Desert Fishes Council, Death Valley, CA, November 2003.

Nonnative fishes in the southwestern USA versus northwestern Mexico: time-lagged invasions as a predictor in desert fish communities.

P.J. Unmack & W. Fagan

W.L. Minckley began efforts to create a comprehensive database on southwestern fishes in 1994 to allow accurate and quantitative investigations into various aspects of native and nonnative fishes. Unfortunately, Minckley passed away before the first paper stemming directly from the database was published, but his legacy via this database continues to grow and be expanded upon. Minckley was intensely interested in the Pacific Coast rivers of northern Mexico, especially their biogeography and conservation, hence the reason for their inclusion into the database. This presentation stems directly from Minckley, who in 1991, first put forth the hypothesis we are testing today. Here we quantify the historical development of nonnative fish assemblages of two North American desert drainages, one with many nonnative fishes (Gila Basin, principally southwestern USA ), and one with few (Yaqui Basin, principally northwestern Mexico). Each river is similar in size, physiography, and ecology, but because of differences in the timing of regional development, Minckley hypothesized the richness and geographic spread of nonnative fishes in the Yaqui are time-lagged relative to the Gila, and that a slow, but steady increase of nonnative fish occurrence is underway in the Yaqui, similar to what has occurred in the Gila. We found increases in regional richness of persistent nonnative species over time have been roughly linear in both basins. Meanwhile, previously established species have continued to spread, such that the cumulative number of reach records for nonnative species has increased roughly exponentially in both systems. For all comparisons, a time lag of 40-50 years exists between the Gila and Yaqui. The majority of nonnative fishes are piscivores, and many have high levels of parental care, a life history trait affording considerable advantages over native fishes. These results predict the presently abundant fauna of the Yaqui may become increasingly imperiled, with a future similar to the Gila, where most native fishes are either extirpated, threatened, or substantially reduced in range and nonnative fishes dominate most fish communities. We recommend immediate actions to identify and protect high priority portions of the Yaqui from further nonnative fish invasions before further degradation occurs.