Expertise
Problems in mixed fisheries
Alexander Kempf and Gerd Kraus | 22.11.2022
When fishing for pelagics with midwater trawls, in most cases only one species is caught at the same time. The situation is different in bottom trawl fisheries where the catch consists of a mix of species. If the stock status differs among these species, fisheries management becomes challenging.
Mixed fisheries are characterised by catching different species in the same haul. Typical examples are bottom trawl fisheries in the North Sea (more information can be found in Fisheries Overviews of the International Council for the Exploration of the Sea (ICES)). For example, many other species like cod, plaice or whiting can be found in the catch when fishing for haddock or sole. And this leads to problems.
Species catched together can have a different stock status (more information under ICES advice on fishing opportunities: What's behind it?). The North Sea haddock stock is currently in good condition. In contrast. the North Sea cod stock is outside safe biological limits and has to be protected via low total allowable catches (TACs) and remedial measures. Since 2015 a landing obligation has been introduced in the EU and UK (more details under Discards in marine fisheries). This means apart from exemptions species under TAC management are no longer allowed to be discarded. When the first quota is exhausted (in this example the quota for North Sea cod), the whole fishery has to be stopped even if enough quotas for other species are left. The species whose quota is exhausted first, is called „choke species“, because it chokes the whole fishery.
What „Choke species“ mean for fisheries
One may think that problems in mixed fisheries can be solved easily by more selective fishing gears. However, often two species show a similar catchability. This means their body length is similar, they have a similar body shape and show a similar escape behaviour in front of the fishig gear making it dificult to separate them. Plaice and sole are good examples for this and a lot of undersized plaice is bycaught when fishing for sole. Also if the species to be protected is larger than the other target species, the choice of the optimal mesh size becomes difficult. If haddock and whiting were bigger than cod, one could catch the first two with larger mesh sizes and let cod escape. Unfortunately, exactly the opposite is the case and relatively smaller mesh sizes are needed to catch haddock and whiting increasing the risk for bycatch of juvenile cod.
If quotas for cod are exhausted and the fishery has to be stopped, severe economic losses are the consequence. The seduction is large to discard cod in order to avoid that fisheries have to stop. This is illegal, however, raised discards from international observer data point towards such a behaviour (e.g. Cod (Gadus morhua) in Subarea 4, Division 7.d, and Subdivision 20). How can this dilemma be solved?
Methods of resolution
The Thünen Institute is working on the development of managment strategies and technical measures to tackle the problems in mixed fisheries. For example, in some cases inter-annual flexibilities for quotas may help to ease choke effects. It is also possible to set fishing opportunities for choke species as an exception above the maximum sustainable yield level, but still within a sustainable range. We have tested this approach together with our European colleagues in the EU project MYFISH and in ICES (more details in the Report of the Joint ICES-MYFISH Workshop). However, fishing above the maximum sustainable yield level must remain an exeption and is only acceptable if all relevant stocks are in good condition. In our example, it is not an option because North Sea cod is in a critical situation. In such cases only technical measures can help to reduce the unwanted bycatch of choke species.
Innovative gears showing a higher selectivity belong to such technical measures. Using swimming and escape behaviour to separate species already during the fishing operation is an important topic for the gear technology group of the Thünen insitute (more details under Reduction of unwanted bycatch). However, when increasing the selectivity of fishing gears, the catchability of other target species has to be taken into account: If catch rates for target species decrease too much, simulations within the EU-Project Probyfish demonstrated that fisheries become ineffective and need in the final end more fishing effort to catch available quotas. This can lead to unwanted side-effects like more impact on bottom habitats or more bycatch of sensitive species.
Another solution to reduce choke effects can be the closure of fishing grounds (permanent, seasonal or real time in areas with high bycatch rates). Spatial managment measures have become more attractive in recent years also to reach more wide-ranging goals like the preservation of biodiversity. In order to reach expected effects, closed areas have to be of the right size and at the right place. Also effort redistribution and food web effects have to be taken into account to get a complete picture of consequences when closing areas. This could be demonstrated based on simulations with the ecosystem model Ecospace within a Dissertation supervised by the Thünen Insitute and the University of Hamburg (Evaluating spatial processes and management strategies under changing anthropogenic influences on the ecosystem of the southern part of the North Sea).
Outlook
The situation in mixed fisheries will be further influenced by climate change. One aspect is that the productivity of stocks is changing. The productivity of North Sea cod is forecasted to decrease further. This will likely worsen the choke situation in North Sea fisheries as shown by our work conducted within the EU-Project Pandora.
A further effect of climate change is that species shift their area of distribution. As cold-water adapted species retreat northward, species from warmer areas move in (more details under Climate change: some come, some go). This may lead to problems when distributing fishing opportunities among countries. The current system of relative stability ensures that each country gets a predefined share of each total allowable catch (TAC). However, this rigid system is not prepared for large scale changes in spatial distribution patterns of fish stocks and associated changes in catch compositions. Countries are able to exchange quotas, however, it is likely that the current distribution of quotas among countries will come under pressure with further progressing climate change.
Therefore, mixed fisheries will stay on the agenda of the Thuenen-Institute. A good cooperation between science, politics and industry is needed to find viable and sustainable solutions for harvesting stocks within mixed fisheries.
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