Pecos pupfish

Despite once existing throughout the Pecos River, C. pecosensis populations have become increasingly isolated due to significant portions of their native habitat being removed. [2][3] Following the introduction of Cyprinodon variegatus (sheepshead minnow), pure Pecos pupfish populations have been further isolated and largely exist in scattered sinkholes, man-made impoundments, and marshes.[3][4] Its natural habitat includes springs, sinkholes, and pools of streams.[5]

Cyprinodon variegatus is thought to have been introduced during the early 1980s, presumably via bait-bucket dumping. Shortly after the introduction of C. variegatus, C. variegatus X C. pecosensis hybrids quickly became widespread, occupying an estimated 50% of the native Pecos pupfish range by 1985. [6][7] The widespread success of hybrids, and quick range expansion throughout native Pecos pupfish waters, has been hypothesized to be the result of selection mechanisms for hybrids during a period when the native C. pecosensis population had experienced a reduction.[8]

Additionally, hybrid populations have been reported to have increased growth-rates and vigor (Heterosis), resulting in Pecos pupfish females to actively choose to mate with hybrids over pure male Pecos pupfish.[9][10] Given this, conservation efforts have been difficult and efforts are being made to prevent C. variegatus from entering into isolated C. pecosensis populations.