Pilot whale stranding: acoustic analysis report

2. Background

On the morning of Sunday 16th July 2023 a mass stranding of 55 long-finned pilot whales was reported on the Traigh Mhòr beach at Tolsta on the Isle of Lewis (Figure 1). The time at which the pilot whales stranded is unknown, although the animals were first reported on the beach at Traigh Mhòr at 6:38 UTC. The Scottish Marine Animal Strandings Scheme (SMASS), University of Glasgow, along with volunteers from British Divers Marine Life Rescue (BDMLR) arrived the same day to assess the situation and respond to it. Of the 55 whales, 54 died or were euthanised on welfare grounds and one was refloated. All carcasses were sampled and necropsy examinations performed on 23 of the 54 carcasses to investigate possible biological, toxicological and physiological causes of the stranding. Findings of the pathological investigations will be published in due course.

Factors known to have contributed to cetacean strandings in Scotland, and around the world in the past include disease, starvation, contaminant burden, entanglement, plastic ingestion, aggressive interactions with other marine animals, collision with vessels, weather, underwater noise and barotrauma (Barnett et al. 2009; Bennett 2001; Coombs et al. 2019).

Underwater sounds have been shown to injure or disturb cetaceans (Nowacek et al. 2007; Weilgart 2007), in addition to inducing temporary or permanent changes in their hearing (Southall et al. 2021). Around the world, cases have been documented of cetacean mass strandings that have been linked to underwater noise, across many species (Hildebrand 2005). Pilot whales specifically are reported to show avoidance responses to some human sounds, including navy sonar (Antunes et al. 2014) and several pilot whale strandings have been linked to anthropogenic noise (e.g. Dalton 2006, Hohn et al. 2006). In Scotland, clearance of unexploded ordnance may have been a factor in a previous mass stranding of around 70 pilot whales (Brownlow et al. 2015). It is therefore important that potential acoustic factors are considered in the investigation of this stranding.