20 Years of the German Small-Scale Bottom Trawl Survey (GSBTS): a Review
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Senckenbergiana maritima 37 (1) 13 – 82 Frankfurt am Main 30.03.2007 20 years of the German Small-Scale Bottom Trawl Survey (GSBTS): A review Siegfried Ehrich, Sara Adlerstein, Uwe Brockmann, Jens Floeter, Stefan Garthe, Hilmar Hinz, Ingrid Kröncke, Hermann Neumann, Henning Reiss, Anne F. Sell, Manfred Stein, Vanessa Stelzenmüller, Christoph Stransky, Axel Temming, Gerd Wegner & Gerd-Peter Zauke With 50 Figures, 18 Tables, and 1 Appendix Keywords: fish assemblages, predator-prey interactions, geostatistics, benthos, seabirds, nutrients, hydrography, temporal variation, sampling method, scale, survey design, IBTS, North Sea Abstract [Ehrich. S. et al. (2007): 20 years of the German Small-Scale Bottom Trawl Survey (GSBTS): A review. – Senckenbergiana maritima, 37 (1): 13 – 82, 50 Figs., 18 Tabs., 1 App., Frankfurt a. M.] The German Small-scale Bottom Trawl Survey (GSBTS) was initiated in 1987 in order to provide com- plementary investigations to the International Bottom Trawl Survey (IBTS) in the North Sea, using the same methodology but focussing high-intensity sampling on selected survey areas. Over the last 20 years, the initial number of 4 survey areas (10 ×eschweizerbartxxx 10 sng- nautical miles; “Boxes”) has been increased to 12, which are distributed over the entire North Sea. This paper describes the survey methods of the GSBTS, summarizes the scientific outcome of the first 20 years, and suggests that international fisheries research institutions would join the GSBTS. The major outcomes of the survey include to date: – Documentation changes in the distribution of fish species and in species assemblages (e.g. changes in species richness, shifts in the southern species component). – Geostatistical evaluation of GSBTS data. – Analysis of spatial scale effects: the relevance of GSBTS survey results for interpreting large-scaled abundance and distribution data from the IBTS. – Description of benthic habitats, composition of invertebrate fauna and its variability. – Process studies, especially investigation of predator-prey interactions between fish through analyses of stomach contents. – Characterization of the typical hydrographic conditions in the survey areas and their variability, and description of the nutrient supply. – Observations of seabirds and their feeding habits. Authors’ addresses: Siegfried Ehrich [corresponding author: [email protected]], Anne F. Sell, Manfred Stein, Christoph Stransky, Gerd Wegner, Federal Research Centre for Fisheries, Institute for Sea Fisheries, Palmaille 9, D-22767 Hamburg, Germany Sara Adlerstein, University of Michigan, School of Natural Resources and Environment, 3010 Dana Building, Ann Arbor, MI 48109 – 1041, USA Uwe Brockmann, University of Hamburg; Institute of Biogeochemistry and Marine Chemistry, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany Jens Floeter, Axel Temming, Institute of Hydrobiology and Fisheries Science, University Hamburg, Center for Marine and Atmosphaeric Sciences, Olbersweg 24, D-22767 Hamburg, Germany Stefan Garthe, Research & Technology Centre (FTZ), University of Kiel, Hafentörn 1, D-25761 Büsum, Germany Hilmar Hinz, Ingrid Kröncke, Henning Reiss, Hermann Neumann, Senckenberg Institute, Department of Marine Research, Südstrand 40, D-26382 Wilhelmshaven, Germany Vanessa Stelzenmüller, Institute of Marine Science (ICM-CSIC); Passeig Marítim de la Barceloneta, 37 – 49; 08003 Barcelona, Spain Gerd-Peter Zauke, Carl von Ossietzky Universität Oldenburg, Institut für Chemie und Biologie des Meeres (ICBM); Postfach 2503, D-26111 Oldenburg, Germany © E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), 2007, ISSN 0080-889X 14 – Analysis of the effects of different parameters on catch rates for bottom fish and on the estimates of abundance indices (e.g. vessel and gear effects, towing time, hydrographic conditions, time of day, number of hauls per area). In continuing this interdisciplinary survey with simultaneous sampling of all faunal and environmental compartments and especially in making it an international effort, we see the possibility of contributing data for the implementation of the ecosystems approach to fisheries management. Particularly, the follow- ing aspects can be addressed and would further increase the scientific value of the GSBTS: – Combining the survey data with highly resolved data from the commercial fishery to separate the ef- fects of fishing from natural variability. – Further interdisciplinary analyses of the entire data set. Main aspects include benthos-fish-bird-com- munity changes over time and their relation to historic fisheries impacts, and the coupling of biological and physical habitat characterisation. – Collection of accompanying data (phyto-, zoo- and ichthyoplankton data) in order to make the GS- BTS a true ecosystem survey in detecting temporal changes in nearly all major levels of the food web. Contents Introduction .................................................................................. 