Do oil and gas platforms affect commercial fish species and biodiversity?

Life below water in the North Sea

This article was featured in Eurofish Magazine 1 2025

The North Sea, home to many different species including the threatened Atlantic cod, remains one of the most important fishing grounds in the world, but it is also a hotbed for oil and gas exploration. Numerous oil and gas platforms are reaching the end of their lifespan in the North Sea and are expected to be decommissioned and removed from the seabed. However, increasing evidence suggests that offshore platforms may function as artificial reefs and even benefit marine biodiversity.

In this article, the authors summarise findings from two separate studies conducted in 2021 and 2023. The first was a broad assessment of several platforms, while the second was a detailed assessment of a single platform. Both studies revealed increased fish abundance and elevated biodiversity of various fish species near the platforms. In addition, fish were often larger near the platforms. These findings suggest that removal of offshore platform foundations from the seabed may affect marine life negatively. Policymakers, NGOs, and other relevant stakeholders are encouraged to consider the potential negative outcomes for marine life caused by the upcoming decommissioning of offshore platform foundations in the North Sea.

Offshore platform ­installations started in the mid-1900s in the North Sea

Since the mid-1900s, the North Sea has been a hotbed for offshore oil and gas (O&G) extraction. Currently, about 50 platforms are extracting fossil fuel in several O&G fields in the Danish part of the North Sea. Increasing evidence suggests that the platform foundations support fish abundance and may benefit marine biodiversity. The submerged hard structure provided by the foundations allows various marine organisms such as invertebrates and algae to attach and grow. This may create a diverse marine ecosystem, hosting many small mobile species. As a result, larger species are attracted, because of the developing foraging opportunities. In popular terms, this scenario is known as a reef effect. When offshore platforms are operational, they are typically surrounded by a 500 m no-fishing zone. The no-fishing zone creates an area that resembles a marine protected area. Combined with the reef effects, the no-fishing zone allows a platform foundation to act as an offshore habitat for several—potentially overfished—species. Near the platform foundation, numerous species may grow in both abundance and body size. Some of the species hosted by platforms are Atlantic cod (Gadus morhua), mackerel (Scomber scombrus) and dab (Limanda limanda).

Elevated fish abundance near several platforms

We examined the impact of offshore platform foundations on fish abundance, fish body size, and species richness during the summer of 2021. The study was completed in the central North Sea, about 200 km from the nearest coast. The platforms had been operational for decades and were each surrounded by a 500 m no-fishing zone. Researchers sampled fish abundance and biodiversity at distances varying from 5 m to 10,000 m from the platform foundations. The results were clear. Fish abundance was highest just 5-20 m from the platforms and declined upon moving away from the platform foundations. The study also found that species richness often increased closer to the foundations, with several species like Atlantic cod, observed within 5 m from the foundations. However, cod abundance declined steeply further away from the platforms. Cod size also varied with proximity to the platforms. Average fish lengths were around 37 cm at a distance of 10 m but dropped to 28 cm at 110 m from the foundations.

The summer 2021 study was the first scientific examination of fish near platforms in the Danish part of the North Sea. Indeed, the study indicated that platform foundations play a role for fish abundance and biodiversity. The study covered several platform foundations and revealed some general trends. Often, fish abundance and species richness declined quickly with distance from the platforms, indicating that the greatest platform effects are observed within the first 100 m of the platform. The 2021 sampling was, however, opportunistic and lacked systematic sampling and balanced sample sizes. Another study was therefore planned and conducted during summer 2023.

Increased fish ­abundance, biodiversity, and fish body size near a platform in the North Sea

Following the broader assessment of fish abundance near different platforms, researchers targeted a single platform to better quantify fish abundance patterns in detail. In 2023, adequate sampling was conducted systematically and planned carefully in advance. The assessment aimed to examine the impact of an offshore O&G platform on fish abundance, fish size and species richness. The platform selected is called Skjold and is more than 40 years old and surrounded by a 500 metre no-fishing zone. Researchers sampled fish abundance and biodiversity at distances ranging between 1 m and 600 m away from the platform foundation. The data clearly revealed that fish abundance, body size, and species richness were highest near the platform. In fact, 100% of the observed species occurred within 20 m of the platform, with the highest fish diversity recorded just 1 metre away from the foundation. Species like Atlantic cod were much more abundant near the platform, and their numbers declined sharply at further distances. Cod sampled near the platform averaged 43 cm in body length, greatly exceeding the body length of the cod sampled 50-100 m from the platform. Thus, consistent with the 2021 study, cod were simply larger near the platform. 

