Yellow perch might be recovering from decimation
Written by Milwaukee Journal Sentinel - jamoke   
Tuesday, 14 March 2006 04:53

When Lake Michigan makes headlines these days, it seems the news is almost always bad. Overflowing sewers. Closed beaches. Crashing fish populations. But a bit of good news is flowing out of the world's fifth-largest freshwater lake - yellow perch could be on the rebound.

Recent state and federal fish surveys show that the 2005 class of perch, perhaps the lake's most popular native fish in terms of eating and angling, is the largest in more than a decade.

The perch population in Wisconsin's Lake Michigan waters had dropped by as much as 90% since the 1980s, likely because of a combination of factors, including a disruption in the food chain linked to a flood of invasive species. The Wisconsin Department of Natural Resources also has pointed to unfavorable weather patterns and historic over-fishing as potential reasons for the problem.

But one recent DNR fish survey shows juvenile perch numbers in the fall were more than double the previous record year of 1989, the year the annual seine surveys began.

DNR biologist Brad Eggold said there are likely a number of factors for the apparent rebound, but a big one is the state's decision in the mid-1990s to stop commercial perch harvests on Lake Michigan except for the bay of Green Bay. Around the same time, the DNR dropped the recreational daily catch limit from 50 to five.

That has allowed more perch to live long enough to reproduce, Eggold said.

Despite the good news, a DNR report on the survey cautions that the perch population remains "a major concern," and there are no plans to lift the daily catch rates.

Members of the class of 2005, after all, are still not much bigger than a finger, and there is no guarantee their numbers won't be decimated in the next few months from a lack of food. Or they could be eaten by bigger fish, such as salmon or trout.

While Eggold says the survey results "were extremely promising," nobody is predicting the population will return to the glory days of the 1980s. Too much has changed in the lake since that time, thanks largely to impact of foreign species, such as the zebra mussel.

"It may be that recovery to population levels that we enjoyed prior to the early '90s is not a realistic expectation given the altered environment," states the DNR report.

The United States Geological Survey, meanwhile, conducted its own lakewide fish survey and came up with similarly encouraging numbers for baby perch. The federal survey showed they are at their highest levels in more than 30 years.

Yet the USGS survey, which looks at multiple species populations, was not all good news.

Numbers of exotic mussels on the lake bottom, for example, appear to have exploded over the past year.

The zebra mussel is the invasive species that has grabbed the most attention, but it is its slightly larger cousin, the quagga mussel, also a native of the Caspian Sea, that appears to be having the most impact under the waves.

The first zebra mussel was reported in Lake Michigan in 1990, and the quagga mussels didn't turn up until the late 1990s. But the quagga recently surpassed the zebra mussel in numbers. A likely reason is the fact that the quagga is a hardier creature that can thrive at depths beyond 200 feet. Zebra mussels typically keep to the shallow water close to shore.

These filter feeders are a worry to lake managers because they live by stripping plankton from the water. That loss of plankton can have a direct or indirect impact on every species in the food chain because the mussels themselves are not a reliable source of nutrition for most Lake Michigan fish.

The annual USGS Lake Michigan fish survey doesn't target mussels, but they turn up in the nets nonetheless, and this year's study estimates the mussel population, in terms of weight, has nearly tripled in the lake since last year.

The USGS study offers no explanation for the apparent explosion, and the study author said in an interview it may just be a fluke that so many were caught in the nets last year.

But Tom Nalepa, a biologist with the National Oceanic and Atmospheric Administration, has conducted his own mussel studies and says the mussels' range has expanded, and it is clear the quagga mussel is now a bigger threat to the lake than the zebra mussel.

"It doesn't matter whether it's shallow depths or deeper depths, it's pretty much all quaggas, and what were seeing now is quaggas are expanding much deeper into the water than zebra mussels," he said.

Quagga mussels can now be found at depths of about 300 feet, Nalepa said.

That's about the maximum depth typical in the southern third of Lake Michigan.

"Now you're looking at almost the entire (southern) lake bed as potential habitat for mussels," said Steve Pothoven, a NOAA biologist at the Lake Michigan field station in Muskegon, Mich.

The big worry is the quagga mussel will squeeze out what is left of a tiny shrimp-like creature called the diporeia, a critical food source for several fish species.

Diporeia used to be found at densities as high as 20,000 per square meter on the lake bottom. Now, in vast stretches on the southern end of Lake Michigan, they have essentially disappeared.

A direct link between the rise of foreign mussels and the decline of diporeia has yet to be made, but few biologists doubt one exists.

Nalepa said he has yet to analyze the data for an update to the diporeia study he did six years ago, but "I'm plugging into the spreadsheets a lot of zeros, and I think it's going to (show) a remarkable loss."

The alewife is another fish species biologists are watching. Their numbers have crashed in recent years, but while this year's study shows adult alewife numbers remain severely depressed, it appears the downward spike has stopped. That's important because the invasive alewife is the primary food source for chinook salmon.

The DNR reports that anglers in Wisconsin waters last year landed an estimated 419,000 chinook, the most since salmon surveys started in 1969, and 33% above the five-year average.

The catch: The average size of the salmon is continually shrinking, likely because of the depressed alewife population.

 
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