Investigating the behaviour of deep-sea fish in the vicinity of an oil and gas installation

Acoustic telemetry of wild fish using baited transmitters was conducted at the Snorre A offshore oil and gas installation in the Norwegian sector of the North Sea to investigate fish behaviour and spatial distribution to determine whether fish associated with the platform were resident or transient. Traditional capture, tag, and release approaches were not feasible in the deep waters surrounding the installation. Instead, a non-invasive approach was employed in which baited acoustic transmitters, equipped with depth and activity sensors, were deployed and ingested voluntarily by fish in situ at depths of 300–310 m above the seafloor. A total of 12 acoustic tags were deployed, six of which were successfully ingested and retained for sufficient durations to generate behavioural data over a five-week period. Tag detections were recorded by three acoustic receivers positioned in a triangulated array along the edge of the 500 m safety zone surrounding the platform. Depth, activity, and movement data revealed clear differences in behaviour amongst tagged individuals, suggesting ingestion by multiple fish species. Based on depth distributions and known species composition in the area, at least three tags were likely consumed by ling (Molva molva), two by tusk (Brosme brosme), and one by saithe (Pollachius virens). Synchronisation tag data indicated a typical detection range of 500–700 m, providing approximately 80% coverage of the safety zone. Consequently, estimates of fish residency in the area should be considered conservative. Despite this limitation, four of the six tagged fish exhibited prolonged presence near the installation, indicating resident rather than transient behaviour. The acoustic telemetry system performed reliably under offshore conditions, generating approximately 180,000 detections and providing rare and valuable data on fish behaviour around an offshore installation. Overall, this study demonstrates proof of concept that baited acoustic telemetry is a viable and welfare-friendly method for studying deep−sea fish behaviour in offshore environments, reducing behavioural bias associated with invasive tagging methods and offering new opportunities to assess fish residency and exposure to offshore petroleum activities.