Survivorship of the Reintroduced Kihansi Spray Toads Nectophrynoides asperginis in a Soft Release Experiment at the Kihansi Gorge, Tanzania

The Kihansi Spray Toad (KST) Nectophrynoides asperginis is endemic to the unique wetland habitat surrounding the Kihansi Falls, Tanzania, with which it was associated. The species was declared extinct in the wild in 2009 by the International Union for Conservation of Nature (IUCN), following the loss of its unique habitat due to the diversion of the Kihansi River for hydropower production. This study is an initial attempt to determine the survivorship and possible sources of mortality of newborn and adult KSTs placed in cages in their original habitat in the Upper Spray Wetland (USW) and Lower Spray Wetland (LSW) areas. The cages were made of wooden frames covered with galvanized wire mesh to prevent the toads from escaping while allowing their prey (live invertebrates) to enter. There was a significantly higher survivorship of adult toads in the USW (29.2%) compared to those in the LSW (4.2%) (χ2 = 36.7, DF = 1, P < 0.0001). The newborn toads showed equally high survivorship rates in both USW (70%) and LSW (80%) (χ 2 = 0.1441, DF = 1, P = 0.7042). Newborn toads had a higher survivorship (75%) than adults (17%) (χ 2 = 16.7, DF = 1, P < 0.0001). Thus, we conclude that newborns were the more suitable life-stage for successful reintroduction of the KST back into its natural habitat. Therefore, we recommend continual reintroduction of newborn toads until the population stabilizes. Further studies should be conducted on other factors affecting the population dynamics of released KST, such as predation, diseases and both inter- and intra-specific competition. Maintenance of the habitat and constant monitoring programmes for the reintroduced KST should be continued.