This study describes the abundance and distribution of harbour seals, Phoca vitulina, in the Netherlands in relation to environmental factors (both natural and human related), and considers the effects of the Offshore Wind farm Egmond aan Zee (OWEZ).
Harbour seals are common in Dutch waters. They are a relatively well studied species, but information on the seals’ habitat use (preference) and on which factors influence their distribution (both natural and anthropogenic) is lacking. In the current study, we utilise data on the movements, and therefore the behaviour, of harbour seals via satellite telemetry. We collected data in the framework of this study, tracking seals to the north and south of the wind farm. Next to this, we have also included data collected in earlier studies, thus the total data set used includes the tracking of 89 individual animals between 1997 and 2008.
This extensive amount of data (almost 29.000 locations after filtering) allowed us to develop a habitat preference model. With regard to the environmental conditions, the study demonstrates that seals in Dutch North Sea waters show a preference for the following:
Areas close to the haul out sites
Relatively shallow areas
Sediments with low mud content
It is estimated that the preference observed is not only governed by the factors themselves but most probably also by the preference of the seals' prey for these environmental factors.
Dive data collected during tracking was used to discriminate between foraging and other behaviour. Locations identified in periods when animals were assumed to be foraging, were used to model preferential foraging habitat. Both models (habitat preference model and preferential foraging habitat model) were applied to estimate the relative abundance of harbour seals within the NCP (Dutch Continental Shelf). This was done by combining these habitat models and numbers counted during aerial surveys. This resulted in two maps: one describing relative abundance, the second describing preferential foraging grounds.
The original design, was not specifically suited to study the effect of the wind farm as the distance to the closest haul-out was over 40 km’s and the probability of tagged seals roaming from the haul out to the wind farm was slim. Furthermore, only one wind farm area was studied. Also, the study did not include the construction period, thus it was not designed with the intention to measure the effects of construction on the seals. Despite this, we have found indications that the seals’ habitat use is influenced by this type of human activities. These should be studied in more detail when planning new wind farms in the seals’ aquatic habitat:
- Seals are less abundant near shipping activity: Only large shipping vessel were investigated, however, the seals on average, are less abundant in the direct proximity of the large shipping routes.
- Pile driving activities could have influenced the seals distribution: Pile driving for OWEZ and then Prinses Amalia Wind Park took up almost a year. The seals tracked during these constructions did not visit the area, thus effects cannot be excluded. This ranges up to at least 40 km’s north of the wind farm, and over 1-00 km south of it.
- The effect of the wind farms in operation could not clearly be defined in this study: Both in the periods before and after construction, tagged seals extend their distribution towards the wind farms.