This study investigates the habitat requirements and distribution patterns of four commercially exploited flatfishes: Pleuronectes platessa (plaice), Solea solea (sole), Microstomus kitt (lemon sole) and Limanda limanda (dab). The main goal of the study was to promote species specific knowledge of habitat requirements of the adult life stage of these four flatfish species, which will be a prerequisite for the identification process of 'Essential Fish Habitats' and the integration of those into a more ecosystem based approach to fisheries management.
The first section introduces a new methodology that objectively identify sites which consistently harboured high densities of fish over considerable periods of time from large datasets such as groundfish surveys. Following assumptions of habitat selection theory these habitats should display consistent favourable conditions while other habitats with variable or low densities of flatfishes should exhibit less favourable conditions. This method was applied to groundfish survey data of roundfishes in the Irish Sea (cod, haddock and whiting) and to flatfishes in the English Channel (plaice, sole and lemon sole).
The habitat conditions of the high consistent abundance sites of the three flatfishes in the English Channel were compared to the conditions encountered at stations of variable and low flatfish abundances and with sites where no flatfishes were caught by the groundfish survey. While consistent high density plaice and sole sites showed similar habitat conditions typical of near shore environments, lemon sole was predominantly found over areas with deeper offshore conditions. Sediment associations also differed between species with plaice occurring over sand while sole was found over sediment ranging from muddy to sandy substrates. Lemon sole was found at sites with slightly higher gravel content than plaice and sole. No clear relationship was found between flatfish abundance emergent epifauna and prey availability. However, the absence of a relationship between prey availability and flatfish abundances was thought to be related to an inadequate sampling strategy.
 An underwater video survey was conducted to explore the habitat characteristics of selected consistently high abundance sites of adult plaice in more detail. The sampling technique enabled exploration of habitat characteristics such as substratum morphology and heterogeneity as well as the presence of benthic macrofauna. No obvious relationships were found between substratum characteristics and plaice densities determined at these sites.
Clear relationships were only observed with the presence of benthic macrofauna.
 In a small scale research area in the German Bight the seasonal and interannual variability of an epifauna community was investigated to determine temporal and spatial stability of benthos important as potential flatfish prey. Fluctuations in abundance and biomass were linked to the migratory behaviour of mobile species in an out of the research area. While seasonal trends were related to temperature fluctuations, interannual changes were thought to be caused by differences in food supply to the benthic community created by changes in front formation.
The influence of changes in the benthos, in respect to prey availability, on the distribution, body condition and resource acquisition of dab was investigated. Dab abundance and prey availability followed similar decreasing trend during consecutive summer periods however during winter a steep increase in dab abundance was apparent while prey availability and body condition continued to decrease. This increase in dab abundance was linked to migratory behaviour of dab due the cold conditions in the shallow areas of the German Bight and due to spawning behaviour. The prey choice of dab was significantly influenced by the abundance of prey organisms in the environment, while the position in the environment, mobility and palatability seemed to be less important.
The main findings of these studies were discussed within a common framework and recommendations for future research were given.