Assessing the impact of the Westdiep Sea farm on avifauna: Evaluation of the monitoring approach through power analysis and recommendations for future monitoring
Daelemans, R.; Vanermen, N.; Courtens, W.; Van de walle, M.; Verstraete, H.; Stienen, E. (2025). Assessing the impact of the Westdiep Sea farm on avifauna: Evaluation of the monitoring approach through power analysis and recommendations for future monitoring. Reports of the Research Institute for Nature and Forest, 2025(46). Research Institute for Nature and Forest (INBO): Brussels. 34 pp. https://dx.doi.org/10.21436/inbor.131028768
Part of: Reports of the Research Institute for Nature and Forest. Research Institute for Nature and Forest (INBO): Brussels. ISSN 1782-9054, more
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| Keywords |
Gavia stellata (Pontoppidan, 1763) [WoRMS]; Podiceps cristatus (Linnaeus, 1758) [WoRMS]; Sterna sandvicensis Latham, 1787 [WoRMS]
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| Authors | | Top |
- Daelemans, R.
- Vanermen, N., more
- Courtens, W.
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- Van de walle, M.
- Verstraete, H., more
- Stienen, E.
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| Abstract |
In 2020, an environmental permit was granted for the development of the Westdiep Sea Farm, a longline aquaculture project located in the nearshore waters of the Belgian North Sea (BNS), approximately 4.5 km offshore Nieuwpoort. The 4.54 km² project area is designated under the Belgian Marine Spatial Plan for extractive species such as mussels, oysters, and seaweed. The area lies entirely within SPA-B1, a Special Protection Area under the EU Birds Directive that is recognized for its ecological importance as a foraging and resting habitat for species including Sandwich tern (Thalasseus sandvicensis), great crested grebe (Podiceps cristatus), and red- throated diver (Gavia stellata). The presence of aquaculture structures and increased vessel traffic associated with the sea farm may lead to avoidance in disturbance-sensitive seabird species, while other species might be attracted due to enhanced food and resting opportunities. To comply with permit conditions, a seabird monitoring program was established to detect such responses, following a Before-After Control-Impact (BACI) design with four ship-based surveys annually over a five-year period. However, the first year of baseline monitoring revealed that seabird densities within the project area were generally low and highly variable, raising concerns about the program’s ability to detect statistically significant changes in seabird densities over time. To further investigate these concerns, a simulation-based power analysis was conducted. Based on species- and location-specific data characteristics derived from the INBO seabird dataset (2001-2023) and the initial four baseline surveys, a large set of randomised datasets was generated. In these datasets, various increases and decreases in seabird densities were simulated under different monitoring strategies, namely seasonal surveys, peak-density surveys, and monthly surveys. Statistical tests were then applied to assess whether these changes in seabird densities would be detected as significant, enabling us to estimate the probability of detecting a true change in seabird density under the proposed monitoring design—i.e. the statistical power. Results from the power analysis indicate that the current monitoring design (16 post-impact surveys) is unlikely to achieve the targeted statistical power of 80% to detect a 50% change in seabird numbers for most species. Even under optimized scenarios that align surveys with month of peak seabird densities, statistical power remained limited, although notable improvements were observed for species such as great cormorant and Sandwich tern. Increasing the sampling effort, for example by conducting monthly surveys, did not lead to proportional gains in power and is not feasible within the available resources. Therefore, we recommend adjusting the timing of the 16 planned surveys to coincide with periods of peak abundance of key species in the area. Scheduling surveys between January and April is expected to maximize impact detection potential for red-throated diver, great crested grebe, and common scoter. In addition, these targeted surveys will be supplemented by observations from INBO’s long-term seabird monitoring campaigns and anecdotal sightings reported by the crew of the sea farm. This revised monitoring design is expected to generate valuable insights, not only about the ecological impact of the current project but also for building the scientific and policy frameworks that will guide future offshore aquaculture developments. |
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