ON FACTORS STRUCTURING THE FLATFISH ASSEMBLAGE IN THE SOUTHERN NORTH-SEA

Ten species of flatfish were studied to see to what extent interspecif ic competition influences their diet or spatial distribution and wheth er the potential of these flatfish species to avoid interspecific comp etition through resource partitioning is constrained by specific morph ological characteristics. For this, seven morphological characteristic s were measured, diet composition was determined from gut content anal yses and overlap in distribution was determined from the co-occurrence in trawl hauls. Canonical correspondence analysis revealed the morpho logical characteristics that were most strongly correlated with the di et composition. Based on these findings the mouth gape was considered to be the most important morphological constraint affecting the choice of food. Two resource dimensions were distinguished along which inter specific competition can act on the flatfish assemblage: the trophic d imension (diet composition) and the spatial dimension (distribution). Resource partitioning was observed along both dimensions separately an d, more importantly, the degree of resource partitioning along thr two dimensions was negatively correlated. Especially thp latter was consi dered strong circumstantial evidence that interspecific competition is a major factor structuring the flatfish assemblage. Resource partitio ning along the two resource dimensions increased with decreasing mouth gape, suggesting that interspecific competition mainly acts on the sm all-mouthed fish, i.e. juveniles. (C) 1998 Elsevier Science B.V. All r ights reserved.