Devil's Hole Pupfish

USFWS Photo

Because they are a short lived species of one year or less, Devil's Hole pupfish are counted at least twice a year (spring and fall counts). Two to three observers record pupfish within the shallow shelf, while scuba divers count the pupfish to a depth of 100 feet within the cavern. Counts during the morning and afternoon are conducted to obtain an average. The spring population varies from 150-250, and the fall population from 400-500. This large disparity between spring and fall populations is due to severe environmental conditions, low oxygen levels, scant algae-producing sunlight within the cavern during winter months, and the short lives of these pupfish which naturally decline during the winter. The pupfish population within Devil's Hole is at it's highest during the fall which is the conclusion of the breeding season. The pupfish in the Refugium are counted by observers from the surface and/or a scuba diver in the deep section. The average spring population is 90 which increases to about 120 pupfish in the fall. The pupfish within the Refugia appear to live longer and obtain a larger size than those within Devil's Hole. This is because the refugia receive more sunlight, which supports more algae, and the cooler water in the refugia contains a higher oxygen content. Because the pupfish live longer within the Refugia, their populations tend to fluctuate less between the spring and fall.

Reaching a size of only 2.5 cm or less, the Devil's Hole pupfish are the smallest of some 25 species. Their body shape is similar to other pupfish with a large head and eyes and a long anal fin. It's pelvic fins are completely lacking, as are the vertical bars of other pupfish. Females lack the ocellus on their dorsal fin. Breeding males become solid dark blue and show the characteristic black terminal band on the caudal fin. In Devil's Hole, spawning occurs mainly on the shallow shelf. Peak spawning appears to coincide with the peak algal growth in Devil's Hole: between April and mid-June. Females lay small numbers of eggs, 4 to 5, on substrate which are fertilized by the male. In Devil's Hole, the males are non-aggressive and exhibit no territorial behavior. This is in contrast to pupfish as a group, which are highly aggressive and territorial; recent observations of the Devil's Hole pupfish within the Refugia populations reveal that they too have become highly aggressive and territorial. The Devil's Hole pupfish is omnivorous, eating algae and detritus obtained from the substrate.