First estimates of entanglement rate of humpback whales Megaptera novaeangliae observed in coastal Icelandic waters

Abstract

Entanglement in fishing gear is a significant anthropogenic source of large whale injury and mortality. Although entanglements have been reported in the eastern North Atlantic, their frequency has not been previously estimated. This study used systematic scar analysis to estimate the frequency of non-lethal entanglements among individual humpback whales Megaptera novaeangliae off coastal Iceland, from 2005 through 2017. Images of the caudal peduncle and fluke insertions of 379 individuals were analyzed for wrapping injuries and notches known to be indicative of entanglement. The results indicated that at least 24.8% (n = 94, 95% confidence intervals [95% CI]: 20.5-29.1%) of individuals had a history of prior entanglement when first encountered. Depending on the metric used, the whales subsequently acquired new entanglement-related injuries at an average rate of 1.9% (95% CI: 0.6-3.2%) or 16.3% (95% CI: 3.0-29.3%) per year, with no statistically significant change over time. Furthermore, evidence suggests that at least some entanglements occurred locally. Observations of whales with gear still entangling the body confirmed the patterns of injury studied here. These results are lower than scar-based estimates from other parts of the world, but the cause of this difference requires further study. Scar-based methods underestimate the frequency of prior entanglement because some injuries heal beyond recognition, do not involve the caudal peduncle, and may occur on whales that die before they are studied. Long-term monitoring of humpback whale entanglement in Icelandic coastal waters is important for evaluating the local effects of fisheries, as well as the viability of the endangered Cape Verde breeding population.