We intensively sampled fishes from two reaches of the Rio Grande within the Albuquerque Basin (Albuquerque and Belen valleys) during 1998–2001, compared our findings with those from 1984, and compiled a list of all fishes known from the two reaches. Structure of assemblages (richness, diversity, abundance distributions–taxon dominance versus rank abundance) was similar between reaches in 1998–2001, but the faunas had low taxonomic similarity. The nonnative white sucker (Catostomus commersonii), western mosquitofish (Gambusia affinis), and native red shiner (Cyprinella lutrensis) dominated the Albuquerque Valley. Faunal change in the Albuquerque Valley between 1984 and 1998–2001 indicated all native fishes besides the red shiner declined, particularly the Rio Grande silvery minnow (Hybognathus amarus) and longnose dace (Rhinichthys cataractae). In contrast, the red shiner alone dominated the Belen Valley, nonnative fishes were less dominant, and aside from the Rio Grande silvery minnow and longnos... Resumen Muestreamos intensivamente a peces de dos secciones del río Grande dentro de la cuenca de Albuquerque (los valles de Albuquerque y de Belén) durante 1998–2001, comparamos nuestros resultados con los de 1984, y compilamos una lista de todos los peces conocidos de las dos secciones. La estructura de ensamblajes (riqueza, diversidad, distribuciones de la abundancia–dominancia de taxon contra rango de abundancia) fue similar entre las secciones en 1998–2001, pero la fauna tuvo poca semejanza taxonómica. El pez exótico, matalote (Catostomus commersonii), el guayacón mosquito (Gambusia affinis), y el pez nativo, carpita roja (Cyprinella lutrensis lutrensis) dominaron el valle de Albuquerque. El cambio de fauna en el valle de Albuquerque entre 1984 y 1998–2001 indicó que todos los peces nativos además del carpita roja declinaron, particularmente el carpa chamizal (Hybognathus amarus) y el carpita rinconera (Rhinichthys cataractae cataractae). En cambio, sólo el carpita roja dominó el valle de Belén, los ...

Northern Middle America (NoMA) is considered a transition zone between the Nearctic and Neotropical biogeographic realms. In this region, Nearctic and Neotropical freshwater fishes create regional faunas of mixed origin, but their general biogeographic patterns have not been quantified. To identify such patterns, we delineate biogeographic regions (BRs) and major biogeographic barriers of NoMA and summarize patterns of faunal similarity among BRs. We used clustering analysis on a presence-absence matrix of primary and secondary freshwater fishes to group 97 level-6 HydroBASINs units spanning NoMA into BRs. We assessed statistical support of clusters using one-way analysis of similarity and implemented a species-indicator analysis. We delineated biogeographic barriers with the software Barrier 2.2 and determined faunal similarity among BRs using beta diversity-Jaccard dissimilarity and producing a minimum-spanning tree. Seven statistically distinctive and geographically coherent BRs were delineated and described. Barrier analysis identified three major barriers within NoMA. The first barrier combines the Sea of Cortés (Gulf of California) and Sierra de Juárez-Cerro Gordo highlands. The second combines the Trans-Mexican Volcanic Belt, Sierra Madre del Sur, and Sierra Madre de Chiapas highlands. The third combines the Río Grande Rift, Sierra Madre Occidental, and Mesa Central highlands. Faunal dissimilarity was very high among BRs, with lowest dissimilarity (92%) between the Balsas-Nacaome and Grijalva-Usumacinta BRs. Boundaries of NoMA BRs do not correspond with political boundaries. We concluded that bioregions of NoMA are faunally distinct, with limited overlap due to presence of strong, long-standing geographical barriers enhanced by aridity in the North.