Therc exist strong differences in size frequency distribution, fecding habits and in habitat use between the populations of "Blennius fluviatilis" and "Micropterus salmoides" coexisting in a small reservoir in NE Spain. Blcnnies are short-sizcd than largemouth bass, its
diet is based upon benthic invertebrates, mainly chironomids larvae and inayfly nimphs, and use a habitat without vegetation, madc of
large bedrocks and situated in a precise point in the study site. Largemouth bass, on the contrary, use mainly the rcservoir vegetated
edges -from which exclude the rest of the fish spccics-, feeds on water coluinn invertebrates (chironomids pupae and zigopterans ninfae)
and fishes and are much larger than blennies. These facts, togethcr with a similar behavior of blennies in a ncar site, which lacks
largemouth bass (Matarrafia river), make us to suppose the Iack of strong interactions between both species, in the sense that one of
them were displaced from his optimun habitat. Nonethcless, the presence oí' young largcmouth bass in blennies habitat rnay be the cause
of an inversion in the fceding rriicrohabitat of thc last specics.
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