Modelling the Whole-Ecosystem Impacts of Trawling

Heath, Michael and Wilson, Robert and Speirs, Douglas (2015) Modelling the Whole-Ecosystem Impacts of Trawling. [Report]

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    Abstract

    Trawling has been controversial since its introduction in the 17th century. In 1882 the Fishery Board for Scotland was established and assigned powers to ban beam and otter trawling where necessary to protect traditional static gear fisheries. Under these powers, large parts of the inshore waters off the east and west coasts of Scotland were closed to trawling. The Firth of Clyde remained closed until 1962. More recently, in April 2015 solicitors acting for Greenpeace obtained High Court permission for a judicial review of Defra’s alleged failure to adjust its policy on allocating annual landing quotas to reflect reforms to the CFP. It is claimed the reforms stipulate that greater preference should be given to sustainable low impact fishing methods at the expense of high-impact methods such as trawling. In Scotland, the exclusion of trawling activity from a network of marine protected areas established in July 2015 has also been highly controversial. There is no doubt that some trawl gears can be extremely destructive of fragile habitats and slowly regenerating fauna such as coral. Over expanses of mud or sand, however, it has been claimed that trawling may be a positive factor, akin to ploughing the fields in terrestrial agriculture, and enhancing the productivity of the ecosystem. There have been many scientific studies, both in the field and using mathematical modelling, of the impact of trawling on the seabed. Similarly, we know very well that harvesting of fish and shellfish, whether using trawling or static gear, has consequences for marine food webs. However, there have been few, if any, scientific studies which have put these two aspects of trawling together and then compared the seabed impacts of trawling with the consequences of harvesting. In this project we used a mathematical model to compare and contrast the whole ecosystem effects of harvesting fish and shellfish with the consequences of other aspects of trawling activity, especially the ploughing of seabed habitats. The model is not detailed to the level of individual species or exact locations. Rather it gives results at the level of a whole regional sea area, such as the North Sea or the whole of the west of Scotland. The project had three main components. First, was the extension of an existing mathematical model of a marine ecosystem to include explicit representation of the ploughing effects that different gears have on seabed habitats. Second, an analysis of a large international data set on activity, landings and catches by different fishing gears in northwest European waters, and the mapping of these onto different seabed habitats to generate inputs to the model. Finally, we carried out a series of sensitivity experiments with the model. These experiments investigated the whole ecosystem effects of seabed ploughing by different gears, using food web indicators relevant to the EU Marine Strategy Framework Directive, and compared them with the impact of one scenario for implementing a landing obligation, and the potential impacts of a reduction in overall fishing activity. For the North Sea, the results show that even if all ploughing effects were eliminated, the effects on the whole ecosystem would be equivalent to only a 1% or less change in overall harvesting rate of fish and shellfish. This is a very small effect compared to the changes in effective harvesting rate implied by the improvements in gear selectivity required to achieve the landing obligation. For the west of Scotland region, the model showed that the food web was more sensitive to the effects of ploughing by fishing gears than in the North Sea, but the effect was still small compared to the consequences of activity reduction overall. The greater sensitivity of the west of Scotland to seabed ploughing arose because the disturbance rate of muddy sediments was around 5-times higher than in the North Sea, almost entirely due to the activities of TR2 Nephrops trawling. Despite our conclusion that the regional scale food web effects of seabed ploughing are small compared to the primary consequences of harvesting fish, this is not to say that there are no effects on regional biodiversity, or significant effects at local scales on specific habitats or vulnerable species. In particular the study identifies the TR2 gear fleet as being responsible for the majority of ecosystem-wide consequences of seabed ploughing. This gear has a particularly high ploughing rate and its activity is focussed on muddy sediments where the nutrient chemistry processes are more vulnerable to ploughing than in sandy and coarser sediments.