FIS003: Whole-ecosystem impacts of trawling

Start Date: Jan 2015

End Date: Sep 2015

Main Contractor(s): University of Strathclyde

Other Sponsor(s): European Fisheries Fund

Extended Title: Modelling the whole-ecosystem impacts of trawling

Main Research Category: Environment / Ecosystem

View Final Report


Project Objectives

Trawling impacts on the marine environment in a variety of ways, ranging from direct ploughing of sediments leading to sediment re-suspension and release of nutrient stored in sediment pore waters, mortality of seabed fauna, and disturbance of predator-prey communities by removal of targeted fish and shellfish species. Teasing apart the relative importance of these factors for whole-ecosystem impacts by direct observation in the field is very difficult due to other confounding environmental variability. Here we propose to utilise a computational end-to-end ecosystem model to resolve the factors attributable to trawling by means of a global sensitivity analysis. The model concerned represents the entire range of fauna and flora in a shelf sea region along with the underlying sediment biogeochemistry. Some modifications to the model will allow us to explicitly represent trawling impacts, and then we can run a series of parameter sensitivity analyses to determine which aspects of trawling have the greatest impact on ecosystem status as measured by a set of high-level ecological indicators. The analyses will be performed on two versions of the model, one representing the North Sea and the other the West of Scotland since these regions have very different sediment compositions. The results will be used to advise on future ecosystem friendly trawling practices.


Specific Objectives:

Objective 1. Representation of physical impacts of towed gear in the model. Start – 1 January 2015, end – 15 March 2015.

In this part of the project we will conduct an analysis of the association between seabed sediment types and fishing effort by towed gears in the North Sea and West of Scotland regions. We will use this, together with existing data on the porosity and permeability of sediments in relation to grain sizes, to parameterise an increase in sediment water diffusion rate in the StrathE2E model as a caricature of the physical impacts of trawling.

Objective 2. Representation of direct effects of towed gear on benthos. Start – 1 February 2015, end – 30 April 2015.

Mortality rates associated with physical damage to benthic fauna during the passage of a typical trawl have been summarised in the literature [5]. In our case, we need to translate these into a mortality increment on the benthos categories in the StrathE2E model, based on the proportion of the regional seabed area which is swept by towed gear in a given time interval. We expect this increment to be gear specific, and depend also on the seabed sediment type.

Objective 3. Sensitivity analysis and scenario experiments. Start – 1 April 2015, end – 15 June 2015.

The aim of the sensitivity analysis will be to determine whether there are particular aspects of the towed gear impacts which have a disproportionately large impact on the ecosystem. This will enable us to advise on how gear may be designed or used to minimise its ecological footprint – for example if it turns out that the ploughing effect is the major factor causing impact, then we can recommend that gears be used in a semi-pelagic model instead of being allowed to settle heavily on the seabed. Alternatively, it may turn out that the deposition of discards is actually the largest effect on the benthos, in which case we can conclude that the issue of trawling impacts is in fact tied up with that of implementing a landing obligation. 

Objective 4. Reporting and Knowledge Exchange. Start – 15 June 2015, end - 31 July 2015.

The final month of the project will be devoted to finalising the report on the project and communicating the outputs to industry and policy stakeholders. We aim to achieve knowledge exchange goals in short workshops which we will arrange in the final month of the project. 


Summary of Project Outcomes

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.


Recommendation from the project

Without denying the damaging effects that ploughing of the seabed by trawl gears can have on biodiversity, and local scale seabed integrity and fragile fauna, the key message from the project is that effects on food web indicators at a regional scale are small compared to the effects of harvesting and landing biomass. By region scale, we mean the whole North Sea, or the west of Scotland shelf.

At the regional scale the main benefits for ecosystem food web status are likely to come for managing the overall levels of activity, or the selectivity (power) of fishing gears, rather than focussing on specific gears which are perceived to be particularly damaging to the seabed.

Notwithstanding the above, a key action point to come out of the project concerns the high ploughing impact of the TR2 gear, especially in the west of Scotland region. Anything that can be done to alleviate this feature of the gear will improve its image and the sustainability of the fishery that relies on it, will be a positive measure. The vast majority of the regional impacts of seabed ploughing arise from the activity of this gear alone.