Abstract
RPS was commissioned by the Institute of Estuarine and Coastal Studies (IECS) to conduct a survey for marine mammals off the Holderness coast for the proposed Humber Gateway offshore wind farm and to report on the results. Data from aerial and vessel-based bird surveys were also supplied, analysed and discussed.
Two species of Pinnipedia - common seal and grey seal, and one species of cetacean - harbour porpoise were confirmed present in the survey area throughout the year. One other cetacean species was seen but not confidently identified.
The species occurred in the following descending order of abundance: harbour porpoise, grey seal and common seal. Very few common seals were seen. The aerial survey produced 30 sightings of unidentified seals many of which were probably grey seal. Grey seal may therefore be slightly more abundant over time than harbour porpoise but the numbers of these two species overall were very similar.
Seasonal trends were distinct for grey seals with the lowest relative numbers observed in the winter and highest in summer and autumn. This coincides with the pupping season (November-December inclusive) when most grey seals move to within close proximity of the birthing grounds (rookeries) the nearest of which is the south shore of the Humber Estuary. Other major east coast rookeries are the Farne Islands and Holy Island to the north.
The different survey methods returned different seasonal patterns in relative numbers of harbour porpoise. The surveys conducted most regularly (aerial and vessel-based bird survey) indicated that peaks occurred in summer and autumn. The dedicated marine mammal surveys were conducted only once in summer and autumn and twice in spring, and were less useful in determining seasonal trends. These surveys were also affected by poor conditions in the summer and autumn.
The records of common seals were too few to draw conclusions on seasonal patterns. Overall numbers of common seal off the Holderness coast are considered to be low for
English waters.
Grey seal and harbour porpoise numbers in the study area are considered to be low to moderate in terms of other areas of the English coastline. Seasonal peaks in numbers also need to be considered in the assessment of the proposed wind farm on these two species.