In addition to cod, other species also exhibited distinct abundance patterns around the studied platform. For example, dab (Limanda limanda) abundance was more than 15 times higher next to the foundation than at 600 m from the foundation. Similarly, saithe (Pollachius virens) was only observed within 50 m from the platform. However, unlike cod, no clear body size patterns were observed for dab and saithe. Thus, the role of the platform foundations seems to differ between species. The site-specific approach allowed researchers to confirm general trends initially observed in the 2021 study. Using systematic sampling in 2023, this study provided a more quantitative understanding of the correlations between foundation proximity and fish abundance, fish body size, and species richness. These findings indicate the significant positive effects that platform foundations often have on local fish communities. To preserve and improve marine biodiversity more generally, it remains important to understand the cumulative effects of numerous offshore foundations on marine life in the North Sea.

Future perspectives of Atlantic cod in the North Sea and removal of ­offshore structures

Atlantic cod abundance in the North Sea has fluctuated greatly over the past decades and remains at historically low levels. Many cod populations have experienced significant fluctuations due to overfishing and climate change, despite recovery plans. Improving conditions for cod and other fishes is crucial for their recovery. Enforcing stricter fishing regulations are likely to help, as regulations directly control overfishing in crucial habitats and elsewhere. Optimised regulations could help, for example, by reducing fishing pressure allowing populations to recover. Atlantic cod fisheries in the North Sea follow technical recommendations, but fishing quotas ultimately rely on international agreements, for instance, between the EU, Norway, and the UK. Our studies in 2021 and 2023 confirmed that cod abundance and body size are significantly higher near platform foundations and face sharp declines further away. This suggests that offshore platform foundations exhibit reef effects, providing shelter, feeding, and nursing opportunities for cod and other species. Therefore, repurposing obsolete platforms as artificial reefs may represent an opportunity for expanding productive areas where cod is protected. However, this approach would need specific guidelines and updated policies.

The legislation in the North Sea currently states that most platforms should be fully removed after ceasing operations, a process called decommissioning. The platforms are taken to land for recycling, seeking to avoid environmental impacts. Policymakers are aware of the direct ecological impacts of full platform removal activities. However, perhaps better strategies should be evaluated for decommissioning. In the US and elsewhere, decommissioning often includes Rigs-to-Reefs approaches. Rigs-to-Reefs is a programme that aims to maintain the artificial reef effects created by foundations after their operational life. Rigs-to-Reefs can be done in multiple ways, based on the local characteristics and requirements. One method involves tipping the structure over and laying it on its side on the ­seabed. Another method involves separating the top of the platform, and either placing it next to the base on the seabed or taking it to shore for recycling. There is also the option of removing the whole platform and taking it to a designated reefing area or to shore for recycling.

A study done in the North Sea examined the extent to which decommissioned platforms support marine life. In the study, cod aggregated near the inactive platform, and fish were larger in body size than those recorded away from the inactive platform. The study suggested that inactive platforms likely offer favourable conditions for marine life when left completely in their original location (figure 4A). Platform removal is often expensive and also disruptive to marine life. Therefore, we suggest that policymakers consider revisiting existing regulations based on recent ecological evidence. We encourage policymakers to consider repurposing platform foundations as artificial reefs, as our North Sea studies from 2021 and 2023 showed positive impacts of platform foundations on fish abundance, body size and species richness. For such a Rigs-to-Reefs program, each platform should be assessed individually and must not release harmful components, even if the platform is inactive and left in the marine environment. With the right approach, leaving submerged structures at sea may offer greater ecological benefits than complete removal. However, further studies are needed to reveal which decommissioning options might deliver the best biodiversity outcomes for the North Sea.

Rania Jeanbeyh, Bachelor student in General Engineering at the Technical University of ­Denmark, Denmark

Bruno Ibanez-Erquiaga, Ph.D. ­student at Technical University of Denmark, National Institute of Aquatic Resources (DTU Aqua), Denmark, and affiliated with Association CONSERVACCION, Peru

Jonas Teilmann, Professor at Department of Ecoscience, Aarhus University, Denmark 

Jon C. Svendsen, Senior scientist at Technical University of ­Denmark, National Institute of Aquatic Resources (DTU Aqua), Denmark

References:

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Acknowledgements

The authors kindly acknowledge the Danish Underground Consortium (TotalEnergies, Noreco, and Nordsøfonden) for granting permission to publish the data from the studies. We would also like to express our gratitude to DTU Offshore for its support. This research has received funding from the Danish Offshore Technology Centre (DTU Offshore) under the ABN program. The funding sources had no role in study design, data collection and analysis, the decision to publish, or preparation of the manuscript. The authors thank everyone that has contributed to cruise planning, data collection, data analyses, interpretation etc.