Anthropogenic and killer whale scar data for gray whales in northwest Washington

Description

These data were used to evaluate anthropogenic and natural scar occurrences of Pacific Coast Feeding Group (PCFG) gray whales off northwest Washington, United States, from 2014–2020 (Fig. 1). The objectives of the study were to document the frequency of entanglement, vessel strike, and killer whale scars on PCFG gray whales, to evaluate differences in scarring by sex, and to compare those observations to previously documented scarring rates for gray whales in the Western North Pacific (WNP), near Sakhalin Island, Russia (Bradford et al., 2009; Weller et al., 2018). There were 139 individual PCFG gray whales photographed during small vessel surveys conducted throughout the study period. Photographs of each whale were analyzed to determine the source and certainty (i.e. “definitive” and “potential”) of observed scars in 23 pre-defined body regions of PCFG gray whales (Fig. 2). A manuscript of our results will be submitted for publication to the Journal of Cetacean Research and Management in February 2024.

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PCFG gray whales were photographed using SLR digital cameras during small-vessel surveys. These surveys were conducted off northwest Washington, United States: from Neah Bay to Sekiu Point; and from Neah Bay to Cape Flattery and then south along the Pacific Ocean coast to Sea Lion Rock (Fig. 1) from 2014–2020. Cascadia Research Collective identified each photographed whale using methods described in Calambokidis et al. (2002). For each gray whale sighting, photographs were evaluated for the visibility and scar source presence in each of the 23 pre-defined body regions (BR). Body regions were adapted from Bradford et al. (2009), adding the left and right sides of the caudal peduncle (BR10 and BR11; Fig. 2). Visibility was scored as full, partial, or no/poor visibility, and scars were determined to have originated from an entanglement, vessel strike, or killer whale attack. If an unknown scar had suggestive, but not definitive, evidence of being due to one of the three known scar sources, it was documented as “unknown, potentially [scar source type]” (Fig. 3). Each scar was independently reviewed by a three-evaluator team (authors RW, EA and JS) that formed consensus on scar determinations. Scars definitively and potentially assigned to a scar source were then independently evaluated by three experts (authors AB, SN, and RS). A decision tree considering the agreement of scar source and certainty for scars of interest was used to assign final scar determinations by comparing the three-evaluator consensus with the expert evaluations (Fig. 4). If an equal number of definitive and potential evaluations of the same scar source were present in the four independent scores (e.g., two of each), preference was given to the potential scar assignment. Scar and visibility assessments from gray whale sightings throughout the study period were compiled into a composite score for each whale. This composite recorded the presence or absence of each scar type as well as the greatest visibility assignment for each body region of the whale. Composites were then used to analyze the proportion of whales with scars from each source, and the body regions those scars occurred in, to calculate differences in scarring by sex and to compare scarring rates between feeding locations. Bradford, A.L., Weller, D.W., Ivashchenko, Y. V., et al. 2009. Anthropogenic scarring of western gray whales (Eschrichtius robustus). Mar. Mammal Sci. 25(1): 161–175. doi:10.1111/j.1748-7692.2008.00253.x. Calambokidis, J., Darling, J.D., Deecke, V., et al. 2002. Abundance, range and movements of a feeding aggregation of gray whales (Eschrichtius robustus) from California to southeastern Alaska in 1998. J. Cetacean Res. Manag. 4(3): 267–276. Weller, D.W., Bradford, A.L., Lang, A.R., et al. 2018. Prevalence of killer whale tooth rake marks on gray whales off Sakhalin Island, Russia. Aquat. Mamm. 44(6): 643–652. doi:10.1578/AM.44.6.2018.643.

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