Demonstration of Subsurface Passive Acoustic Monitoring (SPAM) to Survey for and Estimate Populations of Imperiled Underwater-Calling Frogs

Patrick Wolff | ERDC-CERL



Department of Defense (DoD) management and recovery of threatened and endangered species (TES) necessarily relies upon an understanding of their occurrence, abundance and distribution on installations. Many DoD installations spend considerable time and effort surveying for amphibians because they are one of the most imperiled taxa and many populations are rapidly declining. Several at-risk frog species (anurans) vocalize primarily or exclusively underwater negatively biasing traditional survey methods. The objective of this project is to demonstrate how subsurface passive acoustic monitors (SPAMs) can be used to document the occurrence and relative abundance of at-risk, underwater calling anurans on DoD installations. Specifically, researchers intend to 1) demonstrate and validate the ability of SPAMs to detect anuran calls generated both above and below the water’s surface, 2) examine the influence of environmental factors (e.g., depth, temperature, substrate type, foliage density) on acoustic detectability, 3) demonstrate the effectiveness of SPAM data for population estimation through modeling and empirical validation experiments, and 4) make direct comparisons between the detection and effectiveness of SPAMs as monitoring technology relative to traditional survey methods for amphibians on DoD installations.

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Technology Description

Passive acoustic monitoring (PAM) is an increasingly robust survey methodology to sample for acoustically active species. PAM is particularly effective over traditional survey methods, such as visual or listening surveys, in that they can provide continuous coverage of a particular area for extended periods of time, and the recording sensors are not impacted by inclement weather or limited accessibility to a survey site to the same degree as humans. These passive acoustic data can provide a continuous, persistent record of species behavior, occurrence, and abundance. Traditional PAM technology can be converted for submerged use (SPAMs: subsurface passive acoustic monitors) to record underwater vocalization of species through the use of hydrophones and specialized recording devices. In its present form of a systematized workflow going from acoustic recorder through data visualizations, SPAMs have been a staple of the U.S. Navy and marine mammal monitoring for nearly two decades, and are being increasingly used for natural resource management efforts by the U.S. Air Force and U.S. Army. The increases in technological capability (memory size, battery life, and computer processing speed) have led to massive increases in the spatial and temporal scale addressed by acoustic surveys. If successfully validated for use in freshwater systems, this technology could provide the first reliable method to survey for underwater calling frogs on military installations.

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Emerging research is just now highlighting the wide range of taxa that vocalize underwater, from numerous and diverse fish species to freshwater turtles and arthropods. With the rapidly increasing number of species being reviewed for inclusion on the USFWS’s Endangered Species Act, the DoD will need to rely upon a wide range of diverse and reliable survey technologies and approaches. By demonstrating the effectiveness of SPAMs to survey for underwater calling freshwater animals, researchers will provide to the DoD a powerful, low-cost, low-effort tool to survey for a suite of aquatic animals across DoD installations and branches. Not only will this technology assist with the surveys of rare, underwater calling frogs such as the Gopher Frog, Spotted Frog, and Chiricahua Leopard Frog, but the existing technological framework that the researchers validate here can be easily adapted to survey for various other freshwater and terrestrial species. By demonstrating the use of these systems and developing guidelines pertaining to their deployment and analysis, DoD natural resource managers will be able to successfully survey for these at-risk species and obtain a better understanding of their distribution and abundance on installations to develop plans to minimize impact on these species and to reduce training restrictions.

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Points of Contact

Principal Investigator

Mr. Patrick Wolff


Phone: 217-373-5847

Program Manager

Resource Conservation and Resiliency