Population growth and behavioural interactions of a critically endangered fish with co‐occurring native and exotic species.

Invasive species represent a threat to biodiversity and ecosystem services and cost millions of dollars to the global economy. The viviparous Mexican fish known as the tequila splitfin (Zoogoneticus tequila) became extinct in the wild as a consequence of habitat loss and degradation, and interactions with invasive species. Tequila splitfins are native to the Teuchitlan River in Central Mexico; they were kept in captivity and reintroduced into their native distribution in 2016. Approximately 80% of the fish in the Teuchitlan River are exotic species, and over 50% are twospot livebearers (Pseudoxiphophorus bimaculatus).We performed an ex‐situ mesocosm experiment to explore whether tequila splitfin populations would establish and increase, and how fish would behave when introduced into sites already colonised by other species. We tested this idea by introducing tequila splitfin individuals into empty mesocosms, mesocosms where individuals of the native Ameca splendens had established, mesocosms with individuals of the native Goodea atripinnis and mesocosms with individuals of exotic invasive twospot livebearers. All heterospecific species have been recorded in tequila splitfin's native range, are viviparous fish and share ecological requirements with them.We found that tequila splitfin abundance (number of individuals that survived and new individuals) was greater when sharing mesocosms with native species. Furthermore, they had reduced activity levels when inhabiting mesocosms with exotic invasive twospot livebearers, in comparison, interactions with natives proved to be beneficial.Our results highlight the need to remove exotic invasive species and protect other native species to increase reintroduction success. Close monitoring is needed during the initial stages of the reintroduction, and several reintroduction events from captive breeding facilities may also be necessary. When planning a reintroduction, it is critical to remove exotic species and make an effort to restore the habitat as close as possible to the original conditions. [ABSTRACT FROM AUTHOR]