Changes in the fish assemblages in the upper Rio Ameca, Mexico

We surveyed the fish fauna of the upper Ameca River basin, a watercourse of the Pacific slope of western Mexico. Many endemic species and the dominanc e of goodeid species characterize the basin. We sampled 17 sites during the dry and rainy seasons in 1996 and 1997. The basin has a historic record of 20 native and six exotic species, most of which were described or encounte red in the last decade. We found a dramatic reduction in fish diversity; cu rrently the upper portion of the basin holds only six native species and fo ur exotics. There has been a reduction of 70% in the native fish fauna dive rsity in recent years. The Ameca River has biological symptoms of ecosystem stress, including reduced species diversity, increased dominance by exotic species, and reduced population stability. We attribute this stress to res ervoir construction, water extraction, and municipal and industrial polluti on in the basin.