Predictable hydrodynamic conditions explain temporal variations in the density of benthic foraging seabirds in a tidal stream environment

Abstract

Tidal stream turbines could have several direct impacts upon pursuit-diving seabirds foraging within tidal stream environments (mean horizontal current speeds > 2 ms À1 ), including collisions and displacement. Understanding how foraging seabirds respond to temporally variable but predictable hydrodynamic conditions immediately around devices could identify when interactions between seabirds and devices are most likely to occur; information which would quantify the magnitude of potential impacts, and also facilitate the development of suitable mitigation measures. This study uses shore-based observational surveys and Finite Volume Community Ocean Model outputs to test whether temporally predictable hydrodynamic conditions (horizontal current speeds, water elevation, turbulence) influenced the density of foraging black guillemots Cepphus grylle and European shags Phalacrocorax aristotelis in a tidal stream environment in Orkney, United Kingdom, during the breeding season. These species are particularly vulnerable to interactions with devices due to their tendency to exploit benthic and epi-benthic prey on or near the seabed. The density of both species decreased as a function of horizontal current speeds, whereas the density of black guillemots also decreased as a function of water elevation. These relationships could be linked to higher energetic costs of dives in particularly fast horizontal current speeds (>3 ms À1 ) and deeper water. Therefore, interactions between these species and moving components seem unlikely at particularly high horizontal current speeds. Combining this information, with that on the rotation rates of moving components at lower horizontal current speeds, could be used to assess collision risk in this site during breeding seasons. It is also likely that moderating any device operation during both lowest water elevation and lowest horizontal current speeds could reduce the risk of collisions for these species in this site during this season. The approaches used in this study could have useful applications within Environmental Impact Assessments, and should be considered when assessing and mitigating negative impacts from specific devices within development sites.

Fine-scale hydrodynamic metrics underlying predator occupancy patterns in tidal stream environments

“…The first set of models consisted of three explanatory variables commonly used to explain animal distributions in tidal stream environments (e.g. Hastie et al, 2016;Waggitt et al, 2016a): time to high tide (M2HT), depth-averaged current magnitude (Mag) and depth. These explanatory variables were combined in a single model.…”

“…Marine predators performing pursuit diving of their prey are likely to forage in environments where controlled diving can be maintained while maximizing foraging efficiency, limiting extensive usage of the fastest-flowing currents within a tidal stream (Ladd et al, 2005;Waggitt et al, 2016a).…”

“…Flow regimes are not homogenous in tidal environments and vary owing to the occurrence of fine-scale, tidally-driven or bathymetry-induced physical processes, including shear boundaries, eddies and boils (Nimmo-Smith et al, 1999;Evans et al, 2013;Jones et al, 2014;Kregting et al, 2016). This heterogeneity creates spatial and temporal variation in the distribution of species, with marine predators regularly associated with certain tidal velocities and physical processes (Johnston et al, 2005;Embling et al, 2012;Jones et al, 2014;Waggitt et al, 2016a;Waggitt et al, 2016b;Benjamins et al, 2016;Benjamins et al, 2017). Therefore, quantifying spatio-temporal variation in animal site usage in relation to hydrodynamic features may help to identify which and when areas may be used (Zamon 2001;Waggitt & Scott 2014;Benjamins et al, 2015b;Hastie et al, 2016;Waggitt et al, 2017a).…”

Large scale three-dimensional modelling for wave and tidal energy resource and environmental impact: Methodologies for quantifying acceptable thresholds for sustainable exploitation

“…Similarly, our understanding of the feeding ecology of a range of protected species, including marine mammals and seabirds, is indicating that species have particular preferred feeding habitats, characterised by factors such as current speed, turbulence and primary production rates (Waggitt et al, 2016a(Waggitt et al, , 2016b, influenced by the presence/absence of oceanographic fronts. There will be an increasing need to take account of the changes to the physical environment in assessments of effects on foraging success and efficiency, and consequences for reproductive success, mortality rates and the dynamics of protected populations associated with Natura 2000 sites.…”

Regional-scale patterns in harbour porpoise occupancy of tidal stream environments

“…Friction between these currents and the seabed in shallow areas also generate complex vertical currents characterized by standing waves and fine-scale turbulence, which further increase foraging efficiency (Hunt et al, 1998;Waggitt et al, 2016a). However, despite these advantages, animals may avoid areas of particularly strong or complex currents due to high swimming costs or prey-handling difficulties (Heath and Gilchrist, 2010;Waggitt et al, 2016b). Such avoidance could be particularly evident in animals exploiting prey on or near the seabed, due to the lengthy searches associated with capturing these items (Butler and Jones, 1997).…”