This reports reviews methodologies for measuring and assessing potential changes in marine mammal behaviour, abundance or distribution arising from the construction, operation and decommissioning of offshore windfarms. The report first describes impacts from offshore windfarms on marine mammals and defines the spatial and temporal scope of investigations in order to detect impacts on marine mammals. Impacts from offshore windfarming occur at very different spatial and temporal scales: Construction work, especially pile driving, may result in short-termed but long ranged impacts, whereas the operation of windfarms is expected to result in local but long-lasting impacts.
The report reviews the standard methods used in studies on marine mammals in relation to offshore windfarms. Statistical power of line transect surveys using aircrafts and ships and Static Acoustic Monitoring using T-PODs is analysed from datasets obtained in German studies.
Aerial surveys provide an efficient method for multi-species surveys covering rather large areas (2000 km2 ) per survey, which are considered to be sufficient to compare impact and reference areas. In areas where abundance of harbour porpoises are sufficiently high, which is expected to be the case in many UK waters, aerial surveys are useful to measure population size and population changes. Based on datasets in areas of high densities, it is concluded that 30 % changes can be detected by 1-3 surveys. Aerial surveys will provide additional data on other species, however, as either densities (dolphins, whales) or detection rates (seals) are low, no quantitative data can be obtained in restricted areas. Aerial surveys are often restricted by weather conditions and thus suffer from a low temporal resolution. The spatial resolution may not be sufficient to detect small-scaled impacts as expected during the operation of offshore windfarms.
Line transect counts from ships may also be used for marine mammal surveys. They have a lower spatial resolution and are even more restricted to suitable weather conditions. Based on datasets in areas of high densities, it is concluded that 30 % changes can also be detected by 1-3 surveys. The efficiency of ship transect surveys can probably be highly improved by towed hydrophones, which record echolocation clicks from porpoises and dolphins. The main advantage of towed hydrophones would be, that it can be used at night and during weather conditions, which do not allow visual surveys.
Static Acoustic Monitoring (SAM) using T-PODs or other devices provide excellent data on harbour porpoises and potentially also dolphin species at a high temporal but low spatial resolution. The spatial resolution can be improved by deploying several SAM devices in the area of interest. Statistical analysis from areas with low and high porpoise densities proved that a 30 % change in harbour porpoise presence can be proved with a sample size of 3-11.
Other methods, as telemetry tracking, photo-identification and haul-out counts for seals are described.
For impact studies in relation to offshore windfarms a BACI (Before-After/Control-Impact) design is generally recommended but recommendations are also given for cases, where no baseline data are available or a BACI may not be possible for other reasons.
It is recommended to combine line transect surveys using aircrafts or ships with Static Acoustic Monitoring. An impact study on offshore windfarms should ideally cover two years before construction, the construction period and at least two years of operation. If longer lasting effects are detected, the study during the operational phase should be extended. It is recommended to conduct line transect surveys in monthly intervals. In areas with a marked seasonal occurrence, surveys may be restricted to periods with high abundance, when sufficient data are more likely to be obtained. Continuous recordings of harbour porpoises with SAM are recommended for all areas, where these animals occur in relevant numbers. SAM will provide data which are needed to detect short-termed changes in behaviour and abundance as expected in response to pile driving, but also to detect changes on a much smaller spatial scale as can be detected by other methods as well as long-term changes in response to construction of operation. For seals and dolphins severe problems in assessing the impacts remain, as their behaviour or low densities make it very difficult to obtain enough data for statistical analysis.