14 The importance of spatial scales .................................................... 66 Survey description ......................................................................... 15 Spatial scales: the relevance of GSBTS survey results Boxes ....................................................................................... 15 for interpreting large-scaled abundance and distribution Sampling methods ................................................................... 15 data from the IBTS ................................................................. 66 Statistical precision of the survey data ...................................... 18 Analysing predator-prey interactions at large and small scales .. 67 Topical studies ............................................................................... 19 Scientific value of the GSBTS and applications Hydrography ........................................................................... 19 in management.............................................................................. 75 Nutrients ................................................................................. 28 Future perspectives ........................................................................ 76 Fish fauna ................................................................................ 33 Acknowledgements ........................................................................ 77 Sediments and benthos ............................................................ 57 References ..................................................................................... 77 Seabirds ................................................................................... 61 Appendix ....................................................................................... 82 Introduction rectangle. With these highly resolved catch data, information is gained to address several specific issues: The event that triggered the “German Small-Scale Bot- 1. spatio-temporal characteristics of fish assemblages, tom Trawl Survey” (GSBTS) was a 2-ship comparative fish- 2. calculation of abundance indices of recruiting year-classes ing experiment carried out for 10 days in June 1986.eschweizerbartxxx sng- Fishing of commercially important species and comparison with hauls within a 15 × 16 nm area in the northern North Sea were those of the IBTS, carried out by both ships in parallel, when the FRV ANTON 3. small-scale variability in the distribution of age-groups of DOHRN was to be taken out of commission and replaced by commercially important fish species, the FRV WALTHER HERWIG, which took over the German 4. the generation of long-time data sets of fish assemblages demersal fish surveys in the North Sea. To compare the cap- to quantify the influence of climatic change or of anthro- ture efficiencies of the vessels using the same “GOV trawl”, pogenic impact, e.g. through fisheries, installation of wind to investigate the effect of changing from the previously used parks or establishment of Marine Protected Areas, “180-feet herring trawl” and to adapt the GOV trawl and its 5. quantification of the response of ground fish to hydro- rigging to “International Young Fish Survey” standards (later graphic conditions (e.g. water mass distribution, stratifica- the “International Bottom Trawl Survey”, IBTS), 179 hauls tion, weather events), were made by both vessels within the ten days and the rela- 6. association of ground fish species to benthic habitats and tively small area (Ehrich 1991). The observed high variability benthos communities, of catch data even for relatively evenly distributed species like 7. quantification of the effects of biological processes such as haddock and between hauls located close to each other raised food availability and predation and uncertainties about the validity of conclusions from the coarse 8. quantification of changes in fishing gear (e.g. sweep length) scale surveys like IBTS in relation to specific questions. and haul parameters (e.g. time of day, towing duration). In the following year the GSBTS was introduced with the Apart from the initial four Boxes, in order to expand the goal of providing highly resolved information on the bottom survey over the entire North Sea, further Boxes were added fish fauna of the North Sea. Initially, four survey areas of 10 and a second vessel, the FRC SOLEA has joined the survey in by 10 nautical miles each, the so-called “Boxes”, were selected. 1989. By now, both vessels jointly conduct the GSBTS and However, in the meantime the