Fish and fisheries research to inform ScotMER evidence gaps and future strategic research in the UK: review

Evidence Gap FF.11: Reef/fish aggregation effects

Review of current knowledge

Extensive fish monitoring survey work has been undertaken in operational wind farms in the UK, particularly in early projects developed as part of The Crown Estate's Rounds 1, 2. and 2.5. Such monitoring was generally a requirement under consent conditions and typically consisted of the undertaking of generic pre-construction and post-construction fish surveys. In some cases, species specific monitoring was also undertaken (lobster and crab surveys, shrimp surveys, elasmobranchs, etc). Monitoring surveys were generally conducted using commercial fishing vessels and gears (demersal otter trawls, beam trawls, pots, gill nets, etc).

In 2014, the MMO reviewed the results of the fish and shellfish post-construction monitoring undertaken in 17 Round 1 and 2 wind farms (MMO 2014a). The review concluded that in all cases the requirements prescribed in the licence conditions were fulfilled, however, clear conclusions could not be drawn from the results. It was considered likely that there had been no moderate or major impacts to fish populations due to impacts resulting from the sites reviewed. It was less clear, however, if there had been minor changes to the fish populations that may have gone undetected by the standard monitoring methods used due to the natural high variability in fish and shellfish populations. In the North Hoyle and Kentish Flats offshore wind farms, minor changes were detected as a result of reef effects and changes in fish communities associated with the introduction of hard substrate. However, this effect was not reported at the other sites included in the review. It was argued this this may be due to the early state of the operational phase at which monitoring was undertaken, a result of the post-construction monitoring being incomplete or due to inadequate survey design (MMO 2014a).

As described above, whilst post-construction fish and shellfish monitoring has been undertaken in numerous UK offshore wind farms, it has not been possible to draw clear conclusions on potential reef/fish aggregation effects from its findings. Extensive information on this subject is however available from studies undertaken in other countries. Some examples of these are given below:

• Lindeboom et al (2011) compiled the short-term (two year) results of monitoring carried out in the Egmond aan Zee offshore wind farm in the Netherlands, including fish monitoring. The study identified only minor effects on fish assemblages, particularly around the monopiles and suggested that some fish species, including cod, may find shelter within the wind farm.
• Stenberg et al (2011; 2015) investigated the long-term effect (seven years after construction) of the Horns Rev 1 offshore wind farm on fish abundance, spatial distribution and diversity. The studies found no evidence of negative long-term effects on key fish species or functional fish groups. Overall, fish abundance increased slightly in the area of the wind farm and species diversity was higher close to the turbines. In addition, it was found that fish associated with rocky habitats were distributed closer to the artificial reef structures introduced by the turbines than other species. The results of the study suggest that the artificial reef structures were large enough to attract fish species with a preference for hard substrate, but not large enough to have adverse negative effects on species inhabiting the original sand bottom between the turbines, including sandeels.
• Hansen et al (2012) studied the small-scale distribution of fish in the Middelgrund and Lillgrund offshore wind farms, located in Øresund Strait, between Denmark and Sweden using a stationary camera system. Fish distribution was examined at approximately 0.25 and 50 meters from the turbines. The findings of the study suggest that in areas with homogenous sandy seabed, the presence of a turbine clearly attracts fish. Whilst for areas with more heterogenous substrate and sessile species the fish aggregation function of a turbine was not as significant.
• Reubens et al (2013a) studied the spatio-temporal distribution of cod and pouting from 2009 to 2011 in relation to three different habitats (wind farm structures, shipwrecks and sandy bottoms). The study found highest catch per unit effort values for both species around wind farm structures and indicated distinct aggregation around turbine foundations.
• Reubens et al (2013b) monitored the residency, site fidelity and habitat use of cod in relation to artificial hard substrate from a wind farm over a year and identified aggregation near the hard substrate of the turbines. In addition, a clear seasonal pattern in presence was also observed, with high number of fish present in summer and autumn and a period of low densities during the winter.
• van Hal et al (2017) studied the impact of hard substrate of the fish community in a Dutch OWF which had been operational for five years. The study suggested attraction of cod, pouting, bullrout and edible and velvet crab to the hard substrate. Flat fish and whiting appeared to be attracted to the sandy habitat within the wind farm. In addition, the results of the study suggested that offshore wind farm structures were only used temporarily for shelter or feeding.
• Krone et al (2017) compared the mobile demersal megafauna associated with common types of wind farm foundations (jacket, tripod and monopile with scour protection of natural rock) in the southern German Bight. Monopiles with scour protection were colonised with typical reef fauna and recorded an average of about 5,000 edible crabs per foundation (more than twice as much as found in foundation types without scour protection). In addition, Krone et al (2017) found strong evidence that the three foundation types functioned not only as aggregation sites, but also as nursery grounds for edible crab.
• Degraer et al (2018) provided a review of the findings of fish monitoring in two Belgian wind farms, C-Power and Belwind, 6 and 7 years after construction respectively. The study did not identify a direct wind farm reef effect on the soft-bottom epibenthos and demersal-benthopelagic fish assemblage. However, species known to be fouling on the foundations such as mussels and anthozoa sp. were found to be abundant in soft sediment samples within the wind farm and absent from soft sediment outside the sites. It was suggested that this could indicate that the reef effect was starting to expand beyond the direct proximity of the turbines. It was noted, however, that this would need follow-up work in order to be validated.
• Methratta and Dardick (2019) undertook a meta-analysis of studies that have examined abundance of finfish inside wind farms compared to nearby reference sites and calculated the overall effect size across studies. In addition, it investigated changes in effect size for soft-bottom and complex-bottom oriented species in association with various covariates. The study included information from offshore wind farms located in the North Sea, Irish Sea, Baltic Sea and Øresund. The research found that the overall size effect was positive and significant, indicating greater abundances of fish inside wind farms. Similarly, positive and significant effect sizes were identified for various covariates for both soft-bottom and complex-bottom species.

Next steps in research

From the results of the studies carried out to date in operational wind farms, it is apparent that changes in fish assemblages and reef effects tend to be more obvious at the scale of the turbines and its surrounding area, however they are not restricted purely to the structures themselves (Degraer et al 2020). Further research is however needed to fully understand the implications of the changes in fish assemblages that have been observed to date, so that simple aggregation effects can be distinguished from potential benefits (i.e. feeding opportunities or provision of nursery or spawning habitat).

These first order effects may be considered trivial in the context of the ecosystem. However, as small-scale changes are the basis of large-scale changes, they can be used to inform impacts at regional scales. In order to assess the impact of reef and fish aggregation effects, it is therefore important to identify appropriate functional and temporal scales of ecosystems or their parts requiring investigation (Degraer et al 2020).

In order to address current knowledge gaps in respect of reef/fish aggregation effects, the following actions are recommended:

• Focusing monitoring efforts on addressing relevant questions at appropriate temporal and spatial scales, avoiding the collection of data-rich, information-poor (DRIP) data (see Wilding et al 2017).
• Consideration of the use of ecosystem-based approaches for assessment and monitoring of impacts within operational sites, allowing the integration of data and information from multiple receptors (i.e. benthos, fish, ornithology, etc).

Recommendations outlined in respect of "Evidence Gap FF.05: Strategic fisheries management" are closely related to potential reef effects and should also be considered in relation to this topic.