Quantifying pursuit-diving seabirds' associations with fine-scale physical features in tidal stream environments

The rapid increase in the number of tidal stream turbine arrays will create novel and unprecedented levels of anthropogenic activity within habitats characterized by horizontal current speeds exceeding 2 ms−1. However, the potential impacts on pursuit-diving seabirds exploiting these tidal stream environments remain largely unknown. Identifying similarities between the fine-scale physical features (100s of metres) suitable for array installations, and those associated with foraging pursuit-diving seabirds, could identify which species are most vulnerable to either collisions with moving components, or displacement from these installations.
A combination of vessel-based observational surveys, Finite Volume Community Ocean Model outputs and hydroacoustic seabed surveys provided concurrent measures of foraging distributions and physical characteristics at a fine temporal (15 min) and spatial (500 m) resolution across a tidal stream environment suitable for array installations, during both breeding and non-breeding seasons. These data sets were then used to test for associations between foraging pursuit-diving seabirds (Atlantic puffins Fratercula arctica, black guillemots Cepphus grylle, common guillemots Uria aalge, European shags Phalacrocorax aristotelis) and physical features.
These species were associated with areas of fast horizontal currents, slow horizontal currents, high turbulence, downward vertical currents and also hard–rough seabeds. The identity and strength of associations differed among species, and also within species between seasons, indicative of interspecific and intraspecific variations in habitat use. However, Atlantic puffins were associated particularly strongly with areas of fast horizontal currents during breeding seasons, and European shags with areas of rough–hard seabeds and downward vertical currents during non-breeding seasons.
Synthesis and applications. Atlantic puffins’ strong association with fast horizontal current speeds indicates that they are particularly likely to interact with installations during breeding seasons. Any post-installation monitoring and mitigation measures should therefore focus on this species and season. The multi-species associations with high turbulence and downward vertical currents, which often coincide with fast horizontal current speeds, also highlight useful pre-installation mitigation measures via the omission of devices from these areas, reducing the overall likelihood of interactions. Environmental impact assessments (EIA) generally involve once-a-month surveys across 2-year periods. However, the approaches used in this study show that more focussed surveys can greatly benefit management strategies aiming to reduce the likelihood of negative impacts by facilitating the development of targeted mitigation measures. It is therefore recommended that these approaches contribute towards EIA within development sites.