Indicating fisheries and seabed impact trade-offs

New advice presents measures of fishing pressure, its impact on the seabed, and the value of fish catches and landings, laying the groundwork for exploration of trade-offs.
Published: 6 July 2017

​​​Issued today, the advice for the European Commission's Directorate-General for Environment (DG ENV) centres on bottom-contacting fishing and the relationship between what happens to the seabed (the benthic zone) relative to what is caught and landed at port. For this, three sets of indicators are proposed: fishing pressure – a fishery's 'footprint' of effort – the impact of gear on the seabed, and the value and weight of fish landed. The first two can then be compared with the latter to assess trade-offs between conservation and exploitation. This will help to ensure sustainability, one of the underlying principles of EU's Marine Strategy Framework Directive (MSFD) and Common Fisheries Policy (CFP).

The release​ comes in two parts: the first describes these indicators and the methods used to create them. A more in-depth demonstration of trade-off scenarios run for the Greater North Sea ecoregion ​forms the second part.

The advice builds on work carried out last year on how maps of fishing intensity (effort per unit of area) could be used to assess the state of seabed habitats across the same ecoregions. Produced using vessel monitoring system (VMS) and logbook data, these maps formed the bedrock for three interconnected workshops – WKBENTH, WKSTAKE, and WKTRADE – and a process that has eventually culminated in the new advice. The first workshop's main task was to evaluate ways of modelling the sensitivity of different seabed habitats to pressure, and produce maps and indicators for measuring what effect fishing has on the seabed. The second workshop took these maps and indicators and, along with information on catches and values of landings, brought together stakeholders to explore how trade-offs could occur. The third workshop then considered how to inform managers of these trade-offs.

Indicating trade-offs

The advice lists four annual pressure indicators, along with a multiple year one. The annual indicators show that pressure from fishing is highly clustered. The multiple year one addresses 'unfished areas', and is likely significant for habitat in that a persistent absence of fishing will lead to a greater chance of habitat recovery.

In measuring impact, two methods for producing the two indicators are explained. One method uses the concept of 'longevity' to stand for 'biodiversity': for example, if an area is trawled every year, the only organisms living there are ones that last one year, and so there is less diversity. Greater diversity would be indicated by a time-frame longer than a year enabling more types of organisms to live there. The second method uses 'biomass' (weight) as a proxy for the how well a seabed community is still functioning: as it takes time for organisms to reproduce and grow, the more often a trawl hits an area, the lower the general biomass and magnitude of functions the seabed is performing will be.

Four landing and value and weight indicators for fish landed were also produced. One main trade-off scenario was then described. This is, if the objective were to reduce overall seabed impact, fishing pressure could be moved to core grounds (more fished waters) from the peripheries. Because of the already greater seabed disturbance in fishing "hot spots" areas in deeper areas of the North Sea, for example, a little more fishing pressure in these already affected areas will not be as bad as increasing pressure in areas that have not yet been as affected. This would also mean a smaller reduction in landings compared with shifting pressure the other way (from core to periphery).

North Sea example

Scenarios are presented in the second section (annex) of the advice, where a demonstration trade-off analysis is run for the Greater North Sea ecoregion over the period 2012-2015 using the indicators previously explained. This is the first time such an exercise has been conducted by ICES.

"Now that we've got these indicators, managers could use them to compare the weights and values of the fishing activity relative to what's going on in terms of benthic disturbance the activity is causing," said Mark Tasker, Chair of the group drafting the advice.

"What we found was that the highest values of landings come from a relatively low percentage of the fished area, and we've mapped where these are in the North Sea, and can relatively easily do this elsewhere. If you use only a fifth of the area fished, you're already up at around 80% of the total landings. At 60% of the area, you're very close to 100% landings. This demonstrates quite elegantly that we were to stop fishing in the smaller part, the 40%, we would only lose a small part of the overall revenue generated by the fishing activity."

Another option shows that if five or ten percent of the least fished areas had restricted access, the revenue generated by the landings would not suffer much but that there would be a substantial gain in seabed habitat state, and the functions it provides us with. These areas are shown in the advice.

The demonstration advice comes with a number of caveats, the main one being that the inshore fleet data is not included.  ​​

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​Seabed profile; photo: Silvana Birchenough

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Indicating fisheries and seabed impact trade-offs

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