Spatial and temporal interaction patterns of spiny dogfish ( <i>Squalus acanthias</i> )

Abstract Aggregative behaviour is an important strategy for many fish species, especially for chondrichthyans that depend on close physical interactions between male and female conspecifics for mating and internal fertilization to occur. The Vulnerable spiny dogfish (Squalus acanthias Linnaeus, 1758), a temperate water demersal shark, is known for its aggregative behaviour where individuals tend to move in groups based on sex and/or size. Investigating their aggregative behaviour can be a tool to identify life history events such as mating, which is important for species management and habitat protection. We combined acoustic telemetry with network analysis to investigate co-occurrence aggregations among spiny dogfish in a complex fjord network in western Norway between October 2022 and April 2024. Data from 74 sharks tagged with acoustic transmitters equipped with depth sensors were analysed to conduct a four-dimensional co-occurrence network analysis. The analysis revealed that spiny dogfish showed a high centrality degree, interacting with many conspecifics, whereby male-male interactions were strongest. Time had similar effect on interaction patterns for the three interaction types (male-male, female-female, and female-male), while depth had a positive effect on same-sex interactions and a stronger effect on female-male interactions. There was a higher number of interactions between males and females in August and September, and in more confined spatial areas than in other months, potentially indicating mating periods and areas used by spiny dogfish. Protecting spiny dogfish populations, especially when and where mating occurs, is crucial for ensuring sustainable populations and appropriate management measures to prevent future dramatic population declines.