Transferable Stressors in Small Cetaceans: Historical Status, Current Trends and Future Directions

Cetaceans face a wide range of natural and anthropogenic pressures that can impact on the viability of populations. Assessments of cetacean populations frequently consider single non-transferable stressors with direct effects on survival, such as fisheries bycatch, but more rarely consider stressors transmitted between animals via the food web, through close physical contact and from mother to calf. Transferable stressors can affect population dynamics via sub-lethal or lethal effects on individuals. This literature review concerns transferable stressors in small cetaceans, including their transfer routes, prevalence and effects on individuals and populations, as well as the cumulative effects of multiple stressors. We focus particularly on transferable stressors frequently affecting common small cetaceans (harbour porpoises and common, bottlenose and striped dolphins) in European waters. These stressors include harmful algal blooms, viruses (e.g. Morbillivirus), bacteria (e.g. Brucella), parasites (e.g. Anisakis and lungworms), organic and inorganic contaminants and microplastics. Patterns and trends in prevalence are compared across species, areas and stressors, implications for conservation are considered and knowledge gaps are identified. Further research is needed on dose–response relationships and mechanisms of stressor interactions. While available methods have permitted a better understanding of the adverse effects of transferable stressors, integration into population assessments and consequent management plans remains a challenge.