August is an important month on Lake Erie. Fisheries biologists with the Ohio DNR Division of Wildlife use the results from the August trawl survey to anticipate the future population size of Lake Erie fish, especially walleye and yellow perch. Why not use the trawl data from previous months?
While adult fish are easily caught in May, June, and July trawls, catches of spring-hatched fish during these months aren’t consistent. By August, these “age-0” fish have grown large enough to be captured in the trawl; they are too big to slip through the mesh of the net. In addition, August trawl data is more reliable for forecasting fish populations; fish hatched earlier in the year have run the survival gauntlet and are less susceptible to changes in weather, water temperature, and food availability. This allows biologists to use August trawl data from adult, yearling, and age-0 fish to project future population sizes and make meaningful comparisons to previous years.
2014 marks the 45th year of the western basin survey and 24th year of the central basin survey. The western basin is surveyed at 41 sites between the Michigan border and Huron, OH, while the central basin survey consists of 57 sites from Huron to the Pennsylvania border. One 10-minute tow of the trawl is made along the bottom at each site, and the area sampled and the numbers of fish captured are recorded from each tow. The same trawls and sites are used each year to allow comparisons from one year to the next. Following the survey, results are standardized to the number of fish captured “per hectare” (1 hectare equals about 2 ½ acres). The following are some highlights from the 2014 survey.
In the western basin, the 2014 survey resulted in good catches of age-0 walleye and age-0 yellow perch (Figure 1). While catches remain below the long-term survey averages, the walleye hatch (19.6/ha) ranked 10th highest in abundance since 1992. Age-0 yellow perch catches (668.9/ha) were much higher than the long-term average, and ranking as the 4th highest abundance since 1992. This is great news for western basin yellow perch, which have been at lower population abundance than the central basin for several years. Also of note, age-0 white perch catches were well below average.
In the central basin, the survey is split into two halves, a west-central survey conducted from Huron to Fairport Harbor, and an east-central basin survey conducted from Fairport Harbor to the Pennsylvania border. Few age-0 walleye are collected in central basin surveys as most walleye spawning takes place in the shallower, warmer waters of the western basin. However, yellow perch data from the central basin surveys are important for managing this species. In 2014, age-0 yellow perch numbers were well below average in both halves of the central basin. Catches of adult yellow perch were slightly below average in the west-central and slightly above average in the east-central (Figure 2). Age-0 white perch catches were well below average in the west-central and the east-central basins.
So what happens next? During the months ahead, results from Ohio surveys will be combined with surveys conducted in Ontario, Michigan, Pennsylvania, and New York to estimate population sizes and determine how many walleye and yellow perch can safely be harvested from Lake Erie. This harvestable portion is called the Total Allowable Catch (TAC), and when the TAC is finalized, each state and province is issued their ‘slice of the pie’. In Ohio, the daily bag limits are set for walleye and yellow perch based on our portion of the TAC. The TAC is announced every March at the Lake Erie Committee meeting, and Lake Erie bag limit changes in Ohio become effective on May 1st.
We are still learning what makes a strong or a weak walleye or yellow perch hatch. But our August trawl survey allows us to track differences in hatches from year to year, anticipate population sizes in the future, and gain invaluable information for managing Ohio’s largest fisheries.