Interactions between desert pupfish, Cyprinodon macularius, and Gila topminnow Poeciliopsis occidentalis: implications for establishing and maintaining populations

Authors: Anthony T. Robinson, David L. Ward, Ross J. Timmons, Douglas K. Duncan

Year: 2010 (xlii)

Abstract

Desert pupfish, Cyprinodon macularius, and Gila topminnow, Poeciliopsis occidentalis, inhabited similar environments and are often stocked into the same waters when implementing species conservation. Success of these stockings varied greatly and a working hypothesis developed among conservationists that desert pupfish populations were more likely to establish and persist if they were stocked first and allowed to establish before Gila topminnow were stocked. We decided to evaluate the hypothesis by examining records of past desert pupfish stockings and by conducting controlled laboratory experiments. For the wild situations, we evaluated 33 ponds where four general treatments were implemented: 1) only desert pupfish were stocked, 2) desert pupfish were stocked first and allowed to established and then Gila topminnow were stocked, 3) both desert pupfish and Gila topminnow were stocked simultaneously, and 4) Gila topminnow were stocked first and allowed to establish before desert pupfish were stocked. Desert pupfish established populations and persisted in all nine of the ponds where they were stocked in isolation and in all six of the ponds were they were stocked first. However, only 71% of the seven desert pupfish populations remained extant where both species were stocked simultaneously, and only 50% of the six desert pupfish populations remained extant in ponds stocked first with Gila topminnow and then later with desert pupfish. The same treatments, plus one additional treatment (only Gila topminnow stocked) were implemented in laboratory experiments utilizing 2.4-m diameter tanks; each treatment was replicated four times. The laboratory experiments began by stocking 100 fish into each tank, then after one month an additional 100 fish were added to each tank, and at two months the experiment ended and all fish were counted in each tank. Desert pupfish did better when stocked alone (abundance increased in three of the four trials) or when stocked first (abundance increased in two of the four trials). Desert pupfish did worse when stocked simultaneously with topminnow (abundance decreased in three of the four trials) or when stocked after Gila topminnow had established (abundance decreased in all four trials). Gila topminnow increased in abundance in all four treatments and all four trials, but increased the least when desert pupfish were stocked first. Our results support the original hypothesis and we suggest that when attempting to establish desert pupfish populations, they should be stocked in isolation or should be stocked and allowed to establish before Gila topminnow are stocked.