American bullfrog suppression in the Yellowstone River floodplain

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The American bullfrog (Rana catesbeiana) has recently invaded backwater and side-channel habitats of the Yellowstone River, near Billings, Montana. In other regions, bullfrog invasions have been linked to numerous amphibian declines (e.g., Adams and Pearl 2007). Immediate management actions may be able to suppress or eradicate localized populations of bullfrogs because they are present at low densities. Once established, bullfrogs are extremely difficult to eradicate because they are highly fecund (females can produce up to 40,000 eggs per clutch) and are extremely mobile (Govindarajulu et al. 2005). For this study, we have three objectives: (1) To describe patterns of bullfrog spread and occupancy in the Yellowstone River floodplain, (2) To quantify the impacts of bullfrogs on native species, and (3) To test multiple methods of bullfrog suppression, including seining, minnow traps, shooting with an air gun, dewatering, and carbon dioxide (CO2).

Seining a heavily bullfrog populated side channel at Two Moon County Park in Billings, MT.

Scientists use a net to capture invasive American bullfrog tadpoles in the Yellowstone River floodplain.Public domain

We have conducted annual visual encounter surveys (VES) for amphibians along the Yellowstone River, from downstream of Park City to Custer, since 2012. We conduct surveys in randomly selected sites in the Yellowstone River, side channels, backwaters, and off-channel impoundments. Sites are visited multiple times each year to account for imperfect detection. We perform VES by scanning the surface of the water and shoreline and walking or floating the shoreline. In areas where there is evidence of amphibian presence, we use VES, dip net, and seine surveys to determine presence and identity of egg and tadpoles. We also listen for amphibian calls while performing VES. At each site, we describe local (e.g., water depth) and landscape-scale (e.g., distance from road) habitat characteristics. We then use these field data to test if (1) the probabilities of bullfrog occupancy, colonization and extinction are related to local and landscape-scale habitat characteristics, (2) the probabilities of native amphibian occupancy changes as a function of bullfrog occupancy or colonization. In 2014, we plan on complementing these observational data on native amphibian occupancy with experimental data to assess the potential for bullfrogs to drive native amphibian declines.

Bullfrog tadpoles captured with a seine net near Huntley, MT.

Bullfrog tadpoles captured with a seine net near Huntley, MT. Public domain

Results from 2012 and 2013 surveys indicate that bullfrogs are now firmly established in the floodplain, have extended their distribution from 60 km in 2010 to 106 km in 2013, and are spreading to up- and downstream habitats (Fig. 1). Of most concern is that the number of sites with breeding populations (i.e., presence of bullfrog eggs or tadpoles) has increased from 8 sites in 2010 to 45 sites in 2013 (Fig. 2). We found that bullfrogs were associated with habitat features that characterize permanent waters and that describe human-mediated introductions. These survey data indicate that efforts to control bullfrogs in the Yellowstone River floodplain should reduce the hydroperiod of breeding sites and should increase public outreach and education campaign to prevent further bullfrog introductions.

We are also working with USGS conservation geneticist, Pauline Kamath, to describe the genetic structure of introduced bullfrogs in the Yellowstone River. This genetic approach will help us to identify potential introduction sources and pathways. In 2013, we sampled genetic material from bullfrog tadpoles that occurred at 26 sites in the Yellowstone River. Preliminary results indicate that these sampled bullfrogs comprise 2 unique haplotypes. We hope to expand our understanding of bullfrog genetic structure to more habitats in the Yellowstone River and to other areas in Montana where bullfrogs have been introduced. This information will help us to determine if bullfrogs are being moved within the state.

Scientists Barnaby Watten and Andrew Ray (NPS) conducted dose response laboratory trials.

Scientists Barnaby Watten and Andrew Ray (NPS) conducted dose response laboratory trials to assess the concentration of dissolved CO2 in water required to induce mortality in larvae after 24-hr exposure period. These tests were conducted in controlled laboratory facilities at the Leetown Science Center using hyperbaric chambers.Public domain

In the fall of 2011, we tested the lethal dose of CO2 required to kill bullfrog tadpoles. We conducted dose response laboratory trials to assess the concentration of dissolved CO2 in water required to induce mortality in larvae after 24-hr exposure period. These tests were conducted in controlled laboratory facilities at the Leetown Science Center with Barnaby Watten, Andrew Ray (NPS) and others using hyperbaric chambers (Image 1). Results of this work demonstrated that the LC50 and LC99 (lethal concentrations at 50% and 99% of individuals) for tadpoles (Gosner stages 26-42) were 371 and 549 mg CO2 / L of water (Abbey-Lambertz et al. 2014). These data will facilitate the development of a novel tool for bullfrog suppression – the use of elevated CO2 to induce mortality in isolated ponds, backwaters, and side-channel habitats. To our knowledge, no research has been conducted examining the effects of elevated CO2 on amphibians.

We are using information from our field surveys to identify habitats where bullfrogs are breeding. At these habitats, we are using multiple techniques to remove bullfrog adults, tadpoles and eggs (Images 2-4). Techniques include seining, hand netting, pellet guns, dewatering and electrofishing. We are then assessing how bullfrog occupancy and relative abundance in these habitats change over time.