Aggregation, production and spillover: the cumulative impact of man-made structures on fish.

In the latter half of the 20th century, over 1300 man-made structures (MMS) were installed in the offshore marine environment of the North Sea for the oil and gas industry, and many others are now being installed to obtain offshore renewable energy. There is mounting evidence for the ecological benefits of such structures, not least for marine fish. In the United States of America, MMS (particularly oil rigs) are used as reefs to successfully enhance fisheries in the so-called, Rig to Reefs programme. In Europe the opposite occurs: international regulations governing the North Sea (i.e. OSPAR Decision 98/3) require the complete removal of disused offshore installations.

In the North Sea it is well known that fish aggregate at MMS and oil and gas MMS have 500 m fishery exclusion zones. So the structures act as small marine protected areas. At this scale, the total area protected is very small relative to the area occupied by commercial fish stocks in the North Sea. However, there is also evidence that aggregations of fish extend well beyond the exclusion zone, which is either a result of aggregation towards the MMS or enhanced productivity and protection resulting in fish spilling over into the adjacent area. The "spillover effect" is a well-known benefit of marine protected areas but has not been studied in relation to MMS in the North Sea. If this spillover is significant, then the area influence of MMS could be greater than hitherto assumed.

Here, we use an unmanned surface vehicle equipped with state of the art high resolution acoustic surveying techniques to evaluate the scale of aggregation and spillover effect of fish from North Sea MMS. Data from the surveys are used to parameterise a high spatial resolution model of fish dynamics and movements which we will use test hypotheses about the whole North Sea scale effects of networks of structures, including the extent to which they arise from enhanced productivity or the protection from fishing afforded by proximity to hard substrate, and the North Sea scale consequences of their removal..

Our results will be used by oil and gas operators in the North Sea, one of which is a partner in our project, to inform their cumulative impact assessments. This will provide evidence for derogating removal, which as stated by the CEO of the Scottish Wildlife Trust, in a parliamentary committee, could result in "a triple win", with "...substantial savings to the taxpayer and substantial savings to the operator. There could be a net positive effect on the environment in the retention of these sometimes highly biodiverse areas around rigs and other structures."

The key stakeholders who will benefit from this project are the oil and gas operators, the legislators, the fishing industry and the emerging offshore renewables industry. These stakeholders face the issue of conflicting use of marine space, and in the case of the operators, the prospect of decommissioning their offshore structures at huge costs. At the very least, understanding how fish use these structures and their potential contribution to North Sea fish populations links to the industry challenge of cumulative impact assessment (CIA). There is a lack of certainty over the process of undertaking a CIA, with inconclusive guidance and inconsistent definition of scope. The immediate expected benefits for the oil and gas operators will be providing evidence for cumulative impact assessments associated with decommissioning. However, an outcome of some substantial value would be a demonstration of significant benefits to the environment of the offshore structures. Elevated fish densities, particularly of predatory fish such as cod and saithe, are indicators of a healthy marine environment because these fish are supported by several trophic levels beneath them in the food chain. There could be economic benefits to fishing stakeholders, if these structures are shown to enhance local fish production through, for example, spillover effects. However, such economic benefits would be dwarfed by the massive economic benefits to the oil and gas industry if, as has occurred elsewhere, a demonstration of rigs to reef benefits contributes to a change in decommissioning policy.

The stakeholders will specifically benefit from: a) obtaining quantitative evidence relating to the abundance of commercial fish stocks in and around platforms for comparative and environmental impact assessments; b) use of this evidence to inform future engagement with regulators regarding the interpretation and value of existing decommissioning legislation; c) the development of a non-intrusive method which is able to capture repeatable fish population data in a cost effective manner; d) protocols & models to apply the new techniques in future and to other structures such as offshore renewable devices, pipelines and wrecks.

Key stakeholders, including one of the major operators in the North Sea, are partners in the project. We will engage with these partners with regular face to face meetings, facilitated by the National Decommissioning Centre, which is also a partner. We will also attend stakeholder workshops (e.g. annual MASTS/SUT workshops). We also aim to publicise the potential benefits of offshore MMS by conducting a variety of STEM outreach activities (café scientifique, school visits, and science festival displays of our acoustic data and video data).