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Groundfish survey data from the German Bight from 1902–08, 1919–23, and 1930–1932 and ICES International Bottom Trawl Survey (IBTS) quarter 3 data from 1991 to 2009 were analysed with respect to species frequencies, maximum length, trends in catch-per-unit-effort, species richness parameters (SNR) and presence of large fish (Φ 40 ), the latter defined as average presence of species per haul with specimens larger than 40 cm given. Four different periods are distinguished: (a) before 1914 with medium commercial CPUE and low landings, Φ 40 ≈ 2, high abundance in elasmobranchs and SNR conditions indicating highly diverse assemblages, (b) conditions immediately after 1918 with higher commercial CPUE, recovering landings, Φ 40 at > 4 in 1919, and SNR conditions indicating highly diverse assemblages, (c) conditions from 1920 to the early 1930's with decreasing commercial CPUE, increased landings, decreasing Φ 40 , SNR conditions similar to later years indicating less diverse assemblages, and a decrease in elasmobranchs. In the IBTS series (d), Φ 40 remains low indicating an increased rarity of large specimens, and SNR characteristics are similar to the third period. Dab, whiting and grey gurnard have increased considerably in the IBTS series as compared to the historic data. Φ 40 is suggested an alternative indicator reflecting community functional diversity when weight based indicators cannot be applied.

The interdependence between groundfish assemblages and habitat properties was investigated on the Dogger Bank in the North Sea. Abiotic habitat parameters considered included topography, hydrographic conditions, sediment composition, and the biotic habitat variable the prevailing benthic invertebrates. Distinct epi- and infauna communities occurred at different locations on the Dogger Bank. Fish assemblages were clearly linked to both the biotic and abiotic habitat characteristics. Overall, fish and benthic communities revealed similar spatial distribution, represented in the respective clusters of characteristic and abundant species. Distribution patterns corresponded with the prevailing abiotic conditions such as depth and sediment composition, which appear to relate to autecological preferences of individual species. The apparently most generalist species, grey gurnard ( Eutrigla gurnardus ) and dab ( Limanda limanda ) occurred at all stations and dominated in terms of biomass in most cases. The absolute numbers of grey gurnards were related to the abundance of suitable prey, invertebrate and fish species, which stomach analyses revealed as part of the diet in an independent study during the same research cruise. Haddock ( Melanogrammus aeglefinus ) and whiting ( Merlangius merlangus ) were only abundant at deep stations along the flanks of the bank. The occurrence of lemon sole ( Microstomus kitt ), American plaice ( Hippoglossoides platessoides ) and cod ( Gadus morhua ) was also positively correlated with depth, whereas especially lesser weever ( Echiichthys vipera ), sandeel species and solenette ( Buglossidium luteum ) occurred predominantly at the shallower sites. At the same time, individual fish species such as solenette and lesser weever were associated with high densities of selected epi- or infauna species.