Rearing Rana pipiens for Conservation: Two Approaches to Captive Rearing
Doug Adama and Kris Kendell
Species at Risk 2004 Pathways to Recovery Conference Organizing Committee
The northern leopard frog (Rana pipiens), once the most common amphibian in North America, has declined dramatically, particularly across the western portion of its range. In British Columbia and Alberta, recovery efforts are currently underway. The captive rearing of R. pipiens larvae for release has been a significant aspect of each province’s recovery effort; however, because the availability of resources and the status of R. pipiens vary between the two provinces, each jurisdiction has taken a different approach to the rearing of R. pipiens larvae in captivity.
In Alberta, R. pipiens were reared in two seminatural, outdoor ponds at the Raven Brood Trout Station from 1999 to 2003. Tadpoles were reared at densities of 0.005–0.020 tadpoles per litre. Due to the self-contained nature of the ponds, a natural diet of algae and other wetland vegetation was available to the larvae, and a number of steps were taken to reduce depredation from aquatic insects, American mink (Mustela vison), and great blue herons (Ardea herodias). Survival to metamorphosis ranged from 14 to 33%, while average size at metamorphosis was consistent with that of wild populations and ranged from 30.6 to 40.7 mm.
In British Columbia, R. pipiens were reared in 4000-L tanks from 2001 to 2003. The tanks were located outside and were exposed to ambient environmental conditions. Mixed vegetables and bloodworm were provided to supplement a diet of locally collected marsh vegetation. Depredation was controlled by screening the tanks with mosquito netting and thoroughly cleaning the marsh vegetation to avoid the introduction of predators. Initially, tadpoles were reared at a density of 1.0 tadpoles per litre, but this was reduced to 0.25 tadpoles per litre after 30 days. Mean size at metamorphosis in the rearing tanks ranged from 24.8 to 32.9 mm while survival to metamorphosis ranged from 55 to 97%.
Drawing from eight years of data, we discuss the advantages and disadvantages of the two approaches with respect to survival to metamorphosis, size at metamorphosis, and cost.