Rates of entanglement inferred from scarring prevalence of humpback whales photographed in US Oregon waters

Entanglements in fishing gear are a significant threat to cetaceans worldwide and a concern for large whales in US waters. Yet, entanglement events are infrequently observed, and their lethal and sublethal impacts are likely underestimated. Photographic analysis of wrapping scars on whales shows promise to better assess entanglement. Here, we analyzed scars on 571 individual humpback whales photographed in Oregon, USA, waters (2005–2023). We scored 1533 photos of the tailstock and fluke regions for evidence of prior entanglement. We found that scarring prevalence varied by photo type, with perpendicular/forward tailstock photos showing the highest scarring prevalence (respectively 19.1% and 17.6% most likely caused by entanglements) compared to photos of the fluke underside that tended to miss likely entanglement events (57.4% false negatives). Depending on the scoring approach, 8.2 to 27.3% of sampled whales were likely entangled at least once in their lifetime. We found no significant spatial effect on scarring prevalence and a weak increasing (2016–2020), then decreasing (2020–2023), temporal trend. Males had a significantly higher scarring prevalence than females. Simulations of population trajectory and photographic sampling designs revealed that, even in an unrealistically optimistic scenario of new fishing regulations reducing the number of entanglements to zero, 165 ind. yr–1, for 5 yr, would need to be sampled with good-quality tailstock photos to detect the decrease in scarring prevalence with 80% statistical power. Our findings enable recommendations for monitoring impacts of fishing gear interactions with humpback whales using US West Coast waters.