Anthropogenic and killer whale (Orcinus orca) scarring on Pacific Coast Feeding Group gray whales (Eschrichtius robustus) in northwest Washington

Abstract

Gray whales (Eschrichtius robustus) face many threats to their survival which are multi‐faceted and difficult to assess. In this study, we evaluate photographs of Pacific Coast Feeding Group (PCFG) gray whales taken between 2014 and 2020 off the northwest coast of Washington to document the occurrence of scarring from fishing gear entanglements, vessel strikes and killer whale (Orcinus orca) attacks and compare our findings with scarring prevalence in gray whales off Sakhalin Island, Russia. We found that, of the 139 PCFG whales evaluated, 11.5% had scarring from entanglements, 3.6% had scarring from vessel strikes and 25.9% had scarring from killer whale attacks. We found no differences in scarring rates between males and females. Observed rates of scarring from entanglements for PCFG whales were less than rates observed for gray whales off Sakhalin Island, Russia, while scarring rates from vessel strikes were slightly greater for PCFG whales, but the differences were not statistically significant. The frequency of scars due to killer whale attacks on PCFG whales was significantly lower than reports for whales observed at Sakhalin Island. Estimates of anthropogenic and killer whale scarring in this study are likely biased low due to limited photographic coverage of the caudal peduncle and flukes, where scarring from entanglements and killer whale attacks are most commonly observed. The methods used here were similar to studies at Sakhalin Island, suggesting the evaluations had similar biases. If we assume that observations of non‐lethal scar sources on gray whales are proportional to mortality rates from those sources, findings from this study can help evaluate if current models present plausible injury and mortality estimates for PCFG and Sakhalin Island whales given the finding of no statistical difference in non‐lethal scarring rates between the two groups.