Better mathematical model for sustainable fisheries

Sustainable and safe oceans and inland waterways

Photo: Berlinda van Dam / Hollandse Hoogte

Estimated reading time: 7 minutes

Healthy fish stocks are important both for biodiversity and for the economy. Researchers at Wageningen University & Research and their European colleagues are developing mathematical models that show which factors affect the number and size of fish. This information will help in developing a policy for sustainable fisheries.

The EU fisheries policy is complex and dynamic. That is not just because of the huge variety in fleets — from large industrial trawlers for herring in the North Sea to the small-scale anglers fishing for mullet in the Wadden Sea — but also because fish populations vary from year to year. That has consequences for the catch quotas set by the European Union. Moreover, scientific research has shown that marine ecosystems are particularly valuable for conserving biodiversity. A future-proof European fisheries policy that finds the right balance between nature conservation and the economy is crucial for healthy fish stocks. Now, 25 partners in ten countries have joined forces to develop the building blocks for such a future-proof fisheries policy in the European research project PANDORA. The project is headed by the National Institute of Aquatic Resources in Denmark, a highly reputable aquatic research institute.

Efficient catches

At Wageningen University & Research (WUR), Tobias van Kooten is taking part in the project. The senior researcher, who works for Wageningen Marine Research, is a theoretical ecologist. This means he develops mathematical models and uses computer simulations to study ecological problems. According to Van Kooten, PANDORA is all about building up knowledge to improve fisheries management so that fish can continue to be harvested without this negatively affecting sustainability.

If many fish have to share the available food, there will be less food available for each individual fish

Fish stocks are recovering after a period of overfishing, although now in certain stocks individual fish grow more slowly. Photo: Shutterstock

The specific WUR know-how that Van Kooten contributes in PANDORA is the theoretical knowledge of how individual fish grow and how that affects the population dynamics and the amount that can be sustainably harvested. Van Kooten: “Our task within PANDORA concerns density-dependent growth. That is the principle that if many fish have to share the available food, there will be less food available for each individual fish. As a consequence, all the fish in the population grow more slowly.”

Fish grow more slowly

The current models, which were developed under the auspices of the International Council for the Exploration of the Sea (ICES) and which are used to determine the catch quotas, are based on the assumption that there is always more than enough food for each fish and so all fish always grow as fast as is physiologically possible. Van Kooten says this assumption no longer holds. In PANDORA, he focuses on the plaice population in the North Sea.

What is the PANDORA project all about? In this video project coordinator Stefan Neuenfeldt explains the goals of PANDORA and why cooperation with stakeholders like the fishing industry is so important to improve stock assessment models and methods.

“Previous measurements had shown that fish stocks were recovering after a period of overfishing but individual fish were now growing more slowly. A six-year-old plaice these days is no bigger than a three-year-old plaice 20 years ago. The fish need more time — and more food — to reach a size that makes them appealing for the fishing industry. At present, we see a lot of slow-growing plaice in the North Sea. That can have consequences for fish competing for food with plaice, such as dab and turbot. They will also grow more slowly. The affected species are not necessarily economically important fish but they may well be species that have particular value for nature.” Van Kooten is working with fellow theoretical ecologists André de Roos and Jasper Croll at the University of Amsterdam (UvA) on new mathematical models that can predict how many fish of a given species and size can be found in the sea at a given moment. That will give a more realistic picture of how healthy fish stocks are.


Van Kooten and his colleagues at the UvA use information from annual sampling exercises — on board fishing boats and at fish auctions — which provide data on the size and age of the fish. That information is used to improve calculations of how many fish can be caught without it adversely affecting the ecosystem. In addition to the samples, the researchers also use fishing-boat logbooks that record what was caught, in what quantities and where.

In the end, the question is whether we want more fish in the sea or bigger fish

Fishing policies are currently focused on selective fishing, but a new model could aim to take out small fish in order to allow bigger fish to grow better. Photo: Shutterstock

Using all the available data, the researchers have developed specific theoretical knowledge about the relationship between the growth of the fish and the population, and how that relationship affects the availability of food. “Eventually, we will develop a software application within PANDORA where you can input data from samples and calculate what the availability of food is for a given time point and a given species of fish.” Van Kooten stresses that while the model Wageningen and Amsterdam are developing may be based on data from plaice samples, the tool will be applicable for any fish species and can be used in due course for fisheries in all European waters.

Smart harvesting

How can the PANDORA results be used to help determine the new fisheries policy? “At the moment, plaice are only allowed to be caught if they are above a minimum size. The fisheries policy is currently geared to making fishing more selective, for example by modifying the nets so that little fish can swim through the net meshes. The fishing industry has invested a lot in this in recent years. But in a situation where small plaice are growing more slowly due to a lack of food, it might be better to remove some of the smaller fish so that the remaining plaice can grow faster. It is just like a garden where the lettuces are too close together. Eventually you have to thin them out so that the rest can grow more. It’s as simple as that.” Van Kooten realises that abandoning the principle of selectivity, in which only plaice above a certain size can be caught, is a question for Europe’s politicians. Incidentally, it is not the intention that Van Kooten’s model, which he is now refining, should replace the current model for fish stock management used by ICES. However, the knowledge that has been acquired can be incorporated into the ICES models. Whether that is done is up to ICES. “Our research has not yet reached the point where we have an operational stock assessment model. We would eventually be able to use our findings to advise on abandoning the selectivity policy. In the end, the question is whether we want more fish in the sea or bigger fish.”

European research context

Paradigm for Novel Dynamic Oceanic Resource Assessments (PANDORA) addresses the following European policy challenge: Sustainable fisheries management based on conservation of the marine ecosystem Wageningen University & Research group involved: Wageningen Marine Research Countries involved in Europe: Denmark, France, Germany, Greece, Italy, the Netherlands, Norway, Spain and United Kingdom

Duration 2019 – 2022

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