One of the principal environmental concerns about tidal turbines is the potential for injurious collision with marine animals. Until recently there has been little opportunity to collect and analyse data to assess this risk. Three separate deployments of GPS tags on harbour seals in Strangford Lough and Narrows during the development of the SeaGen tidal turbine have provided the first opportunity to measure individual seal behaviour in relation to the presence and operation of a commercial scale tidal turbine. Thirty six seals were tagged over three years, in 2006 (pre-installation), 2008 (during installation) and 2010 (operation). There was no overall difference in the rate at which seals travelled up and down the narrows past the location of the turbine between the three deployments. However these average rates had wide confidence limits as a result of a wide degree of variation in the behaviour of individual seals. This uncertainty limited the use of this data for investigating differences between years. In 2010, when the turbine was operational, there was evidence that the rate at which seals transited the Narrows reduced when the turbine was on, relative to when it was off. The biological importance of this reduction is unknown at present. Over all three years the frequency of transits was higher at slack tide than when the current was running. Although visual inspection of the distribution of locations where seals chose to transit the narrows suggested local avoidance of the turbine site in 2010, high inter-individual variability limited our ability to detect any significant difference. Examining the rate of transits which were close to the turbine (<25m) when it was operating gives an average rate for each seal of 3 transits per year. Scaling up to the total population suggested that 300 such transits may have occurred during the period between April and August. There were 210 emergency shut downs during this same period. The results of this study suggest that the operation of the turbine is affecting the transit behaviour of seals in the narrows and therefore the risk of seals colliding with an operational tidal turbine may be lower than would be estimated by assuming no avoidance and a spatial and temporal uniform density of animals. This series contains the survey report.