20 years of the German Small-Scale Bottom Trawl Survey (GSBTS) : a review

The German Small-scale Bottom Trawl Survey (GSBTS) was initiated in 1987 in order to provide complementary 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 × 10 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. — 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 following 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 effects of fishing from natural variability. — Further interdisciplinary analyses of the entire data set. Main aspects include benthos-fish-bird-community 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 GSBTS a true ecosystem survey in detecting temporal changes in nearly all major levels of the food web.