Hoekendijk, NIOZ Royal Netherlands Institute for Sea Research in cooperation with Utrecht University
• Language: unknown
• Published: NIOZ 2023
• Subjects: Earth and Environmental Sciences; Nearest neighbour distance; density; remote sensing; Phoca vitulina; Halichoerus grypus; herding behaviour; harbour seal
• Online Access: https://doi.org/10.25850/nioz/7b.b.3d

Many species aggregate in dense colonies. Species-specific spatial patterns provide clues about how colonies are shaped by various (a-)biotic factors, including predation, temperature regulation, or disease transmission. Using aerial imagery, we examined these patterns in colonies on land of two sympatric seal species: the harbour seal and grey seal. Results show that the density of grey seals on land is twice as high as that of harbour seals. Furthermore, the nearest neighbour distance (NND) of harbour seals (median = 1.06 m) is significantly larger than that of grey seals (median = 0.53 m). When the observed seal locations were shuffled slightly through spatial simulation, the frequency of the smallest NNDs (0-25 cm) increased, while the most frequently observed NNDs decreased, implying that both species actively display social distancing. As harbour seals are more prone to infectious diseases, we hypothesize that the larger NNDs might be a behavioural response to reduce pathogen transmission. The observed differences in spatial patterns can be used as a practical tool to differentiate between harbour and grey seals in remote sensing applications, particularly in low to medium resolution imagery (e.g., satellite imagery), where morphological characteristics alone are insufficient to differentiate between species.