Re-introduction of European tree frog in Latvia

Summary

The European tree frog (Hyla arborea), considered extinct in Latvia since the late 20th century after wetland loss and beaver extirpation, was the subject of a captive breeding and release program begun by Riga Zoo in 1987 using adult founders sourced from the nearest stable population, in southern Belarus near the Goryn-Pripyat confluence. Adults were hibernated in refrigerated boxes and later an unheated cellar, then bred using hormone injections of a synthetic Luliberin analogue, with each female producing 200 to 1,000 eggs; larvae reared in aerated aquariums metamorphosed at 60-90% survival within 30-60 days, faster than in the wild. From 1988 to 1992, 4,110 juveniles from 14-17 breeding pairs were released at a single protected 350-hectare site in southwestern Latvia's Liepaja district. The first wild calling male was recorded in 1990, tadpoles were found in 1991, and calling males were detected outside the release site by 1993; by 2002 the species had spread to 110 localities, and by 2005, monitoring showed the population's range covered 800-900 square kilometers and continuing to expand, tracked partly through public outreach via booklets, school lectures and media coverage. The main ongoing difficulty is that the population's range has grown too large for comprehensive monitoring, relying instead on occasional public reports. The project rates itself Highly Successful, citing the scale and continued expansion of the self-sustaining wild population re-established from laboratory-bred stock.