Dynamics of the Barents Sea pelagic compartment: species distributions, interactions and response to climate variability
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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 observation methods are suggested, including modifications of current or development of a new pelagic trawl, use of acoustic recordings of krill and the further development of “DeepVision” and software for automated image analyses. Nevertheless, despite samplings limitations these time series gives new insight into the spatial and temporal dynamics in the pelagic compartment and improves opportunities to study key interactions in the pelagic food web.
The period from 1980 to 2015 can be broadly divided into four segments based on differences in oceanographic and biological (krill, jellyfish, 0-group fish and lumpfish abundance time series) variables with break points in 1986, 1994 and 2004. The period 1980-2015, which is the period considered here, is characterized by warming from a cold and relatively low-productive system to a warm (but variable temperature condition) and slightly more productive, to a record warm (with largest areas of warm waters) and high-productive pelagic system.
The pelagic organisms, such as zooplankton, fish egg and larvae drift with ocean currents into the Barents Sea directly and immediately respond to change in water flux, temperature and distribution of water masses. Krill, jellyfish and 0-group fish (such as cod, haddock, herring and capelin) showed interannual variations in abundance and distribution, but demonstrated a general trend over the sampling period as it increased from a cold 1980s to the record warm 2000s. These organisms were found mostly in the warm Atlantic and mixed water masses. In addition to larger occupation area, 0-group capelin showed also northwards distribution shift, which was correlated with increased temperature and areas of Atlantic and mixed waters. The northward shift may have reduced the overlap with young herring, which in turn can have had a positive effect on capelin recruitment, as herring predation on capelin larvae may significantly reduce capelin recruitment success. 0-group polar cod abundance and distribution decreased during the period. Poor recruitment in recent decades and poorer body condition during and after the spawning may have already resulted in reduced abundance in response to the warm climate. Variable recruitment of fish stocks is a major source of variability in stock development and for the dynamics of the Barents Sea ecosystem. The biomass of 0-group fish contributes directly to the pelagic biomass and the plankton-feeding component and subsequently as juveniles and adults over the following years. Among small non-commercially fish Ammodytidae, Cottidae and Myctophidae were abundant in early 1990s, while Stichaeidae, Ammodytidae and Agonidae in recent decade. However, these small fish made up a small fraction (0.02 %) of the total biomass of the pelagic community.
To investigate the large scale spatial organization and biomass fluctuations of the pelagic compartment, monitoring data for the shorter period 1993-2013, including pelagic catches (krill, jellyfish, 0-group fish and small fish) and acoustic measurements (pelagic fish stocks) were aggregated into small grid cells and larger geographic areas. The estimated total biomass of the investigated pelagic compartment, not including mesozooplankton, ranged between about 6 and 30 million tonnes wet weight with an average of 17 million tonnes over the period 1993-2013. Krill was the dominant biomass component (63%), while pelagic fish (capelin, polar cod and herring) made up 26% and 0-group fish 11% of the biomass on average. During 1993-2013, the total biomass of the pelagic compartment remained relatively stable within each of two main periods (before and after 2004), but increased by a factor of two from around 11 million tonnes in the first to around 23 million tonnes in the last period (i.e., after 2004). The pronounced increase likely reflected the warming and was driven mainly by an increase in krill, presumably due to increased advection. Variable recruitment of fish had a strong influence on the variation in pelagic biomass, first as 0-group fish (including demersal species such as cod and haddock) and subsequently over the following years manifested as strong or weak year classes of the dominant pelagic species. The biomass distribution showed a broad-scale pattern reflecting differences in distribution of the main pelagic fishes (capelin in the north, polar cod in the east, and herring in the south) and transport of krill and 0-group fish with the Atlantic water flowing into the southern Barents Sea. The highest average biomass values were found in the Southwestern and South-Central subareas (about 4 million tonnes in each), with krill as the main component. Biomass was also high in the North- Central subarea (about 3 million tonnes) where capelin was the major contributor. In conclusion, it is evident that the pelagic compartment has undergone large changes in the two last decades in going from a colder to a warmer temperature regime and from a low to a high productive pelagic compartment. The results presented support the general expectations under a climate change; increased production in the northern marine systems, and contraction and decline of arctic species while boreal species expand their distributions.