Control of invasive American bullfrog in small and shallow ponds

Setting up cost-efficient control programs for alien invasive species requires the development of adequate removal methods in combination with insights in population size and population dynamics. American bullfrog, Lithobates catesbeianus, is an alien invasive species, which is suspected to cause substantial ecological damage around the globe through predation, competition and pathogen transmission (e.g. Sharifian-Fard et al. 2011). The species is considered one of the top 100 world’s alien invasive species, for which instant and drastic management measures are necessary. However, control of bullfrog populations is extremely difficult due to the species flexible life history strategy and population biology. Moreover, no conclusive management measures have yet been determined. We investigated how double fyke nets could contribute to the management of bullfrogs by assessing the tadpole population size in 12 permanent, small (< 4000 m²), shallow water bodies. Two population size estimate methods were applied, being the (1) catch-depletion and (2) mark-recapture method. Population density varied substantially among ponds, ranging from 950 to 120.804 tadpole individuals/ha. Catchability of bullfrog tadpoles proved to be consistent over ponds and methods, with one catch per unit of effort (one double fyke net for 24h) retaining on average 6 % of the population. Using these insights we projected the number of catch efforts needed to reduce tadpole numbers to a threshold that more than likely affects final bullfrog population size. Predictions indicated that, for both low and high abundance populations, the use of eight double fyke nets is most cost-efficient. What the exact threshold number should be is debatable, but forecasts demonstrate that only half of the budget would be needed when aiming at a drop till 100 than a decrease to 10 remaining tadpole individuals (figure 1). Given the fairly limited cost of the management with double fyke nets as compared to other methods, it may be worthwhile to fully reduce the tadpole population. The outcome of these experiments will be discussed in light of the feasibility of a full eradication programme for invasive bullfrog in the area.