Scientists have found reproductive abnormalities in yellow perch in three Maryland rivers that are either heavily suburbanized or rapidly developing, which they say helps explain why the distinctive black-striped fish are not thriving in those Chesapeake Bay tributaries and may be linked to toxic pollution.
Significant numbers of eggs produced by spawning female perch in the Severn and South rivers in Anne Arundel County and in Mattawoman Creek in Charles County failed to develop completely, according to a three-year survey conducted by federal and state researchers. Male perch in those rivers also displayed more abnormalities than did their counterparts from two mostly rural rivers in the upper bay.
The findings are "strongly suggestive of contaminant problems," said Jim Uphoff, a biologist with the Maryland Department of Natural Resources. Although no specific pollutant has been identified, he and other researchers say they suspect the abnormalities may stem from exposure to pharmaceuticals, heavy metals or polychlorinated biphenyls, a group of toxic chemicals once widely used in electrical equipment.
The study, appearing in the journal Science of the Total Environment, comes on the heels of a federal report last month that found that toxic contamination remains widespread throughout the Chesapeake. Nearly three-fourths of the bay's tidal waters are "fully or partially impaired" by hazardous chemicals, the report said, with problems severe enough in areas like Baltimore's harbor to warn people to avoid or limit their consumption of fish caught there.
"It's raised a lot of questions," Vicki Blazer, a fish pathologist with the U.S. Geological Survey and lead author of the study, which involved researchers from the USGS, DNR and the U.S. Fish and Wildlife Service.
Blazer has found "intersex" smallmouth bass with characteristics of both sexes in the Potomac and Susquehanna rivers, which she suspects may stem from exposure to endocrine-disrupting chemicals in either agricultural runoff or in wastewater discharges. In the case of yellow perch, however, she said, she and her colleagues saw different abnormalities, which may reflect differences in the fish species, in their habitats or in what contaminants are affecting them.
The scientists examined yellow perch from five bay tributaries during their early spring spawning runs from 2007 through 2009. They saw more abnormalities in the ovaries of female fish and in the testes of males in the South and Severn rivers and in the Mattawoman than in the Choptank River, where farming dominates its Eastern Shore watershed, or in Allen's Fresh, a largely forest-lined creek in Charles County.
With the female fish, the long, jelly-like strings of eggs that they release into the water did not appear to be forming completely, Blazer explained. The membrane covering the eggs was "very thin and irregular," she said, which she surmised could mean the eggs were not as protected from harm.
Some of the male fish had more or larger cells in their testes that secrete testosterone, according to the researchers.
The scientists collected water and fish tissue samples, but the analysis of those has yet to be completed, so it's not clear if any particular contaminants were present. While cold weather and heavy rainfall during spawning season also can influence reproduction, the researchers say they don't believe those explain the problems they're seeing. Blazer said other studies suggest that the abnormalities in the fish may be caused by exposure to contaminants, including PCBs, metals and pharmaceuticals.
One chemical that has been shown to produce the kinds of abnormalities seen in both female and male perch is dopamine, Blazer said. A hormone-like chemical naturally produced in animals that transmits signals in the brain, it's also included in medications.
Researchers say more study is needed to see what, if any, contaminants can be identified in the water, in the fish themselves or their eggs.
But one thing all three waterways with the most perch abnormalities appear to share is the extent of development along their banks. Experts have found fish populations begin to show problems when the watershed they inhabit is more than 10 percent developed. In the Severn, where yellow perch once thrived, roughly 20 percent of the watershed has been covered with buildings and pavement, according to state reports. About 15 percent of the smaller South River watershed is likewise covered. State scientists have estimated that 10 percent of the Mattawoman's drainage area has been developed. By comparison, less than 10 percent of the more rural Choptank has been built upon.
"The suburban landscape is not a very good one for fisheries," said DNR's Uphoff. While yellow perch appear to be doing better in much of the bay, he said, sub-par reproduction in some rivers and streams suggests there may be localized problems there related to population growth and development.
Yellow perch spawn in the same water way where they hatched, so whatever is happening to them could be traced to that watershed — and may be affecting other fish as well. Uphoff, noting a "major drop in the fish community" in the Mattawoman, called yellow perch "our canary in the coal mine."