Abstract
Over the period October 2003 to August 2004 E.ON UK Renewables Offshore Wind Limited constructed a 60 MW wind farm comprised of 30 x 2 MW turbines on Scroby Sands, a dynamic sand bar system approximately 2 km offshore from Great Yarmouth, Norfolk.
The development is located about 2 km north of an area used by Common seals Phoca vitulina as a breeding and haul out site and by Grey seals Haliochoerus grypus for haul out. The Schedule to Licence required that a monitoring programme 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. Baseline pre-construction data was gathered in 2002 and 2003, and construction data was gathered in 2004. The post-construction data gathered in 2005 and analysed in this report represents the final stage monitoring programme, thereby fulfilling the FEPA licence requirements.
The species composition of the colony has changed significantly between the baseline and the subsequent construction and post-construction phases reflecting the relative fortunes of the different species. Significantly more Grey seals were present in 2004 than 2003 and in 2005 than 2002 and 2003. This reflects both the national population increase, and the continued increase in the local breeding population, which had a particularly strong breeding season in 2003 (peak count 51 pups). This in itself is most likely to account for the increases in Grey seals using the sandbar on Scroby.
In contrast, there were significantly fewer Common seals hauled out in 2004 than in 2002. Examination of mean, median and peak counts between years also showed that Common seal pup production was notably poor in 2004. The 2005 data was not significantly different from the data collected in 2002 or 2004 suggesting any decline was temporary. However, Common seal counts remained low in comparison to the baseline years (mean counts: 91.4 in 2002, 79.92 in 2003, 52.27 in 2004 and 57.5 in 2005). Low counts could be attributed to a number of factors, including reflection of the national declining trend (with disease outbreaks and cumulative impacts associated with increased anthropogenic disturbance), the impact of severe storms during their breeding season particularly on pup survival, and potential inter-specific competition with Grey seals. Unfortunately, the nature of data gathering means that there is no evidence to support or repudiate any of these factors.
In contrast, the potential for construction related disturbance has been determined at other offshore wind farm sites (e.g. Bockstigen in Gotland, Sweden where significant but temporary reduction in use of haul out sites by Grey seals 1.5 and 2 km away was noted). Indeed, reduced use of Scroby during the construction period was highlighted as being likely in the Environmental Statement. Moreover, the basis for the species-specific response of Common Seals at Scroby is provided by the species being particularly sensitive to disturbance with slightly better hearing than Grey seals, coupled with the apparent habituation of Grey seals at their main haul-out on the southern tip of Scroby to a tourist vessel. Evidence of a causal mechanism was provided by a significant negative correlation between the number of seals hauled out and the number of boats on the wind farm site, although it should be noted that this data cannot be corrected to allow for other potentially influential factors (e.g. wind speed, wind direction and seasonality).
Although the 2005 data implies some recovery from a temporary effect, Common seal counts remain low, and it is recommended that monitoring is continued to confirm and monitor potential improvements back to pre-construction levels. Such recovery time data does not currently exist, and would be of great relevance to other offshore wind farm sites planned near seal colonies (e.g. in the Greater Wash Strategic Area). It is also suggested that vessels visiting Scroby do not pass within 600 m of haul-out sites along the extent of the landward edge of exposed sands in an attempt to minimise any disturbance to Common seals.