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Clearly defined changes in the fishesof the Lower Rio Grande/Rio Bravo, Texas andMexico, have been documented since 1953. Morerecent surveys show that the originalfreshwater fish fauna has been retreating fromthe lower reaches and is being replaced bybrackish and marine invaders. A total of 13localities between Colombia (Nuevo Leon,Mexico) and the delta (580 km) weresurveyed, with a total fish fauna of 142species, which include native, exotic, andintruding species. Changes were analysed bylocality. These changes show a loss of themajority of freshwater species, replacement oflow to higher salinity forms. This shift isvery clear in marine invaders, with somespecies penetrating the whole areainvestigated. Those changes are morenoticeable in the lower-most localities, anddiminish upstream. Data suggest that thechanges in the basin, reflecting shifts inwhole-basin ecology, include elevatedtemperature, salinity, turbidity, and lowerrunoff levels. There are indications of higherlevels of pollution as well. Informationobtained in this study is being incorporated inan Index of Biological Integrity to appearelsewhere. |
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