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The marine renewable energy industry is expanding globally in response to increased energy demands and the desire to curtail greenhouse gas emissions. Within the UK, Wales has the potential for the development of diverse marine renewable technologies, with a strong tidal range resource, areas of high tidal current energy, and a spatially limited wave energy resource. Targets have been set by the Welsh Government to increase the contribution of marine renewable energy to Wales' electricity generation, and the recent introduction of demonstration zones for tidal and wave energy aims to facilitate developers in device deployment. However, uncertainties remain about the potential impacts of devices, particularly for array scale deployments, planned at several sites, and for the extensive structures required to capture the tidal range resource. Here we review present knowledge of potential impacts, including physical, ecological and societal dimensions, and outline research priorities to provide a scientific basis on which to base decisions influencing the trajectory of Welsh marine renewable energy development.

Many soft sediment invertebrates show highly patchy distributions within populations across small (centimetres–10s of metres) and large (10s 100s of kilometres) scales, despite relative large scale homogeneity in the physical environment. The common European cockle Cerastoderma edule is an example of a patchily distributed species. Predation during the post-larval period is probably critical in determining the recruitment success and potentially the spatial distribution of this species. We investigated if aggregation (being ‘patchy’) of C. edule post-larvae increases survival from predation from juvenile shore crabs Carcinus maenas when compared to a uniformly dispersed distribution. We hypothesised that this will be caused by interference competition between C. maenas . Aquarium based experiments were conducted testing two factors of prey arrangement (aggregated and dispersed) and predator number (one or 5 crabs), and their interaction in controlling prey mortality and predator behaviour. Prey mortality was higher by 30–40% for both predator treatments when aggregated, and there was no interaction between factors. This indicates that the presence of conspecifics has no more significant influence on foraging efficiency than prey arrangement. An individual crab spent 30% longer searching when prey was dispersed, indicating that prey encounter rates limit foraging efficiency when conspecifics are absent. When multiple crabs foraged together they engaged in antagonistic interactions (8.5% of the total time) in both prey arrangements with no significant difference, and other behaviours did not explain clearly the difference in prey mortality. These results indicate that juvenile C. maenas are able to maintain a higher foraging efficiency on aggregated prey, despite the presence of conspecifics and associated interference competition. From this laboratory predator–prey system there is no evidence that aggregated cockle post-larvae are afforded greater protection from predation than uniformly distributed cockles. Therefore the migratory behaviour of C. edule post-larvae that may create or maintain dispersed distributions could be significant for post-settlement survival.

Populations of the Common European cockle (Cerastoderma edule) often have highly patchy distributions and variable recruitment success. One of the proposed reasons is that high densities of filter feeders and/or bioturbators are thought to reduce the success of larval settlement and post-settlement survival, but the direct causal processes driving these observations are not clearly identified and validated. Through combined field and laboratory experiments, we test the hypothesis that macrofauna cause decreases in post-larval density through feeding and movement activities. The effect of excluding the bioturbating lugworm Arenicola marina and filter-feeding adult cockles on post-larval cockle densities was estimated in separate field experiments at two locations from the time of initial larval settlement in May 2012 to late summer August 2012. Lugworm exclusion led to a significant increase in cockle post-larval densities whereas the opposite was true for adult cockles, where exclusion led to a reduction in C. edule post-larval density. Although clear effects were observed in the field, experiments conducted in the laboratory failed to detect changes in mortality or byssus drifting of post-larvae as a consequence of macrofaunal activity. This study demonstrates that the presence of macrofauna can have both positive and negative effects on post-settlement density of C. edule post-larvae. Thus the density, distribution and identity of macrofauna have significant effects on the density and spatial distribution of C. edule post-larvae during the post-settlement period. These observations have implications for conservation and fishery management of this species.