Bioacousticians test passive acoustic monitoring to capture sounds of marine life

The direct comparison of these methods conducted by a team of researchers spanning both U.S. coasts will help us to better collect data to monitor marine life for both conservation and management of protected species.

The use of gliders and floats in passive acoustic monitoring helps address the spatial and temporal tradeoff of stationary and towed instruments.

"Towed instruments provide great spatial coverage. A boat can 'mow the lawn' over a large area in a relatively short period of time, while stationary recorders on the seafloor have great temporal coverage, but they record only within a certain range of the hydrophone," said Selene Fregosi, a researcher at the Cooperative Institute for Marine Resources Studies, Oregon State University and National Oceanic and Atmospheric Administration in Newport, Oregon. "A mobile, autonomous recorder can both cover a large area and survey for weeks or months at a time, without the high cost of operating a large research vessel." Understanding how the methods compare and what their strengths and limitations are is a key to the monitoring of sea life.

During the 172nd Meeting of the Acoustical Society of America and the 5th Joint Meeting with Acoustical Society of Japan, being held Nov. 28-Dec. 2, 2016, in Honolulu, Hawaii, Fregosi will explain how the research team tested the two methods simultaneously by deploying their glider and float in the area where cabled, bottom mounted hydrophones were installed. The cabled hydrophone array localized where animals were, and by using those locations researchers evaluated the performance of the glider and float. By recording simultaneously on all the different instruments, data on the capabilities of the autonomous systems was collected.

The most challenging factor in this testing was fighting bad weather to deploy and recover the instruments.

"We had a very short window of time we could work in the area of the cabled array when there wasn't a Navy exercise, so we didn't really have much time to wait for good weather days. Plus, we had communication problems with the glider halfway through the deployment," Fregosi said. "We weren't getting regular messages every 6 hours as usual, so going out to recover it was a bit like looking for a needle in a haystack. But we got lucky that the glider managed to communicate right when we needed it to."

The test of how well these instruments can detect different species of marine mammals, and over what distances, is just the first step towards using acoustic data collected by gliders and floats to estimate marine mammal density and abundance. These are critical measurements for conservation and management of these protected species.

The next step is to take these comparisons and use the baseline detection capabilities of these instruments to better interpret results in a biological context. Fregosi looks forward to putting these tools to work. "I am hopeful that the use of gliders and other autonomous instruments will become more widespread. They can lead to data collection in areas that weren't feasible to study before," Fregosi said.