The tidal-stream renewable energy sector continues to advance rapidly but impacts on marine receptors, such as marine mammals, remain incompletely understood. Estimating absolute density of marine mammals is a common requirement for consenting and post-consent monitoring but numerous factors may prevent collection of sufficiently robust data to generate such estimates. The outcomes of two marine mammal survey campaigns in adjacent waters in south-western Scotland and Northern Ireland were compared. Sites were 57 km apart and subjected to comparable environmental conditions. In the West of Islay site (SW Scotland), very small numbers of harbour porpoises (n = 12 sightings) were detected over 21 surveys, whilst at the Fair Head site (Northern Ireland), many more porpoises were detected (n = 70 sightings over 13 surveys to date; survey programme ongoing). Final density estimates were therefore far less robust for the West of Islay site. Harbor porpoises appeared significantly more abundant at the Fair Head site despite the short distance between the two sites. Such spatial heterogeneity in porpoise abundance raises questions about the utility of surveying very low-density areas. Development of cost-effective, adaptable and scientifically robust surveying strategies should be a priority for developers and regulators.