Input Information for Salmon Collision Risk Modelling Ross Gardiner Marine Scotland Science

Case Study – tidal turbine array in Pentland Firth Initial risk assessment done by a consultant (Xodus) for the developer (MeyGen). The assessment was revisited early in 2013 by a working group of Scottish Natural Heritage (SNH) and Marine Scotland Science (MSS) to inform licence determination. Working group (WG) members Chris Eastham (SNH) Ian Davies (MSS) Erica Knott (SNH)Ross Gardiner (MSS)

Which life stages are at risk? ? Homing adults – netting records plus historical tagging information indicate many salmon destined for the Scottish east coast return via the firth. Plus their larger size makes them a larger target for turbine blades and their larger mass them less likely to be passively swept past. ? Smolts – WG took view that no evidence that a large proportion of Scottish smolts might pass through the Pentland Firth. Risk assessment for homing adults was therefore considered by the WG to be the priority.

Step 1 - potential for interaction Basic Band model used Input values (1) Number of salmon returning to Scottish home waters annually The great majority of these are destined for Scottish east coast rivers. Proportion assumed to do so through the Pentland Firth 90% Dimensions of firth 7000 m wide, generally about 40 m in depth, cross sectional area 330,000 m 2, turbines to be in water 35 m deep with 10 m clearance to surface and 5 m to bed Diameter of a turbine 20 m (or a little less), giving a rotational area of 314 m 2. Salmon distribution across the firth. No robust information to justify any change from the uniform distribution by width assumption that had been made. The results of recent tagging work carried out since the WG’s review would also be consistent with this assumption.

Step 1 - potential for interaction Input values used (2) Salmon distribution vertically in water column. Although the risk assessment provided by the developer assumed a uniform distribution by depth, the published information which was available suggested that it was more likely that salmon would be mainly close to the surface, with about 72.7% at less than 10 m depth and only 16.6% in the depth zone of the turbines, rather than the 57% which assumption of a uniform depth distribution would imply. Dimensions of turbine blades, the rotational speed, and the dimensions of the fish. Proportion of the time the turbines operate 72.7% - they operate on both flood and ebb tides.

Step 1 - potential for interaction Input values used (3) Fish speed relative to the turbine. Fish assumed to be moving passively with the flow, and the mean current speed while operating of 2.5 m.s -1 was used. I’ll come back to this.

Step 1 - potential for interaction These input values; gave estimates for fish passing through the rotational area of the turbine which would be expected to collide of about 30%, in the absence of any evasion or slipstream effect. It is also possible that fish outwith the rotational area could also be injured by turbulence or pressure changes for example. No allowance was attempted for this. gave predictions of 171 salmon expected to collide annually if there is an array of 6 turbines, and 1730 if there is an array of 61 turbines.

Confidence in input values Reasonably high or high confidence that we know the number of salmon returning to Scottish home waters, the dimensions of the firth, the dimensions of a turbine and the rotational speed, the dimensions of the fish, the proportion of the time the turbines operate, and that salmon are more likely to be close to the surface. Lower confidence that the proportion of salmon returning to Scottish rivers through the Pentland Firth is 90% (Could be too high, leading to an inflated estimate. Mind you, fish returning to other countries could also be using this route …) that salmon distribution across the firth is even. that the fish move passively with the water flow…

Comments on assumption that the salmon will move passively with the water flow (1) Adult salmon are strong swimmers, so could significantly affect the speed relative to the turbine. The true value could be faster or slower, if a salmon is swimming actively in the same direction as, or against the flow. The most efficient swimming speed of salmon of length 0.6 to 0.85 m is about 0.6 to 0.85 m.s -1 (i.e. 1 body length.s -1 ) and they can maintain this indefinitely. They are capable of much faster bursts of speed for short times. A salmon of length 0.6 m has a burst speed of about 2.9 m.s -1 at 6 o C and 4.5 m.s -1 at 14 o C. A salmon of length 0.85 m has a burst speed of 3.4 m.s -1 at 6 o C and 5.6 m.s -1 at 14 o C. (These figures cover the main range of likely length of salmon and expected temperature in the Pentland Firth).

Comments on assumption that the salmon will move passively with the water flow (2) It should be noted that increases in speed relative to the turbine would reduce the calculated collision probability (although not any anticipated consequences of collision) and decreases in speed would increase it.

Step 2 – Can we adjust these calculated collision figures to account other possible factors? (1) Evasion Tank experiments Tank experiments which have been carried out in North America indicate that survival of release rainbow trout passed through axial turbines can be very high. However, the test turbines were much smaller, and the blade speed and potential interaction velocity much lower. And, at least in the studies we’ve seen, the predictions for fish of up to 0.6 m in length were based on only small fish of and 0.24 m in length being tested. Such fish would be expected to be more likely to slipstream passively past turbines than more massive objects, such as adult salmon. Studies on fish at operational turbines None to date were transportable to the Pentland Firth situation in our view.

Step 2 – Can we adjust these calculated collision figures to account other possible factors? (2) We decided that there was no robust information to justify adjustment for factors such as evasion, and did not make any. Thanks for listening. Note: Parameter values in this presentation should be treated as only indicative.