Aims and objectives

The DEFINEIT project aims to produce the tools needed to determine the optimal economic level of exploitation of European marine fish resources. As marine ecosystems consist of many species affecting each other in complex ways, estimating the optimal exploitation level requires advanced mathematical models. These models showe the response of the ecosystem to different levels of fishing. However, there are other considerations to take into account than the maximisation of yield when managing marine ecosystems. The exploitation of the ecosystem must be kept at a level which ensures not only maximum long term yield but also that susceptible species are not affected beyond what is biologically sustainable. Further, the marine environment today, like other areas, is affected by global warming. Therefore, predicting future yields must take account of changes in the dynamics of the ecosystem caused by changes in climatic conditions.

To reach the aim, the DEFINEIT project constructs mathematical models of fish stock dynamics which explicitly take account of species interactions, climatically induced ecosystem changes and exploitation. These models are combined with economical models to predict the fishing effort require to reach the optimal yield. Geographically, the models cover a wide geographic area ranging from the Barents Sea to the Mediterranean. 

Seas of the project.

One of the first things the ecosystem modeller must take into account is the fact that fish eat fish. Therefore, increasing the amount of large fish leads to increased predation on small fish. In the DEFINEIT project, we use multispecies models to investigate changes in predation. Further, to account for the effects of changes in climate on the distribution of fish and their food, differences in distribution and the amount of alternative food are examined in detail. As distribution of juvenile fish has changed in the North Sea into areas with greater density of predators, one may expect that mortality of these fish has increased. However, in the DEFINEIT project, we have shown that the effect may be less than expected because the areas with more predators are also characterised by a higher density of alternative prey. The total amount of prey is therefore large enough to ensure that a considerable proportion of the predatory fish become saturated. In contrast, a change in the distribution of Icelandic shrimps led to an increase in mortality as predatory cod aggregated in the areas of high shrimp densities and low density of alternative food. In the Barents Sea, the DEFINEIT project has shown that the abundance of alternative food has changed considerably over time. Species interaction models have historically mainly been applied to North Atlantic areas but within the DEFINEIT project, we will for the first time develop such a model for the Aegean Sea.

To understand the effect of biological and climatic conditions on recruitment, the DEFINEIT project investigates both egg production and the survival of juveniles. Both vary considerably between areas. Projects results for herring show that the relative contribution of the four spawning grounds in the North Sea has changed completely over the past 40 years. For example, the eggs spawned at Orkney historically supplied up to 75% of all eggs spawned in the North Sea. This proportion has changed to less than 30% in later years, as the southern (Downs) herring stock has recovered. The project also showed that several populations exhibit genetic differences on both regional and local scales. The importance of early life history in the formulation of year class strength of cod, herring and plaice has been investigated and revealed that year class strength is not formulated at the same point in the life history in all species or in different areas. The project has further shown that the development in stock reproductive potential deviates significantly from that derived from the traditional estimates of Spawning Stock Biomass. The improved understanding of recruitment variability will be used in both individual stock assessment and included in species interaction models.

To assess the effect of bycatch of non-target species, the DEFINEIT project has worked towards identifying susceptible species using Ecological Risk Assessment. Sensitive species of sharks, rays and skates have been ranked according to key biological and fisheries parameters. This approach identifies the species and fisheries that should be subject to more detailed assessment to advice on precautionary management of the ecosystem. The project will proceed to determine the maximum level of fishing effort consistent with sustainment of these species.

To determine the optimal economic level of exploitation, the DEFINEIT project develops resource indicators that combine economic, social and biological indicators. The maximum resource rent is calculated based on combined economic and multispecies models. Further, a stochastic approach to economic indicators is investigated since variance and uncertainty are critical issues in relation to the economic performance of natural resource systems. A review of economic and socioeconomic indicators has been completed and will be followed by the integration of approaches from other sectors, principally agriculture and environment, where formal risk assessments impact policy and management actions. A set of indicators were evaluated within a two species predator-prey model with known optimal solutions. The preliminary results suggest that indicators can be effective as a part of adaptive management system aiming for economic efficiency.

In the final phase of the project, future stock dynamics, limits to sustainable ecosystem exploitation and the fishing levels delivering maximum sustainable economic yield under selected climatic scenarios will be analysed in unison. This will ensure the delivery of mutually consistent management advice. Further, the general properties of the ecosystems will be used to suggest rules of thumb for management in areas where the amount of data available is insufficient to construct similar models. Project results are disseminated to both the scientific community, managers, stakeholders and the general public to ensure that results are used in practice. The project scientists in total participate in more than 10 expert groups under the International Council for the Exploration of the Sea (
27 JANUARY 2022