Interannual variation of fish assemblage structure in a Mediterranean river: implications of streamflow on the dominance of native or exotic species

Streams in mediterranean-type climate regions are shaped by predictable seasonal events of flooding and drying over an annual cycle, but also present a strong interannual flow variation. The Guadiana River is one of the most important rivers in the Iberian Peninsula. The fish fauna presents 11 native freshwater species, including eight with high conservation status. Several exotic species are present, the most important being the American centrarchids pumpkinseed fish and largemouth bass. As a typical mediterranean-type river, the Guadiana has an irregular hydrological regime with severe drought periods and floods; the interannual variation of discharge presents a ratio of c. 100 to 1. From 1980 to 1995 several dry years were observed, culminating in the drought of 1991/92-1994/95. Analysing the variation of the fish assemblage structure during this period, exotic species (mostly pumpkinseed) progressively increased, strongly dominating in 1995. Indigenous populations dramatically decreased and a previously common endemic cyprinid (Anaecypris hispanica Steindachner) became endangered and one of the most threatened fishes of Europe. However, the following years presented above-average flows with several flood events and an inverse process occurred, with native species increasing their contribution in a short period. The importance of floods as a disturbance factor in the control of lentic or slow flowing water exotics is discussed. Native species apparently possess adaptive responses to high flows which exotics lack. This hypothesis is consistent with probability-of-use curves and preflood-postflood surveys. Results emphasize the importance of floods in the environmental flows of mediterranean-type rivers. In the absence of flooding disturbance, exotic fish populations predictably grow, increasing the pressure on native species; abiotic disturbance may supersede deterministic outcomes of predation or competition and influence community structure by reducing populations of the exotic species.