Southwestern Association of Naturalists meeting, Norman, OK, 17 - 19 April 2003.

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

P.J. Unmack & W.F. Fagan

Here we quantify the historical development of nonnative fish assemblages of two North American desert drainages, one with many nonnative fishes (Gila River, principally southwestern USA ), and one with few (Río Yaqui, principally northwestern Mexico). Each river is similar in size, physiography, and ecology, but because of differences in the timing of regional development, we hypothesized that richness and geographic spread of nonnative fishes in the Río Yaqui are time-lagged relative to the Gila River, and that a slow, but steady increase of nonnative fish occurrence is underway in the Río Yaqui, similar to what has occurred in the Gila River. 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 metrics examined, a time lag of 40-50 years exists between the Gila and the 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 Río Yaqui will become increasingly imperiled, with a future similar to the Gila River, where 84% of native fishes are either extirpated, threatened, or substantially reduced in range. We recommend immediate actions to identify and protect high priority portions of the Río Yaqui from further nonnative fish invasions before further degradation occurs.