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

Section 11: Feeding ecology and population structure

This event presented an opportunity to investigate broader questions around population structure and feeding ecology of a deep-diving cetacean species. The necropsy response enabled the collection of high-quality biological samples from 56 individuals (including three foetuses), allowing analyses across genetics, stable isotopes and fatty acids and metabolomics. This component of work was funded by Defra through the Healthy and Biologically Diverse Seas Evidence group and supported by JNCC. A summary of this work is presented in sections 11.1 & 11.2 with detail provided in a separate report (Plint et al in prep).

The question being addressed here was whether the animals had similar diets and foraging areas prior to stranding, whether multiple pods were involved in the larger group (otherwise known as a pod aggregation or clan gathering), whether foraging ecology had recently shifted, and whether the whales were in good nutritional condition.

11.1 Population Structure

Dr. Rachel Ball; University of Glasgow.

Dr. Niall McKeown; Aberystwyth University.

Long-finned pilot whales are the species most frequently involved in mass stranding events worldwide (Sergeant, 1962; Evans et al., 2005). Their social structure is characterised by bisexual natal philopatry, where both males and females remain in their natal groups for life.

Behavioural studies indicate that long-finned pilot whale pods typically consist of 3–14 closely bonded individuals, which are composed of long-term social companions (Augusto et al., 2017b; De Stephanis et al., 2008; Ottensmeyer & Whitehead, 2003). These pod units aggregate to form pod complex larger social groupings within pod complexes; alloparental care has been observed from both males and females (Augusto et al., 2017a).

At an even broader social level, clans may form, consisting of temporary aggregations of over 150 individuals. Mating behaviour observed within these clan gatherings supports the hypothesis that exogamy (outbreeding between pods) occurs in these larger assemblages (De Stephanis et al., 2008).

Although genetic studies on free-ranging long-finned pilot whales are scarce, research on the Faroese grindadráp (pilot whale hunts) provides valuable insights into their social genetics. These hunts typically involve entire groups, presumed to represent pods although, with median group sizes of 57 individuals these are indicative of pod complexes rather than single pod units ((Nichols et al., 2020)). Genetic analyses of these captures have revealed extended family structures in which males remain within their natal groups but avoid breeding with closely related individuals (Amos et al., 1993).

Results: Genetic analysis of this mass stranding event revealed that the long-finned pilot whales involved belonged to multiple, unrelated pod units, consistent with a multigroup association likely related to breeding. Although relatedness within maternal clusters was high, the average relatedness across the entire group was low, indicating the presence of several genetically distinct family units. No evidence of inbreeding or heterozygosity deficiency was detected in the MSE group, conforming to patterns observed across the broader eastern North Atlantic population. Furthermore, four unique (private) alleles were detected in some individuals within the group, which were not found in any other single or mass strandings, highlighting the potential for such events to result in the loss of rare genetic variants.

Additionally, 53% of juveniles in this MSE were missing their mothers. Some juveniles were assigned to maternal clusters, suggesting potential alloparental care behaviours such as babysitting prior to the MSE. Critically, spatial separation of related individuals across the stranding site demonstrated that physical proximity on the beach did not reflect genetic relatedness. This finding has important implications for rescue operations: responders should not assume that a calf nearest to an adult female is her offspring, and live animal incident management decisions should account for the complex social structure of mixed-pod aggregations rather than treating stranded groups as single family units. Widespread spatial disruption of kin, including separation of most mother–juvenile pairs, may have led to social confusion upon entering shallow waters, contributing to the stranding.

11.2 Stable Isotope and Fatty Acid Analysis

Dr Tessa Plint & Anna Kebke: University of Glasgow, Heriot-Watt University.

Stable isotope and fatty acid analyses are powerful tools in remote ecological monitoring, offering valuable insights into the foraging niche, diet composition, and habitat use of marine mammals. These analytical techniques provide critical information, including trophic position within the food web, habitat preferences (such as coastal, pelagic, or benthic zones), estimations of preferred prey species and their proportional contributions to the diet, as well as seasonal and ontogenetic dietary shifts. Furthermore, they are useful in assessing nutritional condition and foraging success.

The principle underlying stable isotope analysis is encapsulated in the concept "you are what and where you eat," which describes how isotopic signatures integrate information about diet and habitat into animal tissues over time. These isotopic signatures accumulate at varying rates depending on the tissue being analysed, enabling the reconstruction of dietary history over different temporal scales. For instance, liver tissue reflects dietary information over a matter of days, skin over weeks, muscle over months, and bone over years (Connolly et al., 2004; Deniro & Epstein, 1978, Teixeira et al., 2022).

The objective of this analysis was to employ stable isotope and fatty acid analyses to reconstruct the ecological niche, diet, and habitat use of long-finned pilot whales involved in the 2023 Isle of Lewis mass stranding event. By analysing stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulphur (δ³⁴S) across a time series of tissues with different dietary incorporation rates (liver, skin, muscle) and blubber fatty acids, it was possible to gain insights into the trophic level, foraging , and nutritional condition of these animals in the weeks to months preceding the event. The resulting data were compared with samples from single-stranded individuals affected by chronic illness or starvation to identify contrasting values between healthy and compromised individuals, enabling a more comprehensive understanding of the health status of the stranded group.

Results: Results from stable isotope and fatty acid analysis showed that the animals involved in the 2023 Isle of Lewis MSE were in good nutritional condition and exhibited no evidence of a recent dietary shift indicative of a change in foraging depth or habitat use. This finding contrasts with one hypothesis that the whales may have been exploiting prey in shallower waters prior to stranding and suggests there was no sudden (i.e., days prior) alteration in their foraging ecology. This project demonstrated that stable isotope and fatty acid analysis is a viable tool for long-term, remote monitoring of data-deficient cetacean species.