Genetic data indicate a lack of population structure of Risso's dolphins (Grampus griseus) around the British Isles

Risso's dolphin (Grampus griseus) is a widely distributed oceanic species; its global and regional population structure remains poorly understood. To assess their genetic diversity and connectivity in British waters, we analysed tissue samples from stranded individuals collected between 1992 and 2016 using 16 nuclear microsatellite loci and a 433-bp mitochondrial DNA (mtDNA) sequence. Microsatellite analyses (STRUCTURE, AMOVA, and PCA) consistently indicated a single genetic cluster with no significant geographic differentiation among sampling sites, suggesting a panmictic population. BayesAss results further supported high gene flow and limited isolation among regions. The mtDNA dataset identified six haplotypes with low haplotype and nucleotide diversity, showing no spatial structuring within the British Isles. Phylogenetic reconstruction revealed that all British haplotypes were endemic and formed a monophyletic clade closely related to haplotypes from the Azores in the North Atlantic. Together, the nuclear and mtDNA data indicate low genetic diversity and a lack of population subdivision among British Risso's dolphins. These findings imply recent colonisation and high dispersal capability, consistent with patterns observed in other wide-ranging marine mammals. Continued monitoring, broader geographic sampling, and large-scale photo-identification are needed to refine understanding of Risso's dolphin population dynamics and to inform conservation management.