Migration patterns counteract seasonal isolation of <Emphasis Type="Italic">Squalius torgalensis</Emphasis>, a critically endangered freshwater fish inhabiting a typical Circum-Mediterranean small drainage

Freshwater species with small distribution ranges are vulnerable to extinction, especially when they exhibit small effective population sizes. Squalius torgalensis, Cyprinidae, is an endemic critically endangered fish species from southwestern Portugal, confined to the intermittent streams of the Mira drainage. Assessment of genetic diversity and population structure levels was conducted through the analysis of both mitochondrial (cytochrome b) and nine nuclear microsatellite markers. Remarkable low levels of genetic diversity for both markers were observed (He < 0.38, π < 0.001). These results may be related with the restricted distribution range and dispersion patterns of the species together with demographic fluctuations associated with the intermittent features of the river system. Population structure analyses suggest the existence of two incipient divergent populations; one comprising the tributaries located upstream and the other comprising downstream tributaries of the Mira River. Data also supports the occurrence of high gene flow mainly from downstream to upstream, failing to reflect isolation of populations due to seasonal drought. Obtained results should be incorporated in future management plans for the species. These should be centred around the current connection between populations and on the maintenance of suitable habitat patches, not only for reproduction, but also for summer refuges.