The importance of oil and gas platform foundations for fish communities in the North Sea

Aging oil and gas (O&G) platforms have been in place for several decades and will have to be fully removed once they reach the end of their productive life, following current regulations in the North Sea. Concurrently, increasing evidence suggests that O&G platforms might act as artificial reefs, shaping local biological communities by providing suitable habitats for settlement, refuge, and food sources (i.e., growth). At the same time, the usual fishing ban of 500 m around platforms makes them resemble de facto marine protected areas with restricted fishing, including bottom trawling, enhancing seafloor stability, biogenic growth, and supporting higher trophic level species, including fish. Unfortunately, our understanding of the mechanisms underpinning platform effects on fish communities is limited, and platform decommissioning assessments of the ecological outcomes in relation to conservation and fisheries are limited in scope. This complicates predicting ecological scenarios associated with different platform decommissioning options. In this thesis, I aim to provide an understanding of the role that offshore O&G platforms might play for marine ecosystems in the North Sea using fish assemblages as case studies. Specifically, I explore general and species-specific associations between fish biodiversity, abundance, and individual body size with the proximity of offshore O&G platforms in the North Sea. Further, I investigate if the potential associations show diel and depth patterns of variation. My approach involves using scientific angling for 1) assessing fish abundance and body size patterns along distance-to-platform gradients from multiple offshore O&G platforms in the Danish North Sea, and 2) exploring spatial patterns of variation in fish biodiversity, abundance, and body size near an O&G platform in the Danish North Sea. Additionally, I use underwater video recorded with a remotely operated vehicle (ROV) for 3) exploring spatial patterns of fish abundance along depth, horizontal distance from an O&G platform in the northern North Sea, and the diel variation of these patterns. My results are then framed within the contemporary context of the North Sea habitats and the expansion of artificial offshore structures (e.g., offshore wind farms) to discuss crucial knowledge gaps for achieving biodiversity net gain from offshore infrastructure projects.

The results showed overall higher fish biodiversity towards the platforms, with abrupt declines occurring beyond 50 m from the structures. In addition, fish abundance was positively associated with platform proximity, with abundance values being at least 4-fold higher at sites closer to the platforms than at the end of the assessed horizontal range. These spatial patterns of fish abundance varied among the studied species, suggesting species-specific associations with the O&G platforms. For example, Atlantic cod (Gadus morhua) abundance was positively associated with platform proximity, with the highest number of individuals seen at the study sites closest to the platforms. Further, Atlantic cod individuals closer to the platform were significantly larger than the individuals at sampling sites away from the structure, revealing positive associations between cod individual body size and platform proximity. Finally, the ROV footage showed positive associations between platform proximity and fish abundance, which was also correlated to depth and diel patterns. The ROV footage found higher fish species richness and abundance at the greatest depths and closest to the platform, but the fish aggregations expanded along the water column at night. While the ecological drivers behind my observations are not evaluated in this thesis, my results suggest that offshore O&G platforms are acting as important local habitats for fish communities in the North Sea. Importantly, these infrastructures together with the expanding offshore wind farms require developing clear guidelines for achieving best environmental outcomes not only from decommissioning but also from the other stages of their life cycle.