Sonar Technology Helps DNR Fisheries Researchers Monitor Predator-Prey Dynamics of Lake Michigan
September 17, 2009
When state fisheries managers decided in 1966 to introduce Pacific salmon into Lake Michigan, exotic prey fishes -- specifically alewife and rainbow smelt -- had overtaken the Great Lakes, causing devastating affects on people in the region.
Not only was there the rise of alewives and smelt, but the system's top native predator, the lake trout, was in serious decline. The result was a lack of a natural predator-prey relationship in these huge freshwater resources
As a result, windrows of decomposing fish formed where alewives and smelt had washed up on pristine beaches. Water-intake pipes were clogged with decaying fish. And the overall fishery was imperiled
Fast forward 40-plus years and jump aboard the Department of Natural Resources' Survey Vessel (SV) Steelhead and you will discover part of the reason why dead prey fish on the beach are a rare occurrence. And why the recreational fishery for salmon in Lake Michigan is flourishing these days.
In coordination with researchers from the United States Geological Survey's Great Lakes Science Center, the crew of the SV Steelhead conducts an annual hydroacoustic (sonar) and mid-water trawl survey of alewives and other prey fishes in Lake Michigan to get a snapshot of current conditions.
"The acoustic and mid-water trawl survey is done at night starting in mid-August," explained Randy Claramunt, a research biologist at the DNR's Charlevoix Fisheries Research Station. "It usually takes about three weeks to complete."
Once the survey work is completed, estimates of total prey fish abundance are generated by managers and are used to balance predator-prey dynamics. Fisheries managers adjust salmon stocking rates to keep alewife abundance in check with lake productivity levels.
How is it possible to estimate the number or biomass (total weight) of alewives in Lake Michigan in just a few weeks?
"There is a two-part answer to that question" Claramunt said. "The first is through technological advances in fisheries surveys in the last 10 years. The second is our partnership with the USGS."
The acoustic mid-water trawl survey is extremely efficient. The data collection process is almost entirely digital; laptop computers gather sonar counts of prey fish. The opening size of the net and its depth in the water is controlled by boat speed and how far the trawl is let out. Digital sensors that weigh about 15 pounds each are tied to the trawl in mesh bags and communicate with a computer in the pilot house.
"One sensor estimates the depth of the trawl by measuring water pressure, and the headline sensor and two wing sensors use sonar to record the height and width of the opening," Claramunt said. "The information is displayed graphically on the computer so we can adjust speed and cable length to make sure we have the trawl in the correct location to catch the target fish at a given depth layer -- often in several hundred feet of water."
In addition to the fish counts, a small-mesh net equipped with a water-chemistry profiler is lowered to depths of more than 200 meters (656 feet) to get estimates of zooplankton and other small invertebrates such as mysis (freshwater shrimp). Temperature, conductivity and turbidity readings from the surface to the lake bottom are collected at a rate of six times per second.
All this information is recorded digitally and is immediately available for analysis.
"On a given night of sampling we can trawl up to five miles at depths ranging from 30 to 250 feet, collect up to 100 miles of acoustic data and gather information on all trophic levels of the Lake Michigan food web," Claramunt said.
At the same time the SV Steelhead is surveying Michigan's waters of Lake Michigan, the USGS-GLSC crew is surveying the western shoreline using the exact same methods.
"Cooperation with the Michigan DNR makes this survey possible," said Dave Warner, a research biologist with the USGS-GLSC. "If we did not have the ability to use two vessels, then our uncertainty in estimates of prey fish from the survey would be much higher because we would not be able to cover an adequate amount of lake and water."
An acoustic and mid-water trawl survey of Lake Michigan's prey fish community is comparable in scope to ocean surveys for herring and other open-water marine fish stocks.
In addition to the challenge posed by the size of Lake Michigan, prey fish abundance and distribution can vary greatly from year to year. For example, the lake's alewife biomass averages around 38,500 tons (77 million pounds). But the maximum change observed from one year to another from the acoustic survey can be even more than that -- up to 40,000 tons (80 million pounds).
Understanding the factors influencing this rate of change is of central concern to biologists as new invasive species, such as zebra mussels and quagga mussels, and environmental factors such as global climate change pose new threats to the sustainability of the Great Lakes.
Conducting annual surveys of the food web and using the most up-to-date technology -- such as hydroacoustics -- are essential to mitigating the impacts from these ongoing threats and to maintaining a viable and healthy fishery, which is one of the DNR's goals for managing the Great Lakes' resources.