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

Section 13: Conclusions

Although mass strandings are unfortunate events, they offer rare opportunities to examine large groups of individuals sharing a common ecological context, and gain data around the health, behaviour and ecology of an otherwise largely inaccessible species.

This investigation into the mass stranding of 54 long-finned pilot whales at Tolsta, Isle of Lewis demonstrates the value of a multifactorial, multidisciplinary approach for assessing complex events such as this. To understand the role and impact of factors potentially causal to this event, it is necessary to be able to reliably distinguish what is normal from what may not be. This in turn relies on detailed baseline data across a suite of metrics describing health, reproduction, and toxin burden. These provide the context necessary to collate and interpret the findings. Integrating data from detailed necropsies, supported by extensive histopathology and microbiological screening enabled detailed evaluations to be made about individual body condition and disease status. The addition of data on feeding ecology, reproductive status and toxicological burden, combined with an appraisal of environmental and oceanographic factors, offered a robust dataset for evaluating both proximate and ultimate causes of mortality. A summary evaluating plausible causal factors is given in Table 4.

These examinations, conducted across multiple individuals, revealed no significant evidence of infectious disease, traumatic injury, or pathological abnormalities. The animals were in generally good nutritional condition, with low parasite burdens and no substantial toxin or contaminant burdens for the most likely agents. It is therefore possible to exclude disease or poor health as likely primary drivers of the event.

It is likely significant that one animal showed signs of dystocia, which may have acted as a social or behavioural trigger. This species' well documented strong social cohesion may have contributed to group following behaviour, exacerbated by local environmental conditions, including shallow bathymetry, onshore winds, and a soft substrate; all factors likely to have impaired navigation. These environmental variables, in conjunction with social dynamics, plausibly culminated in a high mortality rate once stranding occurred.

Harmful algal bloom screening for saxitoxin and domoic acid tested below the limit of detection for both toxins, indicating that harmful algal blooms were not a contributing factor to the stranding event.

Heavy metal analysis revealed that the adult animals involved in the stranding exhibited higher contamination levels than juveniles, consistent with expected bioaccumulation patterns over time. Measured concentrations fell within ranges reported in previous studies of similar cetacean populations. Although some individuals approached established thresholds for mercury-induced hepatotoxicity, overall contamination levels were unlikely to have contributed to the mass stranding event.

Feeding ecology analysis results suggested the animals showed no evidence of recent dietary shifts, suggesting that prey pursuit in unfamiliar shallow waters was unlikely to have triggered the event.

Genetic and isotopic data revealed the presence of multiple unrelated pod units, some with distinct isotope signatures, suggesting that the whales were aggregating prior to the stranding event (O’Callaghan, 2023; Oremus et al., 2013).Widespread spatial disruption of kin, including separation on the beach, notably among mother–juvenile pairs, might be an indication that that social disorganisation and confusion during entry into shallow waters may have been a contributory factor in the stranding.

Kinship analysis revealed distinct resource-use patterns among pods and the absence of certain members (e.g., juveniles without mothers), suggesting that not all individuals present in the area stranded. When integrated with reproductive assessments which indicated that several females were pregnant with near-term or recently conceived foetuses or had recently given birth, these findings support the hypothesis that the event represented a clan-level aggregation of multiple genetically distinct pods, potentially assembled for breeding and calving. Although data from Scotland are limited, observations from Norway and the Faroe Islands suggest that higher latitudes are used as calving areas by this species. (Donovan et al., 1993; O’Callaghan, 2023) Historical sighting data do not indicate large coastal aggregations for this species, and thus further research would be required to understand if the distribution or presence of these whales may be changing, for example, in response to drivers such as climate change or prey availability.

Acoustic analysis of two underwater recorders located at a site approximately 12.5 km offshore of Tolsta, and at a site on the west of the Lewis, indicated the presence of routine shipping noise and echosounder signals consistent with typical background conditions in Scottish coastal waters. No noise sources of unusual character, magnitude, or duration, such as those known to influence cetacean behaviour, were detected in the data. Furthermore, no marine activities were reported in the area during the relevant period that would be expected to generate acoustic disturbance of sufficient intensity to cause a mass stranding event. It remains possible, however, that disturbance sources not detected may have occurred.

Although no single factor was identified as the definitive cause of the event, it is plausible that once the animals approached the shore, a combination of factors contributed to the stranding. These include the presence of a female experiencing dystocia, the associated social milling behaviour observed the previous day, and the bay’s bathymetric features, which may have made navigation difficult. This pattern aligns with the species’ strong social cohesion, in which individuals tend to remain close to a compromised conspecific. The circumstances are therefore considered more likely to have arisen from a combination of interacting biological and environmental stressors, rather than from a single causative factor.

This study also highlights the value of combining genetic and dietary markers to distinguish pod units in future stranding events. Investigating changes in long-finned pilot whale distribution and habitat use around the UK and understanding seasonal changes on their distribution should be part of future research.

The work summarised in this report will be further disseminated through peer-reviewed publications, expanding on elements of the 2023 stranding event and broader aspects of pilot whale ecology and population health. Together, these data offer a valuable reference point for future stranding assessments and contribute to a deeper understanding of long-finned pilot whale conservation in a changing marine environment.

1 (green) = No or limited evidence; 2 (orange) = Plausible influence.