Fine-scale proximity to offshore wind turbine foundations increases biomass of demersal fish species

Offshore wind turbine fixed-bottom foundations provide artificial hard substrate through the water column that encourages marine flora and fauna to colonise and aggregate around the introduced structures, a well-documented phenomenon known as the ‘artificial reef effect’. The cumulative impact thousands of turbine foundations at multiple offshore sites have on local and regional marine species populations and communities is not fully understood. Knowledge of the extent and magnitude of the reefing effect at a fine scale (single turbines) is a prerequisite to making broader-scale (single or multiple wind farms) predictions of population level and ecosystem changes caused by presence of offshore wind farms. The influence of fine-scale distance (<250 m) to turbine jacket foundations on abundance, biomass and size of demersal fishes was assessed at a northern latitude wind farm. Abundance and biomass of all demersal fishes, flatfish Pleuronectiformes spp. and haddock Melanogrammus aeglefinus were found to have a significant negative relationship with increasing distance from foundations. Haddock were found to aggregate closer to the structures, yet all statistical models predicted a similar magnitude of increase for each group of between ~1.5 and 1.6 times more individuals and biomass at 30 m from the foundations compared to 240 m. The results illustrate that fine-scale proximity to offshore wind fixed foundations has considerable effects on the presence of some demersal fish species. The cumulative or wider ecosystem consequences of these effects are not known, but the further evidence for localised reefing effects can be of strategic interest for optimizing future wind farm project design, included implementation of nature-inclusive measures that could help meet future marine net gain aspirations.