INTRODUCTION Disturbance is thought to play an important role in the structuring of communities of stream fishes (Karr & Freemark 1985; Resh et al. 1988; Poff & Ward 1990) and fish communities exposed to disturbance caused by floods and drought have to persist in environments that are characterized by fluctuating flows (Meffe & Minckley 1987). The debate regarding the stability of stream fish assemblages is not new (Matthews 1998). In many types of water, fish populations appear to fluc- tuate in abundance and species composition from year to year (Starret 1951). Fish assemblages at a given site often change seasonally or monthly, or over short spans of time associated with floods and drought (Matthews 1998). The perceived stability of stream fish assem- blages can relate to the taxa studied, harshness of the environment, magnitude of disturbances, location within a watershed or the spatial limits set for study by the investigator (Grossman et al.; Harrel 1978; Moyle & Vondracek 1985; Ross et al. 1985; Matthews 1986). Stability is the relative constancy of the abundance of the species over a period of time despite disturbance, and may result from: (i) resistance, when relative species abundance is unchanged despite potentially dis- ruptive forces; or (ii) resilience, the rapid return to a former state following a disturbance (Meffe & Minckley 1987). Brazilian semiarid streams are characterized by extremes of flooding and drought (Maltchik 1996a,b), but the effects of these disturbance agents on stream structure and functioning (including that of fish com- munities) are poorly understood (Medeiros & Maltchik 1997, 1998; Pedro & Maltchik 1998). The present study aims to analyse the effects of hydrological distur- bances (flood and drought) on the stability and diver- sity of an intermittent stream fish assemblage of the Brazilian semiarid region over one hydrological cycle. We hypothesize that hydrological disturbance, in particular floods and droughts, will influence the Austral Ecology (2001) 26, 156–164 Fish assemblage stability in an intermittently flowing stream from the Brazilian semiarid region ELVIO S. F. MEDEIROS 1 AND LEONARDO MALTCHIK 2 * 1 UniversidadeFederal da Paraiba, Departamento Sistemática e Ecologia, João Pessoa, Paraiba and 2 Universidade do Vale do Rios dos Sinos (UNISINOS), Centro II, Avenida Unisinos, 950- Cep. 93022-000, São Leopoldo, Rio Grande do Sul, Brazil (Email: maltchik@cirrus.unisinos.br) Abstract Flooding and drought are important hydrological disturbances occurring in intermittent streams located in semiarid regions. The present study aims to analyse the effects of flood and drought on the diversity and stability of an intermittent stream fish community from the Brazilian semiarid region over the 1996 hydrological cycle. Fourteen fish collections were conducted during the rewetting, wet and drying phases, and 808 individuals were collected. Diversity was measured by using Simpson’s Index (S) and community stability was analysed by using the variation in the abundance, then tested by applying Kendall’s (W) test of concordance over the relative abundance rankings. This test supplies information about the stability of communities by means of the concordance of rankings. A community that is stable or near equilibrium has more constant rankings of its constituent species compared with a fluctuating one. The diversity increased progressively during the rewetting phase (S = 0.480–0.693; coefficient of variation = 0.131), reached its highest values during the wet phase (S = 0.745–0.861; coefficient of variation = 0.072), and remained high during the drying phase, with a lower oscillation than in the other phases (S = 0.699–0.801; coefficient of variation = 0.050). The abundance per collection varied significantly (Kruskal– Wallis 6.94, P = 0.031) throughout the hydrological phases. The stability of the community was higher during the rewetting (W = 0.58, P < 0.001) and drying (W = 0.86, P < 0.001) phases than during the wet phase (W = 0.45, P = 0.1818). In the Brazilian semiarid region, flooding disrupted the stability of the fish community studied and increased the diversity, whereas the absence of floods (expressed by the drying phase) caused a higher stability of the fish community and led to less variation in diversity. Key words: Brazil, disturbance, diversity, drought, fish, flood, intermittent stream, semiarid, stability. *Corresponding author. Accepted for publication September 2000.