Abstract
Powergen Renewables Development Ltd1 are in the process of constructing a wind farm comprised of 30 x 2MW turbines on Scroby Sands, a dynamic sand bar system approximately 3 km offshore from Great Yarmouth, Norfolk.
The Schedule to Licence required that a monitoring programme should be carried out to determine the impact of the wind farm on the seals. This was specified as two
aerial photographic surveys from fixed wing aircraft per month at low water for the six summer months (April to September) pre, during and post construction. This
report is concerned with analysing the second year of pre-construction data from 2003 in relation to the 2002 data, and establishing a baseline against which the potential
impacts of the wind farm may be measured.
The mean survey count for 2002 was 150, whereas for 2003 it was only 116 (a 23% decline). However, this difference was not found to be statistically significant within the context of the naturally high levels of variation associated with haul out counts. However, it is reasonable to suggest that this variation might be masking a genuine decline resulting from the 2002 Phocine Distemper Virus (PDV) outbreak. It would be highly unlikely for the seals on Scroby not to have been affected by PDV since it is highly contagious, and the seals nearby in the Wash were the most affected group in the country with an estimated mortality rate of 35% (SCOS 2003).
Although no statistically significant inter-annual changes in abundance could be detected, the pattern of peaks identified in 2002, namely the rise in Common seals seen in late July and the increase in Grey seals in September, did not occur in 2003. Therefore it must be concluded that these peaks are not related to life cycle events (as discussed in the 2002 report) but are instead more likely to be due to stochastic factors such as local prey abundance, height of the sand bank exposed, and the timing of low tide.
The Common seals pupped on Scroby again in 2003, although it appears that the surveys fell just outside of the peak breeding week. Since pups can swim almost from birth, it was likely that they were out at sea when the surveys took place. The peak number of pups hauled out in 2003 occurred in August (42), in comparison with 2002 when a much higher peak count occurred in early July (67). Unfortunately it was not possible to test whether there was a significant difference between these counts. Other studies have also found that peak pup counts vary both inter-annually and between colonies, to such an extent that comparison is often impossible (Anderson 1981, Warner 1983, op. cit. Thompson & Harwood 1990). This is due to changes in dispersal patterns thought to be associated with local foraging conditions. It is likely that this is the case at Scroby; in 2002 the pups dispersed during August, whereas in 2003 they remained present throughout the summer. However, pup counts can be an important means of detecting change, since changes in seal populations can take a long time to become apparent because they are long-lived annually breeding animals. Pup counts can be a more sensitive indicator of change, and may be a useful tool in assessing any impacts resulting from the wind farm. Therefore it is recommended that more surveys be undertaken during the pupping period in order to improve the accuracy of the pup counts.
The distribution of haul out sites also changed between years, although both in 2002 and 2003 the seals hauled out in preferred areas. This is likely to be connected with changes in the shape of the sandbank; locals have reported a large sandbar appearing parallel to Scroby, which would indicate that sand distribution has changed. The seals may have chosen different haul out sites in 2003 because, as a consequence, the sandbank was highest in different areas, or because the prevailing wind direction made certain haul out sites more or less attractive. Another possibility is that the seals prefer hauling out on some substrate types more than others, and that the distribution of these substrates changed between years. This example highlights an important point: whilst aerial surveys can provide an effective means of monitoring the abundance, distribution and breeding success of seals, these counts cannot explain the reasons for any changes identified (Hammond 2002). Other factors such as noise, disturbance, mortality, emigration, immigration, feeding habits or environmental processes would need to be investigated separately, and if any significant changes occur during the monitoring programme, potential causes will need to be investigated independently. However, it is possible to improve the scope of the data currently being gathered so that the identification of such factors is facilitated. Therefore the following measures are recommended:
? Several surveys to be carried out each year during the pupping period so that an accurate index of pup production can be obtained;
? Use of GPS points so that seal distribution may be related to a definable physical area, and also so that changes in the shape of Scroby can be monitored more effectively;
? Tide condition, time of day, and weather to be recorded at the time of each survey, especially during the pupping period;
? Although not required by the consent, we would recommend one aerial survey to be undertaken in November, December, January and February, which would enable a fuller understanding of seasonal trends.