European stranding networks as a tool for monitoring marine mammal populations (Part II): assessing their utility to monitor bycatch mortality

Fisheries bycatch is an important source of direct human-induced mortality for marine mammals worldwide. Despite its significance, monitoring and quantifying bycatch mortality remains challenging. The examination of stranded animals can provide key information to assess the importance of bycatch mortality, contributing to understanding its impact on populations, and identifying the fisheries responsible. However, the utility of data from strandings remains underexplored. Based on an expert consultation using a questionnaire carried out under the auspices of the International Council for the Exploration of the Sea Working Group on Marine Mammal Ecology, this study assessed the potential of stranding networks in Western Europe to support the assessment of marine mammal population status and impacts of stressors. The present paper (the second of two) focuses on information on bycatch collected from strandings, and its utility for monitoring and assessing bycatch mortality. Responses from 45 organizations in 19 countries indicated that extensive information on marine mammal bycatch mortality is collected and can sometimes be used to identify the species or populations most affected by bycatch, reveal spatio-temporal trends in bycatch mortality, and determine the fisheries and/or fishing gear involved. However, the survey identified several limitations that hinder the contribution of stranding data to bycatch impact monitoring and assessment. There is a need to strengthen data collection and reporting, especially for certain taxa and regions, and for wider adoption and harmonization of best practise protocols for diagnosing causes of mortality and understanding its context, e.g. by determining health status, estimating demographic parameters, helping to identify the fishing gears responsible for bycatch mortality, and appropriately communicating the degree of uncertainty associated with interpretations. Carcass drift models could enable stranding data to be used to estimate total bycatch mortality and contribute to assessing the impact on populations. The need for adequate baseline funding for stranding networks to carry out their activities and for streamlined data reporting was widely recognized. Achieving coordinated, harmonized, large-scale and long-term data and samples collection following best practices will be essential to ensure that data from strandings on bycatch mortality can be integrated into bycatch impact assessments to inform management.