Light at the End of the Funnel: Taxonomic Biases of Aquatic Trapping Methods for Studying Wetland-Breeding Amphibians at Variable Depths

Aquatic funnel traps and light lures are widely used for surveying amphibians in wetlands, but researchers must select among various trap designs and configurations for any particular study. We examine the performance of glow sticks and four types of aquatic funnel traps, including three commercial minnows and a handcrafted, deep-water trap, on surveying wetland-breeding amphibians, including the Blue-spotted Salamander complex (Ambystoma laterale and Unisexual Ambystoma), Spotted Salamanders (A. maculatum), and Wood Frogs (Lithobates sylvaticus) in central Maine. We performed this study from April–May 2023 and 2024 in 15 freshwater wetlands, which were trapped with the four types of traps crossed with two light treatments (i.e., glow stick present or absent) for a total of eight treatment combinations. We deployed 1–3 replicates of each combination every night for 1–3 nights at each site, yielding 633 total trap nights. We used linear and generalized linear mixed models to compare capture probabilities, counts, and body size (snout-vent-length and mass) of amphibians among trapping methods. Trap designs exhibited taxonomic biases, with the Blue-spotted complex (mostly unisexual females) displaying greater captures in deep-water traps than commercial minnows, Spotted Salamanders displaying equivalent captures in all trap types, and Wood Frogs displaying fewer captures in deep-water traps than commercial minnows. Glow sticks boosted capture probabilities and counts of spotted salamanders, weakly improved capture probabilities for the Blue-spotted complex, and, in contrast to previous studies, had no effect on Wood Frogs. Trapping methods exhibited weak morphological biases for Spotted Salamanders, which were longer and heavier in deep-water traps and shorter and lighter in traps with glow sticks. Captures declined for all species over the season and with successive trap nights, highlighting an important consideration for optimizing landscape-scale studies for cryptic species. Future research examining sex and species-specific activity patterns at different depths and light spectra may address remaining uncertainties of aquatic trapping protocols for different amphibians.