Since a few years it is known that bats migrate over sea on a regular basis. As numerous land-based studies have shown that wind turbines can cause high fatality rates amongst bats Rijkswaterstaat started a bat monitoring programme for 2015 and 2016 in order to reduce uncertainties about possible impacts. At the same time Eneco commissioned a bat monitoring programme for 2015 and 2016 as part of the Monitoring and Evaluation Programme (MEP) for the offshore windfarm Luchterduinen. In 2016 Gemini conducted a bat monitoring campaign in windfarm Buitengaats and Wageningen Marine Research executed a bat monitoring programme at Wintershall platform P6-A and offshore research station FINO3 in the same year. The joint monitoring effort included 12 different offshore locations and 5 locations at the coast.
The specific aims of these monitoring programmes are an assessment of :
- The species composition at sea and at the coast
- The spatiotemporal pattern of occurrence, including the flight height
- The relation between environmental conditions and the occurrence of bats
- The function of the Dutch Territorial Sea for bats
The monitoring results at the coast showed that Nathusius’ pipistrelle is very common during both spring and autumn migration, but is also regular throughout the summer. It is also the most frequently recorded species at sea, albeit much less frequently recorded in comparison to the coast. At sea it was recorded from late August until late October (and one observation in November), and –to a lesser extent- from early April until the end of June. There were no records in July until mid-August. The observed pattern of occurrence matches previous offshore monitoring studies in the German and Dutch North Sea.
Due to a limited amount of data in spring we analysed the presence/absence of Nathusius’ pipistrelle per night from mid-August until late October. In this period bat activity was recorded during 11% of the nights at sea and during 66% of the nights at the coast. The higher number of nights at the coast may reflect the relative proportion of bats migrating at the coast and over sea, but the numbers at the coast are likely to be higher due to funnelling, whereas migration over sea is likely to follow a broad front due to the absence of guiding landscape features. However, locally densities at sea may be also inflated as bats are likely to be attracted to offshore structures. Consequently, based on bat detectordata alone, we cannot estimate the proportion of bats migrating along the coast and over sea.
Due to the differences in occurrences at sea and at the coast we developed one statistical model for the offshore stations and one for the coastal stations. We modelled the presence/absence per night as a function of various weather parameters, the moon illumination, the spatial coordinates and the night in year in the period mid-August until late October.
The most important predictor for the occurrence of Nathusius’ pipistrelle in autumn at sea and at the coast are low to moderate wind speeds, followed by night in year (the date). At the coast their presence increases rapidly from mid-August and continues to be high subsequently. At sea the occurrence is strongly peaked. The first wave of migrating animals occurs late August/early September and the second late September. Next, high temperatures increase significantly the presence of bats, both at the coast and at sea. Wind direction is also important; at sea wind directions between NE and SE (with a peak at 94 degrees) result in highest presence, whereas this is the case with wind directions between E and SW (with a peak at 170 degrees) at coastal locations. The observed optimal wind direction at sea (94 degrees) implies that bats crossing over sea choose tailwind conditions, whereas the presence at the coast seems to be shaped by funnelling. Therefore, it seems unlikely that wind drift or storms cause its presence off our western coastline. However, it has been suggested that wind drift is the main cause for the occurrence of bats north of the Wadden Islands. We also found a moon illumination effect in both models. Increasing moon illumination raised the probability of presence at sea and at the coast. Rain reduced probability of the presence of bats at the coast. In contrast, we did not find an effect for rain at sea; thus, bats were recorded with and without rain at sea. High cloud cover was negatively correlated with the presence of bats at sea, but was positively correlated with the presence of bats at the coast.
The sea model predicts a higher probability of presence in the northwestern corner of the study area. However, we think that this is an artefact caused by the relatively high number of nights with bat activity at the P6A platform, in comparison to the presence at the other offshore monitoring locations. This may be just be a coincidence, but it is also possible that a spatial pattern of occurrence at sea is actually present. For example if bats follow their general migration direction (WSW) after leaving the Afsluitdijk they will pass closely to P6-A.
The recorded bat activity at nearshore monitoring locations (between 22 and 25 km from the coast) peaks approximately 4 hours after dusk. It seems likely that these animals departed the same night from the coast. However, bat activity at the locations further offshore (between 58 and 69 km from the coast) starts often close to dusk. This means that these animals must have spent the day at the monitoring location at sea, or in its vicinity. This pattern of occurrence means that the observed bat activity at a particular night may depend on their departure decision in the previous night, or even earlier.
Other species recorded during this study included Common pipistrelle which was occasionally recorded offshore, but was common at the coast throughout the monitoring season. Nyctaloids were recorded uncommonly offshore from June until October and from May until late October at the coast. Nyctaloids identified to species level included Common noctule, Particoloured Bat, Leister’s Bat, Northern Bat and Serotine Bat. Pond bats were not recorded offshore but were regular at the Afsluitdijk and rare elsewhere along the coast. Finally, there were some occasional records of Daubenton’s bats and Soprano pipistrelles at the coast.
The results of this study show that the occurrence of bats at sea is highly seasonal which indicates that individuals recorded at sea are on migration. The peak period runs from late August until the end of September. After that it levels off throughout October. Spring migration is much less pronounced but the duration seems to be quite extensive; from late March until the end of June. Records of bats in July and early August are rare. At the coast bats are much more common in general and their presence is both shaped by migratory movements and the presence of foraging individuals from local populations. Therefore, the relevant period to consider the presence of bats at sea off the western coast of the Netherlands and Belgium seems to be from 15 March until 30 June and from 15 August until 31 October, whereas bats should be considered the entire active season at the coast.
Based on the monitoring results of the 2012 – 2014 studies a precautionary mitigation measure was issued using 5 m/s as cut-in wind speed for the wind farms in the Borssele area in the period 15 August until 1 October. The current study however shows that other environmental parameters, in addition to the wind speed, are important as well. The model developed in this study is likely to predict the presence of bats at sea more accurately, despite the fact the model can be improved.
In order to improve the sea model it is recommended to continue monitoring offshore to increase the number of observations in the dataset. The model can furthermore be improved by monitoring in a denser grid to reveal spatial patterns and include information on the availability of insects (bat migration fuel). In addition, we urgently need monitoring data from higher altitudes as bat migration may occur at altitudes beyond the detection range of the current monitoring network at sea.