Endangered Mexican fish under special protection: Diagnosis of habitat fragmentation, protection, and future - a review

Mexico is a megadiverse country and interior water fish are a clear example of high endemism and specialization. These phenomena are the result of diverse geographic and historic factors that have brought about the spatial isolation of catchment basins. The latter are usually in areas of high climate diversity and include transitional zones between the Nearctic and Neotropical regions. The exact number of fish species endemic to Mexico is unknown since many have probably disappeared before being identified. A considerable number of them are believed to have been lost during the second half of the 20th century. In all of Mexico the microendemism of the fish species creates the enormous diversity but in the same way is the main risk factor associated with the destruction of the habitat. The official Mexican norm currently lists 204 fish species considered endangered, threatened, under special protection or probably extinct in their natural habitat, distributed among 16 orders and 33 families. The reasons behind population reductions in these species are diverse. Intrinsic factors such as low fecundity and highly specialized courting behavior can be cited, but extrinsic factors have probably been fundamental in these reductions, including habitat loss due to tributary diversion or discharges of contaminants and toxicants, construction of hydraulic infrastructure, exotic species introduction, and human pressure in the form of commercial and subsistence fishing. However, the problem is very complex and in the same manner the knowledge on the ichthyofauna in the areas of the country is different, but can nevertheless be subdivided by geographic regions of the country. In the North of Mexico has a more in-depth knowledge on the fishes in such a way that the number of extinct or endangered species is twofold greater with respect to the center and south of the country. The main cause of risk is the destruction of habitat since it does not reach the amount of water through international rivers to maintain biodiversity coupled with desertification and competition from exotic species. In Central Mexico species diversity was high, but human pressure and habitat degradation together with high specialization and fishing for subsistence and/or for sale have been fundamental in its disappearance. In the South of Mexico, although it is the area with slighter environmental impact a lack of knowledge about the native ichthyofauna prevails and hence, there is less of a number of species reported at risk. Firm evidence indicates that endemic fish species in Mexico have adapted to compensate for the effect of naturally occurring toxicants, but there is an absence of information on many other aspects associated with their disappearance. The present study reviews habitat, protection measures, and impact of human activities and water quality in relation to the state of conservation of these fish species.