Biodiversity and Conservation 12: 2043–2056, 2003.  2003 Kluwer Academic Publishers. Printed in the Netherlands. Ecological evaluation of local extinction: the case of two genera of endemic Mexican fish, Zoogoneticus and Skiffia * MARINA YOLANDA DE LAVEGA-SALAZAR , EDGAR AVILA-LUNA and ´ CONSTANTINO MACIAS-GARCIA ´ ´ ´ ´ Departamento de Ecologıa Evolutiva, Instituto de Ecologıa, Universidad Nacional Autonoma de Mexico, * ´ A.P . 70-275, Mexico, D.F. C.P . 07510, Mexico; Author for correspondence (e-mail: mvega@miranda.ecologia.unam.mx; fax: 152-55-5616-1976) Received 15 March 2002; accepted in revised form 29 October 2002 Key words: Eutrophication, Extinction, Goodeidae, Habitat fragmentation, Introduced fish, Skiffia francesae, Zoogoneticus tequila Abstract. The Goodeidae is a family of endemic fish from central Mexico. Populations of several species are declining in the wild and two have been reported extinct; Skiffia francesae and Zoogoneticus tequila. Both species were native to just one locality in the Ameca basin. It is difficult to infer the causes of ´ extinction, since limnological data from Teuchitlan prior to the extinction events are not available, and there are no replicate populations. As an alternative approach, we explored the potential links between key environmental variables and events of local extinction of any species in the two genera (four Skiffia spp. and two Zoogoneticus spp.). In 14 localities known to harbour populations of fish of at least one of the six focal species, we conducted limnological surveys in the rainy and in the dry season, and quantified population densities of endemic and introduced fish. In addition, we quantified the concentration of agricultural pollutants in water and mud samples taken from every locality in the dry season. We found that all the focal species face some conservation threat, but we also discovered an extremely small population of the reputedly extinct Zoogoneticus tequila; in contrast, Skiffia francesae was not found. Eutrophication and habitat fragmentation appear to be the main threats to the focal species, with habitat fragmentation exponentially reducing population size and driving populations into refuges where they face stochastic extinction. Introduction Current patterns of fish extinction are probably the result of anthropogenic factors, but apart from the obvious cases of over-fishing, the precise threats to fish populations are not always evident, in part because the ecology of many fish species is not well known. Different types of fishes utilise several different reproductive strategies and foraging modes, occupy every part of the aquatic habitats within aquatic environments, and vary enormously in size. This extreme phenotypic variation makes it difficult to characterise the patterns of fish vulnerability (Allen- dorf 1988). Some factors, however, are often related to local extinction of fresh- water fish. One is channel incision of streams, which affects migrant species and causes isolation between populations of other species, increasing the risk of local extinction (Shields et al. 1994; Angermeier 1995). Loss of habitat because of