Imaging sonar is being used to estimate the population of sockeye salmon in Ozette Lake, located in Washington State. Surveyors are using imaging sonar technology at the lake’s outlet and along its shoreline to quantify the number of adult sockeye returning to the lake.
The lake’s threatened sockeye population is protected under the Endangered Species Act and scientists do not have an adequate understanding of how many fish return annually. Though the number of fish spawning in the lake’s tributaries is well documented, less is known about a second group of spawners—beach spawners.
By providing sound pulses and converting the returning echoes into digital images, the technology is providing scientists and recovery planners with a clearer picture of the sockeye population’s size and reproductive behavior.
Previously, surveyors counted adult fish returns at a weir placed in the Ozette River at the outlet of the lake. But it was not possible to use the weir during high spring flows, when some adults return to Ozette Lake.
As a result, researchers were limited in their ability to count all returning fish. The weirs also forced sockeye to congregate below the structure prior to reaching the lake, which contributed to high rates of predation.
Other historical methods for counting beach spawners were inconsistent and variable at best. As a result, scientists do not have a complete understanding of the population’s abundance.
The new sonar technology is expected to provide more accurate data to researchers. By providing a visual image of the fish, the sonar will enable surveyors to gather data on total run size and beach spawning sockeye abundance. The new sonar also works well in high flows at the lake’s outlet and in the murky waters of the spawning beaches.
Imaging sonar allows researchers to see how deep sockeye spawn in the water column, whether they spawn in areas not previously documented, how they interact with predators, and other behaviors.
The technology was first tested in Lake Ozette in 2011, and by the 2012-2013 spawning season it was used to conduct three different surveys. During these surveys, the sonar captured images of nearly 950 live sockeye in very high densities at critical locations—core spawning areas on Olsen’s Beach and at Allen Beach.
The sonar will also be used to survey habitats that sockeye may use, but where spawning fish have not been identified previously.
The use of imaging sonar in Lake Ozette’s sockeye recovery efforts involves a partnership of NOAA Fisheries West Coast Region, the Northwest Fisheries Science Center, the Makah Tribe, and the Washington State Salmon Recovery Funding Board (SRFB).
source: NOAA Fisheries