Paul A. Smith

Milwaukee Journal Sentinel

SHEBOYGAN - Scientists last year found a record low biomass of alewives in Lake Michigan, heightening concerns about the balance of predator and prey fish in the lake as well as Wisconsin's chinook salmon stocking strategy for 2017.

Chuck Madenjian of the U.S. Geological Survey on Saturday presented a report titled "2016 Status and Trends of Prey Fish in Lake Michigan" at a meeting of the Wisconsin Federation of Great Lakes Sport Fishing Clubs.

The bottom trawling data showed a total lakewide prey fish biomass of 11.4 kilotonnes, the third-lowest on record. The three lowest totals have come in the last three years.

And significant to chinook fishing, the biomass of alewives was estimated at 0.35 kt, lowest ever.

"It's not necessarily a surprise, because we've been seeing a downward trend," Madenjian said. "But it is concerning to have such historically low levels of prey fish."

The USGS has conducted Lake Michigan forage fish surveys annually since 1973. The bottom trawls are done at the same locations and using the same protocol each year. One of the index sites is Port Washington.

It also performs hydroacoustic surveys designed to detect young-of-the-year fish.

Lake Michigan has become less productive in recent decades, principally due to the invasion of quagga and zebra mussels and a reduction in phosphorous levels. As a result, less plankton is available and recruitment of forage fish has been curtailed.

A decline in alewife numbers prompted the Lake Michigan Committee of the Great Lakes Fishery Commission last June to recommend a 62% cut in chinook stocking lakewide. Of the five primary trout and salmon species in the lake, chinook is the most dependent on alewives for food.

After receiving opposition from angling and business groups, the committee consented to a 27% cut in chinook and a 12% reduction in lake trout stocking for 2017.

Michigan, Illinois and Indiana all agreed to cut chinook plants to help balance the fishery the quickest way possible. But Wisconsin officials were persuaded by a lobbying group to hold the state's chinook stocking level and instead cut brown trout by 50% and lake trout by 58%.

The Wisconsin Lakeshore Business Association, whose members include charter captains, marinas, boat repair shops and other businesses, lobbied the Department of Natural Resources to keep chinook stocking at 2016 levels but reduce other species.

The states’ shares of chinook stocking in 2017 are 810,000 for Wisconsin, 300,000 for Michigan, 150,000 for Illinois and 60,000 for Indiana.

Wisconsin's "go it alone" decision will likely be closely scrutinized in the coming weeks and months as additional data become available.

"The committee's goal was to avoid a crash of the forage base and preserve the chinook fishery," said Jay Wesley, chairman of the Lake Michigan Committee. "It remains to be seen how individual state actions will play out."

According to the 2016 USGS bottom trawl data, bloater chubs had the highest biomass of any forage species, at 5.8 kt, followed by deepwater sculpin (3 kt) and round goby (1.1 kt).

As has been seen in recent years, the lake's alewife population is "truncated," or comprised mostly of young age classes. The older fish are likely being eaten, Madenjian said.

In addition to stocked fish, "wild" chinook are hatched each year in Michigan tributaries to Lake Michigan. In 2015, the U.S. Fish and Wildlife Service estimated 69% of chinook salmon in the lake were naturally reproduced.

The hydroacoustic survey showed alewives did reproduce last year, but the year class was 44% of average, according to the USGS. Overall, the hydroacoustic survey found the third-lowest biomass of young fish on record.

Anglers on the Wisconsin side of Lake Michigan experienced good trout and salmon fishing in 2016, but conditions were relatively poor in Michigan waters, Wesley said.

"It points out how patchy the bait distribution has been," Wesley said. "The big fish are congregating in the areas with the food."

One of the key metrics of the fishery — the predator-prey ratio — has yet to be calculated for 2016. The ratio assesses the condition of adult chinook salmon compared to the forage base.

In 2015, the ratio was 0.108, highest on record for Lake Michigan. Fisheries managers would prefer to see the ratio at about 0.05. A high number is indicative of poor chinook condition and low alewife numbers. Values over 0.1 are considered a "red flag" by fisheries managers.

In the five years preceding a collapse of the chinook fishery in Lake Huron, the predator-prey ratio averaged 0.11.

The Wisconsin DNR expects to finish its predator-prey ratio work in the coming weeks. For now, the agency is set on its 2017 stocking decisions. Beyond this year, though, it's uncertain.

"This could be a year-to-year thing," said Brad Eggold, DNR Great Lakes district fisheries supervisor. "We'll be watching it as closely as we can. I'd advise people to keep their calendars clear because we might need more meetings."