Aggregation, production and spillover: the cumulative effect of man-made offshore structures on fish

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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."