Marine tidal-stream renewable energy devices (MREDs) are beginning to move from demonstration to early commercial deployment. However, the ecological impacts which may result when large arrays of these devices are deployed are unknown. This uncertainty is placing a considerable burden on developers who must collect biological data through baseline and post-deployment monitoring programs under the Environmental Impact Assessment process. Regulators and other stakeholders are often particularly concerned about impacts on marine vertebrates (fish, seabirds and mammals) because many of these receptors are of high conservation and public concern. Unfortunately monitoring for most marine vertebrates is challenging and expensive, especially in the energetic waters where tidal-stream MREDs will be deployed. Surveys for marine vertebrates often have low statistical power and so are likely to fail to detect all but substantial changes in abundance. Furthermore, many marine vertebrate species have large geographical ranges so that even if local changes in abundance are detected, they cannot usually be related to the wider populations. Much of the monitoring currently being undertaken at tidal-stream MRED development sites is thus leading to a ‘data-rich but information-poor’ (DRIP) situation. Such an approach adds to development costs whilst contributing little to wider ecosystem-based understanding. In the present article we discuss the issues surrounding the impacts of tidal-stream MREDs on marine vertebrates and address the questions regulators, developers and other stakeholders need to consider when agreeing monitoring programs for these receptors.