by Don Schreiner
A few years ago, while walking along a small rocky Lake Superior beach near Two Harbors, my 9-year-old daughter asked me, "How many fish do you think there are in Lake Superior?" I told her I wasn't sure and it would take a lot of work to figure that out, if it could be done at all. As we wandered farther down the beach, I wondered if there would ever be enough fish to satisfy all the demands placed on the fishery—prey fish to feed predator fish, fish for people to catch, and fish to keep the aquatic ecosystem in balance.
As the Lake Superior fisheries manager for the Department of Natural Resources, I work with sport anglers, commercial netters, charter captains, environmental groups, and others. One of the major challenges of managing diverse demands on the fishery has been a lack of quantitative information on the number of prey fish available to predators. Today, new technologies have advanced to the point where it is feasible to answer the question my daughter asked: How many fish are there?
Consider the case of cisco (Coregonus artedi), an important prey fish also known as lake herring in Lake Superior or tullibee when found in inland lakes. In Lake Superior, cisco have long been the primary native forage species supporting a large lake-trout population. Cisco have traditionally been, and continue to be, Minnesota's largest commercial fishery on Lake Superior, although the harvest has declined significantly from historic highs. Fisheries managers must try to allocate the cisco among predator fish and commercial netters while leaving enough spawners so the population can sustain itself. To do this correctly, we need to determine how big the pie is before we start slicing it up.
At the peak of commercial fishing in the 1920s and 1930s, 400 licensed commercial netters in Minnesota harvested more than 5 million pounds of cisco annually. By the 1960s, cisco populations declined to levels that could no longer sustain a productive fishery due to stress from overfishing, habitat degradation, competition with exotic species (rainbow smelt), and possible predation by invasive sea lamprey.
In the early 1970s, to protect the remaining cisco, the DNR closed the fishery during the entire month of November, when most cisco spawn. This essentially shut down production of roe (fish eggs) used for caviar, an important source of income for commercial netters. In the mid-1980s, cisco populations began to recover, and commercial netters asked the DNR to regulate the harvest of this rebounding fishery so they could once again participate in the lucrative November roe fishery.
Pings and Patterns. Fisheries scientists borrowed a Navy tool and modified its use to estimate the number and size of fish in a lake. The military uses sonar to detect submarines in the ocean. Marine biologists adapted sonar technology to find fish, a technique they call hydroacoustics.
Hydroacoustics gear operates much like a sophisticated version of fish finders used by anglers: A boat tows a plastic disk called a transducer, which connects to a sonar unit that links to a computer and a GPS unit to record the boat's exact location when a signal is received. The sonar unit, through the transducer, sends sound waves called pings in the shape of a cone from the water's surface to the lake bottom. The transducer also measures the strength of the sound waves that bounce back from the bottom. If a sound wave bounces off a fish or another object in the water column, a pattern appears on the computer screen. The number of targets (fish) and target strength in decibels is recorded and stored on the computer. The GPS location for each target is also stored.
In Lake Superior, the acoustics prey survey is normally conducted at night, when fish tend to spread out and come off the bottom to feed, providing single targets that are easier to identify. As acoustic information is collected, a trawl net dragged behind a boat catches a sampling of fish. Biologists use this catch to determine what species correspond to the acoustic echoes. They also relate the size of the fish to the target strength in decibels to calculate the biomass of each fish species. After taking a representative sample of transects in an area, fisheries biologists can use the number of pings and biomass calculations to estimate the total number and biomass of fish present.
Since 2003 the DNR has collaborated with Tom Hrabik of the University of Minnesota Duluth to conduct annual hydroacoustics surveys and estimate cisco abundance in Minnesota waters of Lake Superior. Based on these surveys, the DNR has determined a total allowable catch of cisco for commercial netters in those waters.
Currently, the total allowable catch is 10 percent of the population of cisco greater than 12 inches. The allowable catch may increase if surveys estimate greater biomass or decrease if biomass diminishes. Since the DNR can now set cisco limits based on population size, commercial netters are allowed to fish cisco year-round.
The rebound of the cisco fishery during the past 20 years has increased marketing and sales of fillets and roe. Cisco fillets served at North Shore restaurants and sold at fish markets are popular during the tourist season. Cisco roe is processed into caviar and sold in many countries. Recently, world market conditions have increased the price for cisco roe, making the fishery more lucrative.
But regardless of how high the price of roe goes, the total allowable catch will still only be a certain percentage of the lake's cisco population—and thus the overall health of the cisco fishery can be sustained.
As for my daughter's question, a four-year study turned up an answer. Using hydroacoustic methods, U.S. Geological Survey biologists and scientists from the University of Minnesota Duluth collaborated with fish management agencies in Minnesota, Wisconsin, Michigan, and Ontario to determine the amount of prey fish in Lake Superior. Each year they surveyed one-fourth of the big lake. The surveys focused on pelagic prey fish, mainly rainbow smelt and three species in the whitefish family (cisco, kiyi, and hoyi). The findings: On average Lake Superior had 113 prey fish per acre—that's a total of 2.32 billion prey fish with a total biomass of 121,900 tons.
Four years after my daughter asked me how many fish there were in Lake Superior, I told her we had an answer. She said, "I don't remember asking that, but how many are there?" I told her about 2.32 billion, and she quickly responded, "Wow! I want to go fishing!"