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Why Are Yellow Perch Disappearing From Our Rivers?

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The U.S. Fish and Wildlife Service is getting closer to discovering why yellow perch have been reproducing at lower and lower levels.
Jonathan Wilson
The U.S. Fish and Wildlife Service is getting closer to discovering why yellow perch have been reproducing at lower and lower levels.

It's just after 9 a.m. on the South River just outside of Annapolis, and a team of U.S. Fish and Wildlife biologists are tossing dozens of fish back into the water. The batch, which was caught in nets left overnight in the shallows, is mostly made up of catfish and white perch. That's why they're going back into the river.

Fred Pinkney, the senior member of the team, is looking for yellow perch: stouter fish in the bass family that can grow 10 inches long.

Pinkney finally spots a female in the bottom of the bucket -- the greenish-yellow coloring and dark vertical striping is unmistakable.

"This is a female... very swollen belly there," he says. "So she has not spawned yet. You can see how wide her belly is, and when we go back to the laboratory, you should be able to get a really good look at how many eggs are contained within this one fish. So this is exactly the stage that we're looking for."

Protecting Yellow Perch eggs

The eggs are key. Pinkney and scientists from the U.S. Geological Survey and Maryland Department of Natural Resources are trying to figure out why yellow perch egg hatching success has dropped from 80 percent in the 1950s to less than 10 percent in the past decade.

And that's despite decades of bans on both commercial and recreational yellow perch fishing in many local rivers.

"So the two rivers that are doing really poorly are the Severn and South Rivers, right near Annapolis," Pinkney says.

The team is under a bit of a time crunch. Yellow perch only spawn for three to five days each spring, and water temperature can affect when and how fast everything happens.

The scientists would like to collect 20 fish — 10 females and 10 males. But on this morning, they only get five fish -- all female. Three of the five have already ejected their eggs.

Pinkney says that isn't necessarily a bad omen for this year's hatch; it's just a bit frustrating.

"It's mostly luck of the draw," he says. "It's tricky with temperature, and we had the warm weekend and then it got cool, and it rained. It's just hard to figure out what's going on."

The team decides to reset the nets and come back in a few days, but they'll still bring the five fish back to the lab to collect egg and tissue samples.

Environmental consequences of chemicals

Vicky Blazer, a fish pathologist for the U.S. Geological Survey, and her colleague, USGS biologist Luke Iwanowicz, await the yellow perch back at the Fish and Wildlife laboratory.

They'll take samples of the kidney, liver, brain, and reproductive organs, and the tissue will be examined at the molecular level for abnormalities. The team will pair that data with water chemistry results from the past year, and hopefully, get a clearer picture of why yellow perch are losing their ability to reproduce.

The scientists would also like to test the tissue samples for chemical abnormalities, but there's just one problem.

"We currently do not have funding to look at the contaminant analysis, so they'll go in the freezer and we'll hope for the best," Blazer says with a wry laugh.

She does have theories about what contaminants they'll find in the yellow perch tissue whenever that analysis is done. Blazer says she expects to see classic, legacy contaminants like PCBs and mercury, or the banned pesticide DDT, but she also thinks they might find evidence of contaminants that have popped up on the environmental radar more recently.

"We haven't thought that much about the hormones that all of us excrete, or that are used in things like birth control, or hormone replacement therapy," she says, "[and] all the pharmaceuticals that people are taking now that are getting into the aquatic environment and also into fish."

It's also possible that a single contaminant isn't to blame.

Environmental scientists around the Bay have spent the past decade studying the level of urbanization around local Chesapeake tributaries, and how it coincides with certain fish populations.

There's mounting evidence that once urbanization reaches a certain level — just 10 percent pavement versus natural soil — the drainage and chemistry changes to adjacent waterways are too much for fish to handle.

Fred Pinkney says, unfortunately, the yellow perch reproduction problems are likely connected to what's happening or what could soon happen to other types of fish.

"Perch is an indicator of the level of development within a watershed. And so other fish species tend to track along in terms of number when the perch go down," he says.

The scientists will find out about the water chemistry results in the next few months. But that contaminant analysis, which could help solve the mystery of the yellow perch once and for all, will have to wait for funding.

Luckily, the tissue samples can last for years in the freezer, and Vicki Blazer says with the current federal budget situation, they may have to.

[Music: "Float On (Karaoke Version)" by Stingray Music from Karoake - In the Style of Modest Mouse, Vol. 1]

Photos: Yellow Perch


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