Investigation into the long-finned pilot whale (Globicephala melas) mass stranding event, Tolsta, na h-Eileanan Siar, July 2023

Executive summary

Long-finned pilot whales (Globicephala melas) demonstrate among the highest stranding frequencies observed globally. This species exhibits pronounced social cohesion and matrilineal structure that are thought to predispose populations to collective stranding, though proximate causal factors often remain elusive. These events, often involving substantial numbers of individuals, can generate considerable public and media attention and inquiry into underlying causative mechanisms. They also offer invaluable research opportunities for studying an otherwise elusive deep-water cetacean species.

Two large-scale strandings of long-finned pilot whales occurred in Scottish waters during 2023 and 2024, presenting the opportunity for a multidisciplinary investigation. This report details the comprehensive analysis into the mass stranding event (MSE) of long-finned pilot whales that occurred in Tolsta Bay, Isle of Lewis, Scotland, on 16 July 2023.

Necropsy examinations indicated good overall health of all individuals, with good nutritional condition despite no indication of recent feeding. Microbiological and histological analysis indicated no major underlying diseases. No viral infections were identified, bacterial infections, where isolated, were considered likely to be incidental and parasitic burden was considered to be within normal limits for this species. There was indication of elevated heavy metal contamination in several individuals although the biological significance is unclear and it is considered unlikely this contributed directly to the stranding event. No algal toxins were detected.

Environmental factors were investigated, including wind, tide and bathymetric aspects. The shallow, steeply shelving nature of the bay, potentially combined with onshore winds present during the stranding event, was identified as having had a potential role in hindering the pod's navigation. This species usually occurs in deep, offshore and shelf edge waters and have been rarely reported in the inshore waters of the Northern Minch as evidenced by the scarcity of previous visual sightings records from the region. There presence in this region was a highly unusual occurrence.

The presence of dystocia - a difficult birth typically caused by a large or poorly positioned calf - in a mature female was noted and suggests a possible social trigger for the stranding. Strong social cohesion may have then led the entire group to follow the distressed individual into shallow waters. Once stranded, the animals were unable to refloat themselves due to a combination of surf generated by onshore winds and the soft sand substrate on the beach. Drowning was determined as the primary cause of mortality for many of the stranded animals.

Long-finned pilot whales are highly social animals, and behavioural responses to disturbance, whether natural (e.g., predators) or anthropogenic (e.g., vessels, underwater noise) are well documented in this species. Consequently, the potential role of other factors, including underwater noise were considered in this investigation. A full analysis of impulsive and continuous noise data collected from hydrophones deployed near the stranding site to assess the presence of cetaceans around the time of the event and potential anthropogenic sound sources has been published and is summarised in the pilot whale stranding acoustic analysis report.

This investigation emphasises the importance and value of taking an integrated ecological approach, integrating health assessments, infectious disease screening, toxicological and environmental data analysis, and social behaviour considerations. Data produced by this investigation will improve our understanding of potential factors that could influence pilot whale strandings and inform pilot whale ecology.