Microhabitat use in a mediterranean riverine fish assemblage : Fishes of the upper Matarraña

We examined microhabitat use in Barbus graellsii, Barbus haasi, Chondrostoma toxostoma, Rutilus arcasii, and Salmo gairdneri over a 19 month period in the upper Rio Matarraña, Spain. B. graellsii and Ch. toxostoma exhibited non-random microhabitat use during all seasons and preferentially occupied deep microhabitats with heterogeneous substrates. During the majority of seasons in which they were present, B. haasi and R. arcasii occurred in microhabitats similar to those occupied by B. graellsii and Ch. toxostoma. S. gairdneri was over-represented in high velocity microhabitats with erosional substrates. We did not observe any evidence of interspecific interference competition or avoidance. Substrate composition did not appear to affect microhabitat use outside of its covariation with depth and velocity. Seaonal variation in microhabitat use by B. graellsii, B. haasi and Ch. toxostoma was strongly correlated with seasonal changes in microhabitat availability. S. gairdneri, however, occurred closer to the substrate when average velocities were high. Larger B. graellsii and B. haasi sometimes occupied deeper, higher velocity microhabitats than did smaller specimens. Larger B. graellsii also occasionally occurred farther from shelter than did smaller specimens; the reverse was true for B. haasi. Larger Ch. toxostoma sometimes were found farther from both the substrate and shelter than smaller individuals, whereas smaller specimens occasionally inhabited deeper areas with more depositional substrates than did larger Ch. toxostoma. During Late Summer 1985, smaller Ch. toxostoma also occupied microhabitats with higher velocities than did larger specimens. A comparison of microhabitat use for two species present in both upper and lower portions of the Matarraña indicated that most differences in microhabitat use could be attributed to inter-site differences in microhabitat availability. The data suggest, hovever, that both species shifted to more protected microhabitats in the higher velocity site. Assemblage members generally occupied statistically distinguishable microhabitats and could be classified as: 1) high-velocity upper water column (S. gairdneri), 2) low velocity lower water column (B. graellsii, Ch. toxostoma and R. arcasii), and 3) shelter-oriented benthic (B. haasi). The introduction of S. gairdneri during Winter 1984 did not produce microhabitat shifts in any of the native species. Whether or not the native species affected microhabitat use in S. gairdneri is unknown. Interspecific competition for space, however, did not appear to strongly influence microhabitat use among the native species.