Individual study: Reintroductions of the European tree frog (Hyla arborea) in Latvia
Zvirgzds J., Stašuls M. & Vilnìtis V (1995) Reintroductions of the European tree frog (Hyla arborea) in Latvia. Memoranda Societatis pro Fauna et Flora Fennica, 71, 139-142
This study is summarised as evidence for the intervention(s) shown on the right. The icon shows which synopsis it is relevant to.
Release captive-bred frogs
A before-and-after study in 1988–1997 in ponds on abandoned farmland in Liepâja, Latvia (Zvirgzds, Stašuls & Vilnìtis 1995, Zvirgzds 1998) found that released captive-bred European tree frog Hyla arborea froglets established stable breeding populations at release sites and frogs colonized new breeding sites. Males were recorded calling from 1990 and tadpoles were observed from 1991. From 1993, calling males were heard outside the release site. By 1994 there were seven ponds with calling males up to 2 km away and by 1997 this had increased to 48 ponds within a 20 km radius of the release site. Breeding was recorded in at least 10 of those ponds. At least four generations had been produced in the wild by 1997. A total of 4,110 froglets were released into ponds in a Nature Conservation Area (300 ha) in June–August 1988–1992. Ponds were monitored using call surveys in spring and by counting tadpoles and froglets in autumn.
Use hormone treatment to induce sperm and egg release during captive breeding
A replicated study in 1988–1992 of captive European tree frogs Hyla arborea in Latvia (Zvirgzds, Stašuls & Vilnìtis 1995) found that following hormone treatment of males and females, frogs bred successfully in captivity. In several cases three or four hormone injections were required to induce spawning. Females each produced 200–800 eggs. An average of 60–90% of larvae metamorphosed in captivity. The period of metamorphosis was shorter in captivity than the wild (30–60 vs 90 days). Over 4,000 froglets were produced. Wild-caught frogs were housed in outdoor tanks and were overwintered in a refrigerator. From February, daylight period, UV light and feeding was increased. Two males and one female were placed in separate 35 l aquaria with water and plants. Breeding was stimulated with hormone injections (100 mg luliberin-surphagon/ml of solution) in March or May. Females received 15–20 mg and males 10 mg. The injection was repeated after 24 hours if spawning did not start. Larvae, metamorphs and toadlets were raised in separate tanks.