RELATIONSHIPS BETWEEN MESOHABITATS, ICHTHYOLOGICAL COMMUNITIES AND IBI METRICS ADAPTED TO A EUROPEAN RIVER BASIN (THE MEUSE, BELGIUM)

We investigated the influence of mesohabitats on fish communities and on attributes of a multimetric index of fish integrity in the River Me use basin. Three consecutive 150 m sectors in a Meuse tributary (Ourth e, Belgium), each divided in two or three sub-sections presenting vari ous percentages of mesohabitats (riffles, runs and pools), were sample d by electrofishing. In each sub-section, relative and absolute biomas ses of each fish species were estimated. The presence of limnophilic c yprinids was inversely correlated (r(2) = 0.70 and 0.56 for absolute a nd relative biomass, respectively) with the percentage of riffles. Sal monids preferred runs and their absolute biomass was highly dependent (r(2) = 0.71) on the proportion of this mesohabitat, whereas biomass o f limnophilic cyprinids was highly correlated (r(2) = 0.75 and r(2) = 0.82 for absolute and relative biomass, respectively) with pools. A po sitive correlation (r(2) = 0.58) was established between relative biom ass of predators and the percentage of this mesohabitat. An IBI was ca lculated for the three sectors on the basis of results from the entire Meuse catchment. Scores of most metrics showed low variation among se ctors but values of two metrics (% of individuals as tolerant, % of in dividuals as ubiquitous spawners) were greatest in sector 2, where poo ls predominated. Further, the lowest IBI score (51/65, integrity class : fair to good) was recorded in sector 2 where pools dominated, while sector I (where runs dominated) obtained the highest score (63/65, int egrity class: excellent). Sector 3 which has a balanced proportion of riffles and runs obtained an intermediate score (57/65). Considering t he response of IBI to the natural variation of mesohabitat proportions , it appears that an accurate sampling requires the prospection of a v ariety of mesohabitats (with a majority of runs) for the evaluation of river quality.