Threats to Freshwater Fish

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Abstract
The sole wild population of the small live-bearing fish known as Picote de Tequila (Zoogoneticus tequila, see the figure) lives in a single, 4-m-wide pool in the Ameca Basin in Central Mexico. This pool supports a population of under 500 of these fish, of which fewer than 50 are adults (1). Trinidadian guppies (Poecilia reticulata) introduced to the area, possibly in an attempt to control mosquito larvae, outnumber them by 6 to 1 (1, 2). Exotic species are a well-known threat to freshwater fish populations, particularly those, like Z. tequila, that are vulnerable as a result of their reduced population size and low genetic diversity (3). Habitat fragmentation, habitat loss, and overenthusiastic collecting further compound the risk of extinction for this and many other fish species. Biodiversity loss affects all taxa, but freshwater fish are especially susceptible because many fish species are rare. One way to help conserve rare species is to learn how naturally small and fragmented populations manage to persist in the wild.