Composition and spatial organization of fish assemblages in the lower Guadiana basin, southern Iberia

A survey of fish assemblages (composition and spatial organization) was made in the lower Guadiana basin (southern Iberia) during the summer of 1994. A total of 6937 specimens belonging to Is species were collected in 44 channel units and 30 sites (i.e. a series of channel units), comprising all the freshwater species previously recorded in the area except the exotic Esox lucius and the endemic endangered cyprinid Anaecypris hispanica. The endemic cyprinids Chondrostoma lemmingii and Barbus sclateri were rarely encountered, whereas the exotic Lepomis gibbosus was the predominant species found. The lower Guadiana fish community changed in space from assemblages characterized by the high relative abundances of Leuciscus pyrenaicus, smaller size-classes of Barbus spp. and Tropidophoxinellus alburnoides (all Iberian endemisms), to assemblages characterized by the high relative abundances of L. gibbosus, Micropterus salmoides, Cichlasoma facetum (all exotic species) and the larger size-classes of Barbus comiza and Barbus microcephalus. The former assemblages used smaller (i.e., narrow and shallow) sites and channel units, usually in tributaries and closer to headwaters, with smaller L. pyrenaicus being dominant in sites located in the tributaries discharging to the brackish Guadiana. This group was also related to channel units rich in emergent macropthytes, The latter assemblages were related to the largest sites and channel units, typically found in the main river or in tributary sites far from the source. Besides these two groups, some species were associated to noticeable environmental conditions; Blennius fluviatilis to channel units with current velocity and heterogeneous substrate, typically found in the main river, and Cobitis paludica to channel units with heterogeneous substrate.