Last Thursday afternoon, fisheries biologist Paul Piavis was removing ear bones from yellow perch caught earlier in the week in the Patuxent River, where once there was a thriving fishery for yellow perch and where one might be built in the future.
By removing and sectioning the ear bones and counting the calcified rings, much as one might count the rings on a tree stump, Piavis said biologists can get a read on age and growth rates.
But, he said, ear bones reveal only a part of the mysteries surrounding yellow perch, a species that inhabits the upper reaches of tidal rivers and creeks.
Each winter, Piavis and other Department of Natural Resources personnel run net surveys in a number of Maryland rivers where yellow perch gather before spawning.
"Basically, we are looking at age structures of the populations," said Piavis, whose teams set fyke nets in the Patuxent and Nanticoke rivers last week and soon will move into the Choptank River and the upper Chesapeake Bay.
"But the time frame of study is very compressed," he said, "because the only effective time to sample the population is just before or during the spawning run."
And during that period, which began last week and runs through mid-March, DNR crews can sample only a few of the rivers that hold yellow perch.
In an effort to gather additional information, Piavis said, this year DNR has asked an organization of recreational fishermen, the Coastal Conservation Association, to run a volunteer catch survey in the Nanticoke watershed and the watersheds of other Eastern Shore rivers.
But he also hopes to get reports from the Magothy, Severn, South, West and Rhode rivers, which have been closed to catch-and-keep fishing for yellow perch the last 10 years.
"Much of the information we have gathered [on the middle bay rivers of the Western Shore] is anecdotal," Piavis said. Information from local fishermen has ranged from reports of sediment on egg strands below the Lake Waterford Dam on the Magothy to accounts of catching and releasing 60 to 70 yellow perch an hour on College Creek off the Severn River.
"And while it seems like the populations in those rivers are doing well the last couple of years, we want to know more," he said. "Information from this volunteer survey will give us a ballpark figure, and if we see older, larger fish, that would be a crude way of looking at stock structure."
If strong growth can be documented, it's possible that those Western Shore rivers eventually could be reopened to catch-and-keep fishing, he said.
In general, he said, yellow perch populations "in most river systems are stable, but expanding," and in the Magothy and Severn rivers, populations appear to be up significantly from the numbers of five to eight years ago.
But yellow perch numbers are down considerably from the heydays of perch fishing in the mid-1960s and '70s.
A decade ago, when Piavis went to work for DNR, one theory about the decline of yellow perch in the heavily developed watersheds of the Magothy, Severn and South rivers was that acid rain was at fault.
"That never quite made sense, because yellow perch is an acid-tolerant species," Piavis said. "But with changes in land-use patterns -- residential development, road building, etc. -- there would have to have been increased sedimentation."
Yellow perch produce strands of eggs that attach to vegetation or other objects in the water, which make them susceptible to increases in sediment washed into streams and creeks from heavy rain or snowmelt.
Piavis said a 1970s study showed that a 50 percent coverage of sediment killed the eggs.
Watershed protection programs implemented by state and federal governments and private conservation groups have improved water quality and yellow perch populations.
But late each winter it still is something of a guessing game, trying to figure when and how successfully the yellow perch will spawn.
The yellow perch surveyed on the Patuxent last week included females with roe still tightly packed and large numbers of smaller fish milling in preparation for the spawn.
Once the proper combination of water temperature and hours of daylight are reached, Piavis said, the eggs in the females will increase in size and become less tightly packed.
"The optimum water temperature is 8 degrees centigrade [49 degrees F], but it is not like a trigger, where the fish say, 'Oh, we've reached 8 degrees, let's get going,' " said Piavis. "A lot of it is regulated by photoperiod more than water temperature."
Piavis said the timing of the spawn this year probably will be fairly similar to last year's, when January and most of February were unusually warm and a cold snap carried into early March.
The best guesstimate for the main two weeks of the spawn, he said, is March 8-15.
Pub Date: 2/21/99