Microhabitat use by fish in the lower Rio Matarraña, Spain, 1984–1987

We quantified microhabitat use in Barbus graellsii, Cyprinus carpio, Chrondostoma toxostoma, Esox lucius, Gobio gobio and Leuciscus cephalus during 1984–1987 in the lower Rio Matarraña, Spain. Fluctuations in numerical abundances of assemblage members increased during 1986–1987. These increased fluctuations were due to decreases in the abundances of Ch. toxostoma, E. lucius and G. gobio during the latter 2 years of our study. Only C. carpio increased slightly in abundance during this period. An analysis of microhabitat availability data indicated that 16 seasonal samples could be assigned to 1 of 6 groups ranging from: seasons with zero velocities, shallower depths and an algal/debris substratum to those with high velocities and depths and a substratum composed of algae/debris, rubble and gravel. Most seasonal changes in the substratum were attributable to conditions inhospitable to the growth of benthic algae (i. e., high velocities or low oxygen levels) rather than to actual changes in the underlying lithospheric substratum. Principle component analyses indicated that most species were overrepresented in deeper microhabitats with depositional substrata. Ch. toxostoma, however, tended to occur over rubble substrata in both shallow and deep microhabitats. Most seasonal changes in microhabitat use were produced by seasonal variations in microhabitat availability. However, all species except E. lucius exhibited seasonal variations in microhabitat use that typically involved velocity, depth and substratum composition. Although smaller specimens of most species were found closer to the substratum in 1984–1985, they tended to occupy shallow areas in 1986–1987. Canonical analyses of discriminants and univariate data demonstrated that the distance from the substratum was the best predictor of interspecific differences in microhabitat use. Species could be arrayed along a vertical gradient with L. cephalus and Ch. toxostoma occupying mid-water column positions, B. graellsii and C. carpio occurring in lower-water column microhabitats and E. lucius and G. gobio strongly associated with the substratum. Changes in interspecific microhabitat use were not correlated with changes in species abundances, and hence, interspecific competition did not appear to strongly influence microhabitat use during our study.