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Search 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

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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

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.

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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,

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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.

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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.

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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.

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