The stock structure of the North Atlantic population remains uncertain, despite genetic, morphometric, physiological and observational studies (ICES 1993, 1996; Fullard et al. 2000).
Studies analyzing differences in mitochondrial DNA (mtDNA) and allozymes in pilot whales across the North Atlantic have shown no significant differences between pilot whales from the western Atlantic, Iceland and the eastern Atlantic (Andersen 1993, Siemann 1994). This could be considered as evidence that there is only one stock of pilot whales in the North Atlantic. However, this uniformity may be related the social structure of pilot whales. If they occur in strong matrilineal (derived from mother) schools, as suspected, then the actual population unit is the school rather than the individual, making the genetically effective population size relatively small, in the order of several thousands rather than hundreds of thousands (ICES 1996, NAMMCO 1998b). This small effective population might be expected to have a low mtDNA variability. Andersen (1993) found that schools captured in the Faroes differed in their allozyme composition, suggesting that genetic differences between schools do exist.
More recently, however, a study using neutral microsatellite markers (Fullard 2000, Fullard et al. 2000) revealed significant genetic differentiation within the North Atlantic, and particularly between West Greenland and other both western and eastern regions (Cape Cod, Faroes and UK). This could not be explained by a simple isolation-by-distance model. Instead, the pattern of genetic differentiation suggested that population isolation occurs between areas of the ocean which differ in sea surface temperature, with 1) a cold-water population west of the Labrador/North Atlantic current, and 2) a warm-water population that extends across the Atlantic in the Gulf Stream.
Recently, Monteiro (2013) used both mitochondrial DNA (mitochondrial control region, neutral marker) and two loci of the Major Histocompatibility Complex (MHC, adaptive marker) on samples from USA, Faroes, Norway, UK and Northwest Iberia to revealed a level of genetic substructure in the North Atlantic, with consistent divergence patterns describing Northwest Iberia as a separate group.
Faroese stamp issued in 2010.
Other lines of evidence suggest that there are at least two stocks in the North Atlantic. There are morphometric (body shape) differences between pilot whales caught in the Northwest and Northeast Atlantic (Bloch and Lastein 1993, ICES 1996). This indicates that pilot whales from the two areas are unlikely to be from the same stock. In addition, the depletion of pilot whales off Newfoundland from 1947 to 1972 apparently had no effect on pilot whale abundance elsewhere, indicating that there is probably little or no exchange between this area and others (Mercer 1975, Nelson and Lien 1996).
There may also be differences over smaller spatial and temporal scales. There were significant differences in pollutant concentrations (Aguilar et al. 1993, Caurant et al. 1993) and parasite burdens (Balbuena et al. 1993, 1994, 1995) between schools of whales landed in the Faroe Islands at different times and locations. This suggests that these schools spend different proportions of their time in different areas, which again may be indicative of stock differences and negate the hypothesis of just one resident Faroese population (ICES 1993).
In 1997, the Scientific Committee of NAMMCO concluded that, based on the evidence noted above, it was likely that there was more than one stock of pilot whales in the North Atlantic, and more than one stock subject to harvesting in the Faroe Islands (NAMMCO 1998b). It was apparent that further research was still required to resolve the stock delineation of pilot whales in the North Atlantic and this is still the case. However, the new genetic information strongly point to the existence of substructuring in the North Atlantic and the 2011 and 2012 satellite tagging clearly support that pilot whales caught in the Faroes do not belong to a resident population.
Movements of pilot whales satellite-tagged on October 2nd, 2012, off Vágur, Suduroy, with 125 days of contact (Mikkelsen, pers. commn.).