Genetica van mariene landschappen ter ondersteuning van een duurzaam visserijbeheer van platvis

In 1883, Thomas Huxley put forward that all the great sea fisheries are inexhaustible. This illustrates the classical notion that marine environments tend to be demographically ‘open’ without any chance of complete extinction. Such belief was based on the wide distribution of many marine species with extensive larval and adult dispersal, and large population sizes, even after heavy exploitation. However, the classical view of inexhaustible fisheries has proven incorrect after major declines in many world fisheries the last decades, prompting for a better understanding of fishery stock demography and connectivity using multidisciplinary approaches. Fisheries research has gained from the integration of genetic studies showing the various mechanisms explaining the evolution of population structure in the ocean. The resilience of fish populations and the maintenance of genetic diversity has important implications for the viability and stability of entire ecosystems under heavy exploitation.In this thesis, I aim at integrating novel knowledge on the genetic composition of flatfish stocks with fisheries management tools. Therefore, the genetic structure was investigated of the flatfish turbot and brill, in the Northeast Atlantic Ocean. A combination of conventional population genetic analyses and multivariate statistics was used to assess how environmental factors influence the genetic variation at various spatio-temporal scales. The focus was on several flatfish species with a synchronic sampling design. The seascape genetic approach made it possible to identify the interaction between oceanography and species-specific traits, and their effect on the true dispersal of the species. In the case of turbot, the combination of anonymous and gene-based nuclear markers proved useful for elucidating potential population units as each shows a different level of differentiation. This approach is highly valuable from a conservation perspective, as it allows one to infer the effect of ‘community-wide’ fragmentation. Common geographic patterns were identified in the genetic structure. However, differences in sensitivity to gene flow barriers were reported. This discrepancy was driven by species-specific traits, particularly reproductive behavior. Within the Northeast Atlantic Ocean, I could distinguish two large groups: one comprising the Skagerrak-Kattegat area and a second along the Irish shelf. For turbot, another population might even be present in the Baltic Sea. Seascape genetic analyses based on putatively selective markers suggest an additional gene flow barrier within the North Sea, linked to the Friesian frontal zone. For a species like turbot, with a wider distribution range in the North Sea than flatfish such as sole and brill, the identification of this barrier has an important implication for the delineation of management units. At present, the incorporation of genetic data into statistical fisheries models has been limited. This is mainly because genetic research draws inferences about populations on an evolutionary time scale, while fisheries management is more interested in short-term demographic independence. Fisheries models usually lead management instruments towards short-term gains. But the long-term vision of sustainable fisheries management also needs to take evolutionary consequences into account. Our findings suggest that turbot could serve as a so-called ‘umbrella species’, i.e. a vulnerable species whose environmental requirements encapsulate the needs of most other species. It represents a foundation for appropriately managing “seascapes” in order to preserve diversity. Using turbot for that purpose has two advantages: 1) the impact of environmentally driven connectivity is most straightforward in turbot, and 2) turbot seems most vulnerable to the negative effects of fisheries and can therefore serve as the proverbial ‘canary in the coal mine’ – an indicator of declining environmental quality. This implies that policymakers will have to make trade-offs to serve all marine stakeholders in the best possible way without jeopardising the long-term gains. Scientists can help to address these trade-offs. They can meet the needs of the stakeholders by communicating scientific results on all administrative levels as well as informing policy-makers of these results and the impact they may have on the ecosystem. Scientific research results are normally communicated via peer-reviewed publications and participation in conferences. At the same time, scientists must communicate through their national fisheries institutes, because these institutes play a key role in communicating between politicians, stakeholders and scientists. Such a communication strategy would not only transfer knowledge but will also help to tackle the problems related to a mismatch between the knowledge produced and the knowledge that policymakers actually need. Finally, it has become clear in recent years that unexpected management outcomes stem from human behavior related to uncertainty. The ecosystem is not yet sufficiently understood, and it shall never be completely understood because of its inherent variability and associated uncertainties. To address and reduce uncertainty, managers will have to incorporate information about resource user behavior provided by the social sciences. Such information depends largely on improved interdisciplinary communication among scientists. Often uncertainty in scientific results is reduced via the peer review process for high-level journals, usually restricted to a specific field. By expanding research to involve other disciplines and increasing efforts to improve communication between scientists and other actors, more and better knowledge will be produced and uncertainty will be further reduced. The ICES structure and workflow play a crucial role in creating an atmosphere for trans- and multi-disciplinary research. Working in a transnational context to stimulate interdisciplinary research and communication will encourage rapid and sure progress towards improved sustainability of sea fisheries. <w:latentstyles deflockedstate="false" defunhidewhenused="true"