A Funnel Trap for Sampling Salamanders in Wetlands

In response to our need to sample tiger salamanders (Ambystoma tigrinum) in diverse wetland habitats of the Prairie Pothole Region (PPR), we designed a funnel trap that overcomes many of the problems associated with other sampling methods (Heyer 1976, 1979; Berger 1984; Shaffer et al. 1994). We evaluated our trap in 17 seasonal and semipermanent wetlands at the Cottonwood Lake Study Area (CLSA) in Stutsman County, North Dakota; wetlands of the CLSA are typical of depressional wetlands found elsewhere in the PPR.

Figure 1. A funnel trap for sampling amphibian larvae in wetlands. Overall height of trap and drift fence should be determined by the maximum depth anticipated in the wetland being sampled. The trap should be set 10 cm above the water surface to allow sampling of the entire water column while simultaneously providing oxygen to captured animals.

Our funnel trap has a welded rectangular frame constructed of 0.63 cm diameter steel rod covered with 0.32 cm mesh galvanized screen (Figs. 1 and 2). We constructed a 5 cm wide funnel opening that is raised 7.62 cm above trap bottom on one side of the trap, and attached a hinged mesh lid to the top of each trap. To firmly seat the trap into the substrate, we extended 4 steel rods 30 cm beyond the bottom of the trap; these legs can be omitted and weights may be substituted and used as anchors in wetlands with rocky substrates. An additional feature of our trap is a 200 cm drift fence that directs free-swimming salamanders to the opening of the trap. The drift fence is constructed of 0.32 cm mesh nylon, 91.4 cm wide, with the ends folded over to form end loops. We attached the drift fence to the trap by inserting a threaded steel rod through a small opening at the top of the trap and through the end loop in the drift fence. We then screwed the rod into a 0.63 cm nut welded to the bottom of the funnel opening. The opposite end of the drift fence was supported by inserting a length of 2.54 cm PVC pipe through the end loop and into the substrate. Lead weights attached to the bottom edge of the drift fence maximized contact with the substrate. Floats attached to the top of the submerged portion of the drift fence maintained its vertical orientation. Where water depths exceeded 91 cm we increased the height of the fence by adding a second length of 0.32 cm mesh nylon.

Figure 2. Construction detail of a funnel trap designed to sample amphibian larvae in wetlands.

These traps have proven effective in all wetland vegetative zones and in water depths ranging from 10 cm to over 2 m. During each of three periods (18-29 July, 1-12 and 15-26 August) in 1994, and concurrent with our funnel trap sampling, we sampled five randomly selected semi-permanent wetlands with minnow traps (0.3 × 1.0 m), seines (4.0 × 1.3 m, 0.6 cm mesh), and dipnets (22.0 × 47.5 cm). Although we were able to deploy minnow traps (10 per wetland) in all five wetlands, our funnel traps (10 per wetland) captured more tiger salamanders (mean = 3.70/trap/24 h; N = 150; SD = 7.60) with less variance than the minnow traps (mean = 0.34/trap/24 h; N = 150; SD = 1.34). Seining and dipnetting were not satisfactory sampling techniques because of submergent and emergent vegetation, which severely limit the use of seines and dipnets as a quantitative tool in well-vegetated depressional wetlands of the PPR.

Our funnel traps cost ca. $45 per trap for materials; minnow traps are commercially available for $13.50 per trap. Labor to install funnel and minnow traps is similar. While the higher cost of our trap may limit its use to some extent, greater capture rate combined with the ability to sample the entire water column with reduced mortality may justify these higher costs. When sampling rare taxa our trap may detect populations other sampling techniques overlook.

Literature Cited