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52 GREAT BASIN NATURALIST MEMOIRS No.3 tive fish population. At Hiko Spring, no parasitism was evident until 1965 (Fig. 11). Shortfin mollies (P. mexi- cana), mosquitofish, and largemouth bass were all absent from collections made at Hiko Springs in June 1964. In January 1965 a few mosquitofish were seen and one was col- lected. In February 1965 both shortfin mol- lies and largemouth bass were seen in the Iimnocrene, and in March mollies began to appear in the monthly collections. Both mol- lies and mosquitofish increased in abundance through 1965. Lemea first appeared on Gre- nichthys in March 1965. Incidence of para- sitism increased to February 1966, at which time examination of the population was dis- continued because numbers had declined too low to permit continuation of the study. The population was extinct before June 1967. 30 25 20 E U) - 15 U) 0 .... 0 a.. - 10 0 ~ 0 5 . 1940 1945 1950 BIOTIC INTERACTIONS Interactions of native western fishes with introduced species have resulted in extensive hybridization, especially in trout, plus vari- ous kinds of competitive and predatory con- sequences. One example which is especially interesting, because it was replicated, oc- curred in Manse Spring, Pahrump Valley, Nye Co., Nevada. The endemic, and cur- rently endangered, Pahrump killifish (Emp- etrichthys latas latas) was restricted to the single Iimnocrene which was approximately triangular with maximum dimensions of about 25 X 15 m. In November 1961 six goldfish were introduced into the spring by one of the farmhands. They had reproduced by July 1962 and during that summer the children on the farm removed most of the ....... headwaters of Moapa River G-Q Crystal Springs - Exotic Fish Introduced 1955 1960 1965 1970 Fig. 10. Incidence of parasitism by Lernea on Crenichthys baileyi populations which remained abundant. Ie