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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Adult perch are sampled to determine if genetic or reproductive changes can explain the continued poor perch reproduction in Lake Michigan. © Ron Bruch
Adult perch are sampled to determine if genetic or reproductive changes can explain the continued poor perch reproduction in Lake Michigan.

© Ron Bruch

June 2003

Adrift on the
sea of life

The search to explain declining yellow perch populations in Lake Michigan zeroes in on the fry's ability to survive "the black hole" – those first months drifting around the big water.

DNR Communication and Education staff

Sample often in the first year | Tracking early growth
Alewives snacking on perch? | Similar results around the lake

With trawls, fine-mesh nets, diving gear and scalpels, teams of biologists have been sampling yellow perch populations and dissecting why the popular panfish remains in serious decline in Lake Michigan and Green Bay. There are several theories afloat to explain the losses and lots of researchers committed to finding out which ones hold water.

Yellow perch abundance lakewide (including Green Bay) has declined more than 90 percent since the late 1980s, when the population in the bay alone was estimated at 10 million pounds of fish. Today it is less than a tenth of that.

"With the exception of 1998, we have not been able to confirm good natural reproduction of yellow perch since 1991," says Bill Horns, DNR Lake Michigan fisheries biologist, "although we are cautiously optimistic about the 2002 year-class from Green Bay." Commercial yellow perch harvests that in 1989 were limited to 475,000 pounds from Green Bay and 320,000 pounds from Lake Michigan were reduced in stages to zero in Lake Michigan and 20,000 pounds from Green Bay. The commercial harvest ending June 30, 2002 in Green Bay dropped further to 19,000 pounds and since then, only 16,000 pounds of yellow perch have been harvested from Green Bay.

Similarly, sport harvests in Green Bay and Lake Michigan that exceeded four million yellow perch in 1991 (approximately 800,000 pounds), dropped to 242,000 fish in 2002 as populations declined. Daily bag limits were cut from 50 fish to 10 in Green Bay and five in Lake Michigan. Sport fishing for yellow perch will continue to be closed from March 16th through May 19th in Green Bay and from May 1st through June 15th in Lake Michigan to protect mature females throughout the spawning period.

Researchers from states bordering Lake Michigan including state fisheries biologists and university scientists continue to meet regularly as a Yellow Perch Task Group to collectively investigate factors that may contribute to yellow perch decline. They are examining fishing pressure, exotic invasive species, food sources, fish biology, disease and changing lake conditions to pinpoint the most vulnerable portions of the yellow perch's natural life cycle and explain their reproductive failure in Lake Michigan. The Sea Grant program has provided major support for this research. A similar task group is now being formed to develop a research agenda for Green Bay.

Sample often in the first year

Yellow perch live about seven years and reach full sexual maturity in three to four years. In Green Bay, they spawn in April and early May shortly after ice out, when water temperatures range between 44 to 52° F. Spawning starts a bit later in Lake Michigan.

DNR staff collect perch for aging, health and population surveys during summer and fall. © DNR Southern Lake Michigan Fisheries Team
DNR staff collect perch for aging, health and population surveys during summer and fall.

© DNR Southern Lake Michigan Fisheries Team

In the Great Lakes, yellow perch spawn on sandy, gravelly or rocky bottoms, where 15 to 25 watchful males may closely trail one female. As soon as she releases her ribbony strand of eggs, the males cloud the waters with a sea of milt to fertilize them. The gelatinous skein of eggs can stretch up to seven feet in length and contains up to 210,000 eggs packed into accordion-like folds. Yellow perch are random spawners, and do not construct nests. The egg masses are protected only by the gelatinous matrix that holds them together. The gelatin is apparently noxious tasting and foraging round gobies will spit out the yellow perch egg masses if they suck them in. The egg masses drift freely across the sandy bottom until they lodge in a rocky crevice or settle to the bottom. The adults neither guard the egg masses nor tend their young, abandoning them after spawning.

Typically, eggs hatch in 8-10 days but may take up to a month before fry emerge. The newborn fry are about a quarter-inch long and absorb nutrients in their yolk sac for several days before they search out zooplankton. The ravenous fry will put on 50 percent of their first year's growth in three months and grow rapidly for two years. Thereafter they tend to put on weight faster than length. Under ideal conditions, perch fry float about the lake and come back toward shore in mid-summer and early fall to find shelter and food.

