USVs Contribute to Ship-Based Fish Surveys
A new NOAA Fisheries study demonstrates that unmanned surface vehicles can expand the range and duration of ship-based acoustic fish surveys. Recent advances in autonomous vehicles, together with the development of highly capable fishfinders (echosounders) with low-power requirements, have made long-term acoustic measurements of fish abundance possible.
Manned research vessels and oceangoing robots each have strengths and limitations. Together, they can accomplish much more than either alone.
Ship-based acoustic-trawl surveys combine acoustic measurements with trawl sampling. This allows scientists to identify and measure fish and collect other vital biological data for stock assessments. “Acoustic instruments basically just see a swim bladder, not what fish it’s in,” explains Alex De Robertis, the NOAA scientist who led the study. “But ships are expensive, and can cover only so much sea.”
Unmanned surface vehicles cannot collect trawl samples, and therefore cannot identify or measure the fish seen by the fish finder. But robots cost less to deploy than manned vessels, and they have endurance: powered by wind and sun, they can go farther for longer.
DeRobertis wanted to evaluate the potential for unmanned surface vehicles to make long-term acoustic measurements. His team collaborated with other groups to equip a pair of saildrones with fish finders and deploy them for months. They made ‘follow the leader” comparisons with the state-of-the-art research NOAA ship Oscar Dyson in areas of the Bering Sea where midwater fish are dominated by walleye pollock.
The team found that the saildrones collected high-quality acoustic data over a long-term deployment of 103 days. The numbers of fish detected by saildrones compared well with those of the Oscar Dyson.
However, in some cases, the fish appeared to dive when Oscar Dyson, a noise-reduced vessel designed to minimize fish avoidance reactions, approached. This happened only when the fish were at shallow depths (<100 m) and was more pronounced at night. Fish at deeper depths showed no avoidance response to the ship. So this isn’t really a problem for NOAA Fisheries surveys in Alaska, which typically occur during the day when fish are at deeper depths.
NOAA Fisheries has already begun to explore the many possibilities that unmanned surface vehicles equipped with acoustic fishfinders offer for research that could not be done before.
“We’ve just completed some Arctic work in the Chukchi Sea. Alaska’s Arctic is an ideal area for a drone survey because its midwater fishes are dominated by a single species, Arctic cod. The whole survey was done by robots. Without trawl samples from ships, the data we are collecting are not up to the stringent standards required for fisheries management. But the saildrones are collecting information where we had none before,” De Robertis said.
Visit NOAA’s website for more details.