Several factors contributed to this decline of native fish in the San Pedro River. The most influential was the introduction of non-native fish, which began in the 1880s, when early settlers introduced carp, bass, catfish, sunfish, and other species that served as food sources. In addition to these larger fish species, the settlers also introduced small mosquitofish to control mosquito populations. Some kinds of exotic species also prey directly on the native fish. When an ecosystem loses most of its native species, ecological problems can result.
Native fish are part of natural food webs that have evolved over millions of years.
The longfin dace is one of only two native species of fish currently in the San Pedro River. Unlike some native fish in Arizona, the longfin dace is highly adaptable. It is found throughout the state's streams and rivers, in part because it is able to survive aquatic extremes of violent flooding and severe drying and in part because it tends to hold its own against non-native species. The Longfin Dace can survive in shallow water, where many natives and non-natives can't thrive. Longfin dace feed upon both aquatic invertebrates and plants.