Longline hook testing in the mouths of pelagic odontocetes

Abstract

Several species of odontocete cetaceans depredate bait and catch and, as a result, become hooked and entangled in pelagic longline fisheries. The present study measured how selected commercial longline hooks, including “weak hooks”, behaved within odontocete mouths. Five hooks (Mustad-16/0, Mustad-18/0, Mustad J-9/0, Korean 16, and Korean 18) were tested on three species of odontocetes known to interact with longline fisheries—short-finned pilot whales (Globicephala macrorhynchus), Risso's dolphins (Grampus griseus), and false killer whales (Pseudorca crassidens). Specimens were secured to a stanchion, hooks were placed in the mouth at multiple positions along the dorsal lip, and the force required to pull each hook free was measured. The soft tissue lips of these odontocetes were capable of resisting forces up to 250 kg before failing. The polished steel M-16, M-18, and J-9 hooks straightened at forces between 50 and 225 kg, depending on hook gauge. When straightened, these hooks exposed the sharpened barb, which sliced through the lip tissue, usually releasing the hook intact. The K-16 and K-18 hooks behaved very differently, breaking at higher forces (110–250 kg) and consistently just at the barb; usually, there was measurable soft-tissue loss and often shards of the hook were retained within those soft tissues. The different behaviours of these two hook types—the M and J type polished steel vs. the K type carbon steel—were consistent across all species tested. Mechanical tests were also conducted to determine if hooks could fracture the mandible of these same odontocetes. Only the M-18 and K-18 hooks had sufficiently large gapes to hook around the mandible, and both hook types fractured bone in short-finned pilot whales and Risso's dolphins. These results support other lines of evidence indicating that longline hooks can cause serious injury to these species, and suggest possible steps to mitigate these impacts.

Marine Mammal Interactions With Fisheries: Review of Research and Management Trends Across Commercial and Small-Scale Fisheries

“…Modifications to longlines have been extensively trialed. Hook modifications (McLellan et al, 2015;Hamilton and Baker, 2019), gangions (O'Connell et al, 2015) and decoys (Wild et al, 2017), have (commercial and small-scale fisheries); as per the management categories described by Dawson et al (2013). significantly reduced the rate of killer whale and sperm whale depredation in longline gear.…”

Origin and Persistence of Markings in a Long-Term Photo-Identification Dataset Reveal the Threat of Entanglement for Endangered Northern Bottlenose Whales (Hyperoodon ampullatus)

“…Baird et al (2014) proposed that trailing edge fin scars could potentially occur if whales that became hooked in the mouth thrashed or twisted against the line to break free. Given the low probability of observing beaked whale entanglements, simulation of entanglement mechanics occurring with lines and gear associated with offshore fisheries (e.g., McLellan et al, 2015;Howle et al, 2018) could provide insight on the origin of other unusual scars. Additional analysis of melon and beaks photographs, or aerial imagery of the entire body (e.g., Ramp et al, 2021) would provide another perspective on patterns observed here and comparative estimates of the rate of fisheries interactions.…”

Best Practices for Assessing and Managing Bycatch of Marine Mammals

“…Gear modification is an available and common mitigation approach in the absence of assessment (see below). For example, much research has focused on modifying longline gear to reduce the likelihood that animals get hooked (e.g., Bigelow et al, 2012;McLellan et al, 2015).…”