Prioritizing future evidence needs for marine and freshwater mammal conservation action

Abstract Marine and freshwater mammals are increasingly threatened due to human activity. To improve conservation practice, decisions should be informed by the available evidence on the effectiveness of conservation actions. Using a systematically collated database of studies that test the effectiveness of actions to conserve marine and freshwater mammals, we investigated the gaps and biases in the available scientific evidence base. While there is a growing evidence base covering actions to address key threats (e.g. fisheries and bycatch) to marine and freshwater mammal populations, we identified large geographic and taxonomic biases. There was no relationship between the number of studies and marine mammal species per ecoregion and we found biases towards coastal areas of the Global North, with many regions and species having little or no evidence available. The number of studies per species did not correlate with (1) the threat level, (2) evolutionary distinctiveness or (3) the public ‘popularity’ of the study species. We also found a mismatch between actions tested and the actions suggested as needed in the International Union for the Conservation of Nature (IUCN) Red List. Several of these gaps and biases likely reflect the feasibility of researching marine mammal populations; many species can be difficult to access, with limited baseline information on populations and threats, and testing actions can require costly long‐term monitoring. Prioritizing the most cost‐effective conservation strategies for marine and freshwater mammal species will require a comprehensive evidence base on the effects of actions. Continuing to build the necessary baseline data, and focusing future research and funding towards the priority gaps identified in this study will be important to deliver this target.