Recruitment – the ability of young fish to survive that first year and eventually join the breeding population -- has been paltry for more than 15 years, Horns says."We only saw promising year-classes of yellow perch in Lake Michigan during 1995 and 1998; in Green Bay during 1998 and, perhaps 2002. In fact, our creel survey work shows that in both Green Bay and Lake Michigan, more than 90 percent of the yellow perch that anglers are catching now are from that 1998 year class." Those five-year-old fish are now the prime contributors to the breeding population. Hopes for the future hinge on their breeding success last year through next year.

Tracking early growth

To track the reproductive success of each year-class, DNR dive teams have examined traditional yellow perch breeding grounds on the bottom of Lake Michigan each spring since 1997 to count the number of drifting egg masses. Underwater surveys last June indicated yellow perch are laying eggs in good numbers compared to recent years. Divers found 11.5 egg masses per 1,000 square-meters of surveyed lakebed. "That's a fairly big increase in the number of egg masses compared to some of the earlier years," says Brad Eggold, DNR supervisor of the southern Lake Michigan fisheries. "Those eggs probably hatched as they always have."

Most yellow perch appear to perish between the time they hatch and the time they return to their near-shore spawning grounds to overwinter. "That time between hatching and coming back to near-shore water to survive the first winter – that seems to be doing the perch in," says Eggold. "That's the black hole."

"We just don't know a lot about what happens to these perch during this extremely vulnerable period," says Bill Horns. "Ideally those fish would find refuge in areas where there is vegetative cover, food and rocky protective cover, but that might not be the case. Some of our current research aims to fill that information void."

John Janssen, a senior scientist at the UW-Milwaukee Great Lakes WATER Institute, believes wind and water currents on Lake Michigan may be blowing newly hatched yellow perch across southern Lake Michigan to locations where they have limited food supplies. In mid-summer last year, Janssen and DNR crews towed large fine-mesh nets behind boats to catch yellow perch fry less than half an inch long. Due to steady winds, only 10 days after hatching, the perch fry were most abundant about 10 miles offshore; a few days later, large numbers were found as far as 25 miles offshore. "Less than two weeks after hatching we were netting good numbers of yellow perch fry a third of the way to Michigan," Janssen says. "The young fish find themselves far from home in an environment that may have little food."

The eastside of Lake Michigan tends to have a soft bottom where the microscopic plants called phytoplankton can settle out. Those microscopic plants, mainly diatoms, are the primary food source of the burrowing amphipod, which in turn is the primary food source for young yellow perch. But zebra mussels also prey on phytoplankton, and large populations of this invasive mollusk can consume a considerable quantity of available food. The zebra mussels may also filter the water, increasing the clarity so other fish can more easily see and prey upon young yellow perch.

"If those young perch get blown into a 'food desert' they've got serious problems," Janssen says. It's probable that young yellow perch swim up to the surface after hatching, get caught in the currents and, after 40-50 days wash ashore into rockier shores that provide food and refuge. But if the weather is rough and steady winds prevail, the fry can get into real trouble, really fast. First, they can't readily find their way back home. Second, they become prime target prey for other fish. Substantial numbers may never survive to make it home.

Researchers are exploring other avenues that might explain why so many fish aren't surviving long enough to enter the breeding population. Crews in June capture yellow perch during the spawning season to measure the length, weight, overall condition and sex ratio of the perch population. The fish are also tagged and released so the population can be estimated later in recapture studies. The male-to-female ratio was roughly 9:1 in the most recent sample. A little more surprising, by June only 55 percent of the females had spawned, 34 percent were not yet ready to spawn, and 11 percent were considered "ripe" but had not yet spawned. The percentage of females in these spawning surveys has been so variable that it's difficult to assess whether the results indicate a change in the population.

In August and September fisheries crews use beach seines and bottom trawl nets to sample the abundance and health of fish born earlier in the summer. The abundance of these young-of-the-year perch in early fall has been a fairly reliable predictor of yellow perch population strength in subsequent years.

