Disentangling Natural and Anthropogenic Forms of Mortality and Serious Injury in a Poorly Studied Pelagic Dolphin

Erin Ashe,Philip S Hammond,Rob Williams,Alexandra Morton

doi:10.3389/fmars.2021.606876

Erin Ashe, Philip S Hammond + Show 2 more

Open Access

https://doi.org/10.3389/fmars.2021.606876

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Abstract

Killer whale (Orcinus orca) populations specialize in both prey and prey acquisition tactics around the world and may be a primary evolutionary driver of the habits of small cetaceans. Entanglement in fishing gear is the most significant anthropogenic threat to the survival of cetaceans worldwide. Distinguishing between natural and human-caused sources of mortality and injury is a key task in marine mammal conservation and management. In British Columbia (BC), Canada, mammal-eating killer whales co-occur with Pacific white-sided dolphins (Lagenorhynchus obliquidens). Bycatch mortality rates are unknown here due to lack of systematic fisheries observer coverage. Drawing from more than three decades of first-hand observations of killer whale attacks on Pacific white-sided dolphins, we identify common themes with respect to predatory behavior of killer whales and anti-predatory responses of dolphins. With input from veterinary pathologists, we outline clues to distinguish killer whale rake marks from scars and wounds likely to be caused by fishery interactions. We examined photographs of 415 well-marked Pacific white-side dolphins for evidence of injuries and scars consistent with either killer whale attacks or fishery interactions. In this case study, healed scars from interactions with killer whale predators were ∼8× more common than scars from fishery interactions (3.9 vs. 0.5%), suggesting that predation is a much bigger threat to Pacific white-sided dolphins in the study area than anthropogenic impacts, or that dolphins are much less likely to survive a fishery interaction than a predation attempt. To advance our knowledge on poorly studied species, multiple lines of evidence will be needed.

Highlights

Killer whale (Orcinus orca) populations specialize in both prey and prey acquisition tactics around the world, with prey ranging from fish and seabirds to great whales
The majority (69%) of the predation events took place in confined waters, either a shallow bay or a narrow passageway, with eight respondents indicating that the killer whales used the local topography to actively herd the dolphins into a place where they could be killed
The anti-predatory tactics used by the dolphins generally started with high-speed chases (“squall”) and, when that failed, eight respondents noted that the dolphins entered shallow waters to avoid predation, sometimes beaching themselves on shore in the process

Summary

Introduction

Killer whale (Orcinus orca) populations specialize in both prey and prey acquisition tactics around the world, with prey ranging from fish and seabirds to great whales. Disentangling Natural and Anthropogenic Mortality to exploit particular prey species [e.g., carousel feeding on herring (Clupea harengus) in Norway (Simila and Ugarte, 1991), intentional stranding to capture seals (Lopez and Lopez, 1985; Guinet, 1991), and “wave-washing” seals off ice floes (Visser et al, 2008; Pitman and Durban, 2012)] These foraging tactics may be a primary evolutionary driver of habitat use and behavior of many marine small cetaceans, including their seasonal movement patterns and acoustic repertoires (Morisaka and Connor, 2007; Srinivasan and Markowitz, 2010; Kyhn et al, 2013). Improving our knowledge of the diets of killer whale ecotypes is critical to understand the role predators play in communities, to explore and mitigate conservation conflicts among predator, prey, and fisheries, and to inform resource management programs that are robust to ecosystem considerations (Chasco et al, 2017)

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