Freshwater fishes and water status in Mexico: a country-wide appraisal

Mexico is the southernmost country in North America, and extends into Central America, south of the Isthmus of Tehuantepec. The northern half of Mexico is located on the Temperate belt and is arid in character (Nearctic), while the rest is within the Tropical belt (Neotropical). Climate varies from extremely temperate desert in the north, to tropical humid in the south. Mexico has more than 500 freshwater fish species, about 271 of them country endemics, and approximately 48 endemics from binational basins. There are still some 30-40 fish species not yet described. There are 563 fish species colonizing coastal flood plain species. In addition to the numbers of colonizing fishes, the burden of introduced exotics has also been growing. In 1904, only 4 species were recognized as exotics; by 1997 the number had increased to 94, and by 2008 to 115. The main fish collections in Mexico are at IPN, UNAM, and UANL and are the most representative, being national in scope, although concentrated in the tropics, central region, and general in coverage, respectively. spa

The decline of the native fish fauna has been in focus in recent years, usually as trend-in-time comparisons, where the loss of native forms and increase of exotics and/or colonizer species is evident in many basins, mainly in Rio Balsas, Rio Grande, and Rio Lerma-Santiago. As a result, the numbers of species reported at some degree of risk have been increasing also, from 17 in 1963 to 192 in 2005. The trends in colonizers, exotics, and species at risk among Mexican fishes are parallel. The Index of Biological Integrity (IBI), in either its geographical, or historical form (IBIh), has been applied to the Rio Grande/Rio Bravo basin, USA and Mexico. IBIh values go between 0-91 (average 31). Alien species are regarded as detrimental. Overall, the IBI trends have been similar in all regions, starting from 70-95% in upper reaches, decreasing to less than 0-35% in the lower reaches of West central basins, and then down to 15% or less near the Lower Rio Grande delta. spa

Several alien species of plecos have been recognized in the rivers Balsas, Grijalva-Rio Usumacinta complex, and, also, one in the Rio Grande. Mexican rivers are notoriously dewatered in the northern half of the country. Until 1962, the Rio Grande had an average runoff of 12,000+ millions of cubic meters/year; however by 2002 it was less than 2% of that value. The river went nearly dry along the Big Bend region and was dry for months in the delta region, both in 2002 and 2004. The Rio Grande is mostly dry north of the Rio Conchos junction, its main Mexican tributary, and other tributaries provide now between 1% (Rio San Juan) and 20% (Rio Conchos) of pre-1960 runoff. A modified Index of Biological Integrity for Rio Grande resulted in grades from 70 to 95% of the baseline in upper reaches, less than 35% in lower reaches, to less than 15% near the coast. The Texan version of the IBI was not representative as it suppresses data on euryhaline fishes. The reports of total toxics were masked, since the sum should have included both organics and heavy metals exceeding USA regulations to the total count, but only one of the two was included. spa

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