tequila fish
© Chester zoo/PA
‘A rare success story’: tequila fish are thriving after their reintroduction in Mexico.
Declared extinct in the wild in 2003, species has been reintroduced to its native river after being bred in Chester.

A "charismatic little fish" declared extinct in the wild has been reintroduced to its native Mexico after being bred in an aquarium at Chester zoo.

The tequila fish (Zoogoneticus tequila), which grows to no bigger than 70mm long, disappeared from the wild in 2003 owing to the introduction of invasive, exotic fish species and water pollution.

Named after the Tequila volcano, which looms north of its native habitat, the species was discovered in 1990 in the Teuchitlán River in Jalisco, south-west Mexico.

Now conservationists at Chester zoo and the Michoacana University of Mexico have teamed up to return more than 1,500 fish to the river in a project cited as an International Union for Conservation of Nature (IUCN) case study for successful reintroductions.

Recent studies have confirmed that the fish are thriving and already breeding in the Teuchitlán. Experts say it has created a blueprint for future reintroductions of other highly endangered fish species, with a rescue mission for another, the golden skiffia (Skiffia francesae), now under way.

Prof Omar Dominguez, of the Michoacana University of Mexico, said: "This is the first time an extinct species of fish has ever been successfully reintroduced in Mexico and so it's a real landmark for conservation. It's a project which has now set an important precedent for the future conservation of the many fish species in the country that are threatened or even extinct in the wild but which rarely take our attention."

In 1998, at the outset of the project, the university's aquatic biology unit received five pairs of fish from Chester zoo. These 10 fish founded a new colony in the university's laboratory, which experts maintained and expanded over the next 15 years.

In preparation for the reintroduction, 40 males and 40 females from the colony were released into large, artificial ponds at the university. This exposed them to a semi-natural environment where they would encounter fluctuating resources, potential competitors, parasites, and predators such as birds, turtles and snakes. After four years, this population was estimated to have increased to 10,000 individuals and became the source for the reintroduction to the wild.

A long-term monitoring programme was established involving local people trained to assess water and habitat quality.

Dr Gerardo Garcia, Chester zoo's curator of lower vertebrates and invertebrates, said the successful reintroduction was an important moment in the battle for species conservation.

"It is a real privilege to have helped save this charismatic little fish and it just goes to show that with the skill and expertise of conservationists, and with local communities fully invested in a reintroduction project, species can make a comeback from environments where they were once lost," he said.

"With nature declining globally at rates unprecedented in human history - and the rate of extinction accelerating - this is a rare success story. We now have a blueprint for what works in terms of recovering these delicate fish species in Mexico and already we're on to the next one - a new rescue mission for the golden skiffia is already well under way."