Vertical migrations of fish schools determine overlap with a mobile tidal stream marine renewable energy device

Abstract

Large increases in the generation of electricity using marine renewable energy (MRE) are planned, and assessment of the environmental impacts of novel MRE devices, such as kites, are urgently needed. A first step in this assessment is to quantify overlap in space and time between MRE devices and prey species of top predators such as small pelagic fish. Here, we quantify how the distribution of fish schools overlaps with the operational depth (20–60 m) and tidal current speeds (≥1.2–2.4 m/s) used by tidal kites, and the physical processes driving overlap. Fish schools undertake diel vertical migrations driven by the depth of light penetration into the water column, controlled by the supply of solar radiation and water column light absorption and scattering, which in turn depends on the cross‐sectional area of suspended particulate matter (SPM). Fish schools were found shallower in the morning and evening and deeper in the middle of the day when solar radiation is greatest, with the deepest depths reached during predictable bimonthly periods of lower current speeds and lower cross‐sectional area of SPM. Potential kite operations overlap with fish schools for a mean of 5% of the time that schools are present (maximum for a day is 36%). This represents a mean of 6% of the potential kite operating time (maximum for a day is 44%). These were both highest during a new moon spring tide and transitions between neap and spring tides. Synthesis and applications. Overlap of fish school depth distribution with tidal kite operation is reasonably predictable, and so the timing of operations could be adapted to avoid potential negative interactions. If all interaction between fish schools was to be avoided, the loss of operational time for tidal kites would be 6%. This information could also be used in planning the operating depths of marine renewable energy (MRE) devices to avoid or minimize overlap with fish schools and their predators by developers, and for environmental licencing and management authorities to gauge potential ecological impacts of different MRE device designs and operating characteristics.

Assessing the effects of tidal stream marine renewable energy on seabirds: A conceptual framework

“…The prevalence of conveyor-belt foraging for birds diving to collision risk depths (Δd, Figure 1) within tidal lease sites has yet to be quantified. The extent to which tidal energy devices may act as fish aggregating devices that enhance foraging opportunities for seabirds while simultaneously potentially increasing collision risk is also unclear Fraser et al, 2018;Williamson et al, 2019;Whitton et al, 2020).…”

“…effects of multiple devices and arrays) and interacting effects (e.g. turbines as fish aggregating devices) is a further challenge (Shields et al 2009;2011;Frid et al 2011;Broadhurst, Barr, and Orme 2014;Roche et al 2016;Fox et al 2018;Goyert et al 2018;Williamson et al 2019;Whitton et al 2020). As discussed in the context of collision risk above, this will necessitate combining methods that can assess both habitat-scale and fine-scale variables, including surface features such as boils or wakes.…”

Surface Characterisation of Kolk-Boils within Tidal Stream Environments Using UAV Imagery

“…It is therefore clear that an understanding of turbulent features should inform turbine design and micro-siting in order to more accurately calculate the loading on each device, the rate of fatigue on material components and the range of fluctuations in power output that may be experienced [10,11]. These turbulent features are also important in determining animal presence within tidal stream environments, as they offer potential foraging hotspots for mobile predators (seabirds and mammals) because of the influence of flow variation on the distribution of prey species (fish) [8,12,13]. Therefore, to comprehend the full environmental and ecological impact of tidal energy developments, which can be a barrier to successful consenting and licensing of MRE devices and arrays, fine-scale hydrodynamic habitat characterisation is required by current and future projects [12].…”

“…These turbulent features are also important in determining animal presence within tidal stream environments, as they offer potential foraging hotspots for mobile predators (seabirds and mammals) because of the influence of flow variation on the distribution of prey species (fish) [8,12,13]. Therefore, to comprehend the full environmental and ecological impact of tidal energy developments, which can be a barrier to successful consenting and licensing of MRE devices and arrays, fine-scale hydrodynamic habitat characterisation is required by current and future projects [12].…”

“…While this approach can assess flow characteristics, and thus predict turbine energy yield, it does not have the necessary spatial resolution to accurately capture fine-scale coherent turbulent structures such as kolk-boils [21]. Additionally, tidal stream environments are challenging to survey due to strong tidal flows and inherent turbulence, adding extra logistical, financial and risk considerations to any in situ data collection effort [12,23]. Hence, there is an urgent need for suitable platforms that can safely, and cost-effectively, survey tidal stream environments at the fine-scale spatio-temporal resolutions required [24].…”