Passive acoustic monitoring reveals feeding attempts at close range from soaking demersal longlines by two killer whale ecotypes

Abstract

Odontocetes depredating fish caught on longlines is a serious socio-economic and conservation issue. A good understanding of the underwater depredation behavior by odontocetes is therefore required. Historically, depredation on demersal

Counting sperm whales and visualising their dive profiles using two-hydrophone recordings and an automated click detector algorithm in a longline depredation context

“…This acoustic approach could then be complementary to traditional visual counting from the vessel at hauling (Roche et al 2007; Tixier et al 2010). However, the acoustic method has the added advantage that whales can be counted also when the fishing vessels are not hauling, and thus have no visual observers in action (Richard et al 2022). Also, in some of the deployments reported here, the acoustic method counted more whales than reported by the visual observers, indicating that the latter may underestimate the real hauling interaction rates.…”

“…Finally, (Richard et al 2020) observed plausible evidence of sperm whale depredation on demersal soaking longlines using accelerometers fixed on longlines' hooks, with an event confirmed by the entanglement of a sperm whale. Indeed, settings have specific acoustic signature which could attract attention of odontocetes before soaking (Richard et al 2022). Nevertheless, this study was based on a limited dataset and the occurrence of depredation behaviour on soaking longlines could not be quantified.…”

“…In this paper, the focus is on the sperm whale activity during the whole fishing process, i.e. from setting to soaking as in previous studies on killer whales (Cieslak et al 2021; Richard et al 2022). A passive acoustic monitoring method is described, allowing the dive behaviour of sperm whales to be visualized and classified, and thus to be used as a proxy of foraging activity.…”

“…On the other hand, demersal longlines are thought to be mostly depredated during hauling (Guinet et al 2014; Hucke-Gaete et al 2004; Mathias et al 2012; Straley et al 2014; Towers et al 2019; Roche et al 2007). However, recent studies suggested that sperm whales and killer whales may also depredate from demersal longlines during soaking (Richard et al 2020, 2022; Janc et al 2018; Cieslak et al 2021; Towers et al 2019). Although proofs that killer whales actually depredate on soaking demersal longlines are still not clearly confirmed, pieces of evidence for sperm whales doing so are clearer.…”

“…In order to better describe and quantify depredation on soaking longlines, passive acoustic monitoring (PAM) is of great interest as it may cover a longer temporal scale around soaking longlines. Such approach was recently used to describe a common foraging activity by killer whales around soaking longlines (Richard et al 2022). However, this study only relied on presence/absence of acoustic signals produced by killer whales, and there was no confirmed evidence of killer whales foraging on the seafloor or depredation on soaking longlines.…”