Because young yellow perch tend to swim in closer to shore during the day, the standard sampling method is to walk a long beach seine with quarter-inch mesh straight out from shore. The four-foot-high net is weighted on the bottom and has floats on top to keep it perpendicular to the water. While the on-shore end of the net is held steady, the other end is walked in a sweeping arc toward the shore. Two pulls are taken at each site, and 14 sites are sampled along the coast between Kenosha and Sheboygan. Sampling via these long nets is considered an important companion to sampling with trawling nets drawn behind boats in deeper water.

There is a glimmer of hope in last year's results. For nine years, yellow perch were so scattered that seining averaged well less than one perch per pull of the net. Last year the average was 1.3 yellow perch per pull – the strongest results since the 1998 samples when 3.02 fish were captured per sample. Fish from this year-class that survive through the winter will enter the breeding population in 2004 at the earliest.

Pier fishing for perch. The perch limit for sport angling remains at five fish until studies can demonstrate that the perch population is recovering. © Robert Queen
Pier fishing for perch. The perch limit for sport angling remains at five fish until studies can demonstrate that the perch population is recovering.

© Robert Queen

Those same results were reflected in winter sampling, when gill nets of one to three-inch mesh were placed offshore of Green Can Reef off Milwaukee, one of the best spawning grounds for yellow perch. Because yellow perch do not segregate by size or sex at this time of year, it's one of the most effective times for DNR crews to effectively sample the whole population. This winter assessment is used to estimate the size, age composition and sex ratio of the yellow perch population. Results showed that the 1998 year-class continues to dominate the population as it has for the last several years. It confirms that no other strong year-classes are present to help the Lake Michigan yellow perch population rebound.

Other research teams tested yellow perch blood and hormone levels and examined the sex organs of male and female yellow perch to look for developmental changes that might indicate genetic changes, which could explain the precipitous drops in perch populations. No significant differences between yellow perch from inland lakes and the Great Lakes were noted.

Yellow perch ovaries and testes develop over the summer, fall and winter in advance of the spring spawning season. Hormone and protein levels in the blood are important indicators that the fish are healthy and capable of reproducing. Tests from the winter of 1996 through spring 1997 did not indicate reproductive problems, though it appears the sexual maturity of Lake Michigan yellow perch lags slightly behind fish sampled from inland lakes. The Lake Michigan fish are simply ready to spawn a little later in the season than their inland lake counterparts. Other microscopic differences in the body structures of inland lake and Great Lakes perch are being examined.

Alewives snacking on perch?

Other Great Lakes studies have shown that adult alewives will prey on yellow perch larvae, but no studies have quantified that possibility on Lake Michigan. Back in 1997, alewives were netted for six nights on Lake Michigan for 30 minutes after sundown. The fish were measured and preserved, and the stomach contents from 340 fish were analyzed. No larval perch were found and tests showed that 95 percent of the alewives' diet consists of copepods, a large group of freshwater crustaceans.

Similar results around the lake

Surveys by other states lakewide indicate similar weak yellow perch populations and poor young-of-the-year survival since 1989. Wisconsin, Michigan and Indiana studies estimate the population drop-off at 90-95 percent, while Illinois studies show an 86 percent drop in the waters off their portions of the Lake Michigan coastline.

Though the trends in Green Bay are similar to Lake Michigan, differences in the physical and biological environment mean that somewhat different factors may be at play in that warmer, shallower, more sheltered environment. A series of public workshops in 2002 led to the formation of a new Green Bay Fisheries Research Group, which will develop a research agenda to examine unique conditions in Green Bay. The roles of white perch and cormorants will be of interest to that group, along with the many factors already discussed regarding Lake Michigan.

Agencies around the lake are reaching ever wider to find strategies to help sagging yellow perch populations recover. Worldwide perch experts will convene in Madison for the first time in eight years on July 20-24, 2003 at the Third International Percid Fish Symposium. Experts will provide updates on perch management and breakthroughs in percid aquaculture. Special sessions will examine the status of yellow perch in the Great Lakes, in the hope that sharing results will lead to further strategies and options for bringing back robust populations of this Midwestern staple of fishing, fun and fine eating.