Results

DEFINEIT produced the tools necessary to determine the optimal economic level of exploitation of North European marine fish resources, combining knowledge on species interactions, recruitment processes, vulnerable species and socio-economic aspects. Marine ecosystems consist of many species (including humans) affecting each other in complex ways and estimating the optimal exploitation level requires advanced mathematical models. These models estimate the response of the ecosystem to different levels of fishing. DEFINEIT constructed mathematical models of fish stock dynamics which explicitly took account of species interactions, vulnerable species and exploitation to allow estimation of the effect of different fishing management objectives on the marine ecosystem. These models were combined with economical models to predict the fishing effort require to reach the optimal yield and the effect of pursuing this yield on socioeconomic aspects. Geographically, the models covered a wide geographic area ranging from the Baltic Sea over the North Sea to the Barents Sea and Icelandic Seas. 

Large fish, marine mammals and seabirds all eat fish. Therefore, increasing their numbers leads to increased predation on small fish. In DEFINEIT, we used multispecies models to investigate changes in predation and differences in distribution and the amount of alternative food. As the abundance of the primary food of predatory fish decreases, one might expect that growth of these predators decreases. However, in DEFINEIT, we have shown that the effect is often less than expected as predators increasingly use alternative prey sources. In the North Sea, the total amount of prey is often large enough to ensure that a considerable proportion of the predatory fish become saturated even when one prey type is only found in low densities. 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, DEFINEIT has shown that the abundance of alternative food changed considerably over time, alleviating problems with lack of the primary prey species, capelin.

To understand the effect of biological and climatic conditions on recruitment, DEFINEIT investigated both egg production and survival of larvae and juveniles. Both production and survival vary considerably between areas. DEFINEIT showed that the relative contribution of the four spawning grounds in the North Sea herring population 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 herring stock has recovered. The project also showed that several populations exhibit genetic differences on both regional and local scales and that as a result, sensitivity to fishing most likely differs between individual populations. Investigations of the importance of early life history in the formulation of year class strength of cod, herring and plaice 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 further showed that the development in stock reproductive potential deviates significantly from that derived from the traditional estimates of Spawning Stock Biomass. In spite of this, reference points based on more appropriate estimators of egg production generally did not improve stock recruitment relationships.

To assess the effect of bycatch of non-target species, DEFINEIT worked towards identifying susceptible species using Ecological Risk Assessment. Sensitive species of sharks, rays and skates were ranked according to key biological and fisheries parameters. Their distribution and sensitivity to fisheries was investigated and advice given on precautionary management and maximum level of fishing effort consistent with sustainment of these species. Using size based models, DEFINEIT has shown that in the case where no species differences in selectivity beyond those induced by size exist, the maintenance of sensitive species is likely to incur a significant cost to fisheries.

DEFINEIT developed resource indicators combining economic, social and biological indicators to determine the optimal economic level of exploitation. The Maximum Economic Yield was calculated based on combined economic and multispecies models and is an indicator of how a fishery can be exploited in an economically optimal sustainable way. Adding species interaction decreased the estimates of Maximum Economic Yield relative to predictions not including species interaction. Hence, models not including species interactions are likely to overestimate the potential economic gains of the fishery by moving to Maximum Economic Yield. Evaluating the yield given the constraint that fishing mortality on vulnerable species should stay below sustainable limits did not result in large changes in profitability when species interactions were taken into account. A stochastic approach to economic indicators showed that to maximise the total Net Present Value of the fishery, fishing levels slightly higher than current are needed. However, maximising overall economic benefits will come at a trade-off as some of the stocks are likely to end up outside biological constraints. In general, removal of larger predatory fish is likely to benefit prey species and this result was consistent across all model types used. A method was developed to derive estimated of indicator suitability from individual or groups of stakeholders and this method was used to formalise the process of elicitation of uncertainties, from both experts and stakeholders, for Norwegian and Russian Herring in the Barents Sea.

Project results were disseminated continuously during the project to both the scientific community, managers, stakeholders and the general public to ensure that results were used by the scientific community as well as in practical management. The project scientists in total participated in numerous expert groups under the International Council for the Exploration of the Sea and their work in these groups directly improved management advice for a range of stocks in the study areas. Further, DEFINEIT worked to facilitate the adoption of an Ecosystem Approach to Fisheries Management within the ICES area, in particular through the introduction of multispecies candidate reference points, through the advancement of science to assess climatic impacts on recruitment and enhance management of impacts on vulnerable species and through the development of methods to estimate socioeconomic effects of fisheries management objectives.



Final project report

The DEFINEIT project ended on the 30th of June 2012.
Download the final project report 

http://www.defineit.dk/results
23 SEPTEMBER 2017