Abstract
Ecosystem surveys are carried out annually in the Barents Sea by Russia and Norway to monitor the spatial distribution of ecosystem components and to study population dynamics. One component of the survey is mapping the upper pelagic zone using a trawl towed at several depths. However, the current technique with a single codend does not provide fine-scale spatial data needed to directly study species overlaps. An in-trawl camera system, Deep Vision, was mounted in front of the codend in order to acquire continuous images of all organisms passing. It was possible to identify and quantify of most young-of-the-year fish (e.g. Gadus morhua, Boreogadus saida and Reinhardtius hippoglossoides) and zooplankton, including Ctenophora, which are usually damaged in the codend. The system showed potential for measuring the length of small organisms and also recorded the vertical and horizontal positions where individuals were imaged. Young-of-the-year fish were difficult to identify when passing the camera at maximum range and to quantify during high densities. In addition, a large number of fish with damaged opercula were observed passing the Deep Vision camera during heaving; suggesting individuals had become entangled in meshes farther forward in the trawl. This indicates that unknown numbers of fish are probably lost in forward sections of the trawl and that the heaving procedure may influence the number of fish entering the codend, with implications for abundance indices and understanding population dynamics. This study suggests modifications to the Deep Vision and the trawl to increase our understanding of the population dynamics.
Highlights
Fishery management has shifted focus from a single-species approach towards an ecosystem approach that takes how human interventions and food web linkages affect ecosystems into account [1,2]
Current survey methods have their limitations, including a lack of spatial distribution data due to all species being collected in a single codend, an inability to identify and quantify less robust species that are destroyed by the codend [7] and the difference in the size-catchability performance of the various trawls used [8,9]
Our goal was to evaluate if the current system can be used to identify, quantify and measure small and fragile organisms, and how it can be improved to meet the goal of collecting data to increase our understanding of population dynamics
Summary
Fishery management has shifted focus from a single-species approach towards an ecosystem approach that takes how human interventions and food web linkages affect ecosystems into account [1,2]. The monitoring programmes used as a basis for fisheries management advice have been forced to adapt to meet the data needs for ecosystem-based management by measuring a wide range of ecosystem components [1,3]. The Norwegian-Russian Barents Sea Ecosystem Survey (BESS) is a comprehensive survey that gathers a wide range of measurements from the physical and biological components of the ecosystem [5]. Current survey methods have their limitations, including a lack of spatial distribution data due to all species being collected in a single codend, an inability to identify and quantify less robust species that are destroyed by the codend (e.g. comb jellyfish, Ctenophora) [7] and the difference in the size-catchability performance of the various trawls used [8,9]. It is necessary to develop new tools that can identify and quantify species that are damaged and that can sample all species and sizes at the same time
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