Dual colour and thermal drone surveys improves detection of marine debris entanglements in fur seals

Marine debris entanglement is a growing threat to pinnipeds, with acute consequences including injury, restricted movement, and increased energy expenditure now well documented. The use of drones to monitor populations and estimate entanglement prevalence is growing. Drone-borne colour (RGB) cameras enhance detection of conspicuous debris, but thin or transparent materials such as fishing line are often missed. Thermal infrared (TIR) imaging may improve detection by highlighting heat from wounds or disrupted fur caused by entanglement. We assessed the efficacy of drone-borne RGB and TIR imaging for detecting entanglements in Australian fur seals (Arctocephalus pusillus doriferus) at Seal Rocks, south-eastern Australia. Across 54 drone surveys using representative commercial RGB + TIR sensors and concurrent handheld imaging, we captured imagery of entangled and non-entangled individuals. The resultant orthomosaic images and videos were classified by reviewers with relevant experience, and reviewer performance was assessed using generalised linear mixed models to estimate recall and precision. Entanglements were clearly visible in handheld TIR via elevated heat signatures. Similar thermal anomalies were visible in 81% of RGB + TIR drone detections of marine debris entanglements, with 95% reviewer agreement. RGB + TIR videos achieved the highest estimates and most reliable balance of recall and precision, followed closely by RGB alone, though differences were not always statistically significant. Importantly, fishing line was effectively detected in dual RGB + TIR drone imagery. This study presents the first use of drone-borne TIR imaging to detect marine debris entanglements and demonstrates that combining RGB and TIR video maximises detection performance.