Estimates of Collision Risk of Harbour Porpoises and Marine Renewable Energy Devices at Sites of High Tidal-Stream Energy
The aim of this study was to investigate how often porpoises occurred in two areas of immediate interest for tidal-stream development on the west coast of Scotland. These two sites were the tidal narrows of the Sound of Islay (between the islands of Islay
The potential to extract energy from tidal-streams offers one of the truly sustainable alternatives to fossil fuel use (MacKay 2008). Furthermore UK coastal waters have a potential to provide substantial quantities of this resource (ABPmer 2008). It is likely that introducing energy extraction machines to coastal waters will have consequences (positive and negative) for the receiving environment. However, because these technologies are new, there is uncertainty over the precise nature and extent of these environmental interactions. As a result, the Scottish Government is exploring a "survey, deploy and monitor" approach to allow a staged introduction of the sector while permitting lessons from environmental and other studies to be incorporated before reaching full-scale developments.
One frequently cited and significant area of uncertainty for the developing marine renewables sector is the possibility of injurious collisions between large vertebrates (marine mammals, sharks, turtles, diving birds) and operating tidal-turbines (Linley et al., 2009). While there are some obvious direct parallels to the collisions that occur between flying vertebrates (birds & bats) and wind turbines (Barrios and Rodriguez 2004), there are also many fundamental differences that mean that direct extrapolations are inappropriate (animal sensory modalities, relative animal to turbine size, blade velocity and so on). Likewise, comparisons with other analogous interactions (whale-ship or fish-power station cooling intake strikes etc.) are also too dissimilar to directly inform us of the nature of the problem.
In an attempt to estimate the potential magnitude of future interactions, modelling work was conducted as part of the Scottish Strategic Environmental Assessment for marine renewables (Wilson et al., 2007).This exercise attempted to determine how often marine mammals and turbines would independently share the same locations in space and time and thus how often interactions (that could lead to collisions) were likely to occur. This encounter model focussed on potential rates of interaction between harbour porpoises ( Phocoena phocoena) off the west of Scotland and a development of fictitious (but typical) 16 m diameter three bladed turbines. The model predicted that co-occurrences between animals and turbine blades could be relatively common at around thirteen interactions per turbine per year. This model necessarily made a variety of assumptions both about animal behaviour and turbine design but nevertheless the level of potential interaction clearly warrants further investigation. Among the key assumptions was that the density of porpoises in sites of interest for tidal-stream energy extraction are similar to the rest of the west of Scotland (Block 'N', 0.394 km -2 estimated at that time from the SCANS-II survey, Macleod 2006).
The studies outlined in this current report has focussed on the validity of this density assumption. At present there is uncertainty over whether porpoises either target or avoid marine habitats subject to high rates of tidal flow. Studies carried out in Shetland (Evans 1997), Wales (Calderan 2003; Pierpoint 2008), the Bay of Fundy, north America (Johnston et al. 2005), Devon (Goodwin 2008) and, to an extent, western Scotland (Marubini et al. 2009) all indicate that porpoises preferentially target / are found in elevated densities in areas of high tidal-streams. In one of the most directed studies of porpoises in a tidal-race environment, Pierpoint (2008) found that porpoises were present in Ramsey Sound (Wales) particularly during the ebb tide and were there for 70% of the observation periods. These animals appeared at highest densities in the area of maximum tidal flow and in water depths between 25 and 57 metres. Likewise Gordon et al 2011, compared densities and encounter rates for porpoises at two Welsh tidal current sites with those for other European sites and found them to be amongst the highest reported.
In marked contrast, Embling et al. (2010), analysed results from dedicated cetacean surveys from the southern Inner Hebrides. They found that porpoise distribution was best explained by tidal currents with the higher densities predicted in areas of low current. A follow-on study encompassing the entire Hebrides (Booth 2010) found that depth (especially waters between 50 and 150 m), steep slopes and proximity to land were all important in explaining areas of high porpoise density. Relationships with current speed were less important than these other variables. In only two of six years of data did current speeds appear important in his modelling work and in these the relationships between current and porpoise occurrence were contradictory (2005: less current, more porpoises, 2008: more current, more porpoises). Given that relationships were not apparent in the other four years of survey effort, Booth considered that if current speed was important it would have to be at scales finer than those his surveys were designed for ( i.e.at eddy and tidal rips).
Therefore, the relationship between porpoise occurrence and areas of strong tidal flows remains confused. This is primarily because surveys have either been focussed entirely within areas of strong flow or at larger scales where water-flow characteristics were not a primary consideration in survey design. Accordingly at this time it is difficult to determine how often encounters between porpoises and tidal-stream energy devices are likely to occur. In this project, therefore, we specifically targeted two tidal narrows (Sound of Islay and Kyle Rhea) of interest to the tidal-stream energy sector on the west of Scotland to look specifically at porpoise occurrence. The results would help inform the considerations of how often porpoises are likely to come into close association with tidal-stream energy devices.
For these investigations we used a variety of standard, modified and new techniques to investigate temporal and spatial patterns of porpoise occurrence. Specifically: Section 1 outlines the use and results from conventional boat surveys (using visual and acoustic techniques) adapted for collecting unbiased data in water moving at speeds similar to vessel speed. Our main aim of this effort was to derive comparable estimates of porpoise density in tidal-stream habitats relative to more typical west of Scotland coastal waters. Section 2 describes the use of two new acoustic methods for porpoise detection and tracking that were specifically developed for tidal sites. The aim here was to advance cutting edge acoustic tools so that they could be used in moving water where conventional methods cannot be successfully applied. Section 3 is an overall discussion and considers the implications of these results for rates of interactions between porpoises and tidal-stream energy technologies. Two appendices follow this report. One describes one of the novel acoustic tracking methods in more detail. The second briefly shows the results of further data on porpoise distribution collected outside of this contract work but as a direct result of it.
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