Entanglement scarring rates in three species of rorqual whales from the Gulf of St. Lawrence

David Gaspard, Christian Ramp, Julien Delarue, Scott Landry, Richard Sears

Abstract:
Lethal and sub-lethal effects of entanglement are a serious threat to survival and recovery of cetacean populations. However, it has been considered that rorqual whales, due to their size and strength more easily break free of fishing gear and are therefore less vulnerable to entanglement. In addition, their more pelagic distribution may reduce encounters with coastal fishing gear. Based on standard photo-identification pictures, which only cover the flanks of each whale, the proportion of blue and fin whales with entanglement scarring was previously estimated at around 10%. In this study, we analyzed blue (n=203), fin (n=322) and humpback whales (n=120) photographs taken on a feeding ground in the Gulf of St. Lawrence from 2009-2016 for entanglement scars. Effort was made to photograph all body regions above the water line, including the peduncle, based on protocols developed for humpback whale scarring rates. Results clearly show that entanglement is underestimated when only using standard photo-ID photos; when the peduncle is visible, the proportion of individuals with scarring is 37% (n=37) for blue whales and 43% (n=196) for fin whales. Photo-ID pictures alone would have given an estimate of 8% and 6% of blue and fin whales with entanglement scarring. An estimated 80% (n=76) of humpback whales have entanglement scars, similar to estimates for Gulf of Maine humpback whales. Our findings likely underestimate the extent of entanglements since the mouth and pectoral fin were not captured in these photos. The threat of entanglement for these rorquals must be taken into account in recovery strategies; actions to improve entanglement assessment and reduce entanglement risk are required.