Determining the “world’s rarest fish” involves distinguishing between species with a wide geographic range but low numbers, and those confined to a tiny, isolated location. The most accurate measure of rarity is the total number of individuals in its natural habitat, which points to a single, critically endangered desert dweller. This tiny fish holds the record for the smallest natural population size of any vertebrate on the planet.
The Title Holder: Smallest Known Population
The fish that currently holds this title is the Devil’s Hole Pupfish (Cyprinodon diaboli). Its extreme rarity is defined by having the most restricted population size and range. This critically endangered species lives entirely within a single, water-filled limestone cavern in the Mojave Desert. Biannual spring population counts monitor the species’ fragility. In April 2024, the count recorded 191 observable individuals, marking the highest spring total in 25 years, yet still representing an incredibly small total population for an entire species.
This number has fluctuated dangerously over the past decades, dropping to an all-time low of just 35 fish in 2013, highlighting its constant vulnerability. The pupfish is a small species, typically reaching a maximum length of about 30 millimeters (1.2 inches). Males display a bright metallic blue color during the spawning season, having adapted to survive in an environment lethal to most other fish.
The Extreme Limits of Its Habitat
The exclusive habitat of the Devil’s Hole Pupfish is a geothermal pool known as Devil’s Hole, a detached unit of Death Valley National Park in Nevada. This single cavern is the only place on Earth where this species exists. The water is sourced from a deep aquifer, maintaining a constant, geothermally heated temperature of around 33° to 34° Celsius (91° to 93° Fahrenheit) year-round.
This stable, warm temperature is near the upper limit of thermal tolerance for most fish, demonstrating the pupfish’s extreme specialization. The fish are confined to the upper 24 meters (80 feet) of the cavern’s water column. They rely on a single, shallow rock shelf near the surface for survival. This small, submerged shelf, measuring approximately 3.5 by 5 meters, is where the fish forage and spawn.
The limited surface area of this shelf is the root cause of the population bottleneck, restricting the available food supply and spawning territory. Dissolved oxygen levels are exceptionally low in the water column, typically ranging from 1.8 to 3.3 parts per million. The primary food source for the fish is the algae and diatoms that grow on this sunlit shelf, meaning the entire food web is dependent on this extremely small area.
Conservation and Monitoring
Due to its critically restricted population and habitat, the Devil’s Hole Pupfish was officially listed as an endangered species under the Endangered Species Act in 1967. The protection of its habitat was secured by a landmark 1976 Supreme Court decision. This ruling limited groundwater pumping nearby to maintain the water level in the Hole, recognizing the federal government’s right to protect the water resources necessary for the species’ survival.
Monitoring the population involves a highly coordinated effort between the National Park Service and the U.S. Fish and Wildlife Service. Biannual counts are conducted by SCUBA divers and surface observers. Divers descend to about 24 meters to physically count the fish, providing a direct census of the entire wild population. The habitat is protected by fencing and continuous remote monitoring to prevent human interference and track environmental changes.
A significant safeguard against catastrophic loss is the establishment of refuge populations at facilities like the Ash Meadows Fish Conservation Facility. These captive populations are maintained in artificial habitats that mimic the unique conditions of Devil’s Hole. They serve as an insurance policy should the wild population face sudden extinction, and also help researchers understand the complex environmental dynamics of this unique ecosystem.