Seabird use of tidal stream environments and potential interactions with tidal energy devices
As climate change becomes a more pressing reality in the 21st century, governments around the world are setting targets to meet electricity production demand from renewable energy sources such as tidal streams. However, tidal stream environments are also important habitats for marine wildlife, including seabirds. Risk of collision and habitat displacement are the major negative effects of concern. The following thesis therefore examined the use of tidal stream environments by diving seabirds and their potential interactions with tidal stream energy devices at designated sites. Focus was given to two diving seabird species that are potentially vulnerable to negative effects from tidal turbines, the European shag Gulossus aristotelis and razorbill Alca torda. High resolution biologging and telemetry (Time Depth Recorders (TDRs) and GPS) data from individuals breeding at colonies within a major tidal energy site, the Pentland Firth, Scotland, were analysed. Year-round camera trap monitoring and diet sampling to provide longterm understanding of impacts at another commercially operational site, Bluemull Sound, Scotland, were trialled. Assessing the spatial overlap of seabird foraging distributions (areas of “high risk”) with tidal development and lease sites revealed the importance of species and scale. I found that the smaller the site, the less overlap there was. While this was the case for both species, the difference was more dramatic in European shags, where overlap with the overall development area was 99%, while overlap with the lease sites within was 0.72%. The influence of industry-relevant environmental variables (tidal flow velocity and bathymetry) on foraging behaviour were examined using statistical modelling techniques. Within the tidal stream environment, foraging behaviour and consequently risk level was again species-specific. European shags were strongly influenced by seafloor depth, and therefore used the water column in its entirety, centring on depths between 32 and 42m. This range placed them at high risk of interacting with seabed-mounted horizontal axis turbines that are deployed between 25- 50 m depth. However, as dive frequency decreased substantially at tidal flow velocities >1.5 ms-1, collision risk to shags may be minimized by increasing the speeds at which turbines begin to operate from 1.0 ms-1 to 1.5 ms-1. By contrast, the majority of razorbill dives were shallower than 10 m, putting them at low risk of interacting with seabed-mounted horizontal axis turbines. Finally, camera trap images and dietary samples collected at a European shag roost outside of the breeding season showed a positive influence of the tidal cycle on shag abundance as well as primarily benthic foraging. Roost monitoring promises therefore to be a cost-effective complement to high resolution survey tools. As regards potential for interactions with tidal energy devices, this thesis found that the risk of collision specifically with currently deployed turbines in the Inner Sound is likely to be low. However, due to up-scaling to tidal arrays as well as the increasing sizes and numbers of offshore renewable developments, the assessment of displacement and cumulative effects will become increasingly important.
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