| Summary: | Long, consistent and uninterrupted monitoring resulting in time series of biological and environmental data are needed to understand the relations between environment and species, and species interactions, which also affect fish stock production and thereby fisheries (Dragesund et al. 2008). Traditional fishery science in the Barents Sea has mainly focused on the commercially important species (e.g. cod, haddock, capelin and herring). However, the conducted surveys provide a substantial amount of data on additional species, and thereby the opportunity to study a wider range of species, species interactions, community structure and ecosystem processes required for an ecosystem-based management (Michalsen et al. 2013, ICES 2016). This aspect has been the focus of this thesis, which aim is to 1) evaluate monitoring data for use in ecosystem studies, 2) understand past and current changes in the pelagic compartment of the Barents Sea ecosystem, and 3) determine the effects of the recent warming on the pelagic compartment and its components. Since 1965 the international 0-group fish surveys and since 2004 joint Barents Sea ecosystem surveys have provided an early estimate of year class strength and huge amounts of additional data. The thesis is based on pelagic catch data and information from these autumn surveys reported in a series of nine papers. The monitoring data were quality checked and only pelagic trawl station of satisfactory quality were used to establish time series of 0-group fish abundance (9 species) and biomass (4 species), biomass and abundance of juveniles and adults lumpfish, biomass of krill and jellyfish and spatially resolved biomass time series of pelagic compartment. Through this work, the databases have been updated and now data are available for the scientific community. An evaluation of sampling equipment and the observation methods indicated limitations of past and current monitoring that may have lead to increased variance and biases. Further development of survey equipment and ...
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