(To hear a radio show about this mystery, click here.) Welcome to CSI-Chesapeake Bay.
Inside a morgue-like room furnished with a steel table spread with hypodermic needles and glass vials, Dr. Vicki Blazer, a fish pathologist, wields a scalpel on a smallmouth bass.
Blazer, a veteran investigator of underwater mayhem, is trying to solve a mystery. She works for the U.S. Geological Survey’s National Fish Health Laboratory in Leetown, West Virginia, and she’s trying to discover the cause of a mass killing that wiped out an estimated 80 percent of the smallmouth bass in the Shenandoah River in 2004 and 2005.
She slices a cut along the bottom of a 16-inch long smallmouth bass, moves aside the pale pink intestines, then finds what she’s searching for.
“And then we have this organ right here, it’s actually a paired organ, kind of white – that’s the testes,” Blazer says, lifting the paper-clip sized organ onto a scale for weighing.
In hunting for things that might have killed the bass, Dr. Blazer stumbled across something odd: more than 80 percent of the males she examined were growing eggs inside their testes.
They were “intersex” – in other words, fish with both male and female sexual characteristics. And it wasn’t just a few fish in the Shenandoah River. The sexual confusion was widespread among more than 500 fish she examined in the Potomac, Susquehanna and eight other rivers in the Mid-Atlantic region. Significantly, the gender-bending bass were much more common in civilized areas than in streams out in the woods, where about 10 to 14 percent of smallmouth bass have intersex characteristics.
Dr. Blazer (at left in photo above) suspected that drugs – including birth control pills – flushed down toilets, when combined with other chemicals flowing out of waste treatment plants, were mimicking female hormones and screwing up both the sexual development of the fish and their immune systems.
“I don’t think it’s a coincidence that we are seeing fish kills in the same places we are seeing intersex and some of these indicators of endocrine disruption,” Dr. Blazer said. “I think that potentially some of the same chemicals that would cause intersex can also affect immune response.”
A lot of the fish that died in the Shenandoah had fallen victim to a variety of common bacteria, viruses and parasites. “So it suggests to me that whatever’s there, the fish are susceptible to because they are immunosuppressed,” Dr. Blazer said.
She said that means they have weakened immune systems – not unlike a person with cancer or AIDS – and can be killed more easily by common germs found many places in the environment, she said.
But in a plot twist to this mystery, the drugs-in-sewage theory was recently dealt a setback. Dr. Blazer tested fish near two sewage plants in Western Maryland and found the same high prevalence of feminized males upstream as well as downstream.
That left pesticides, herbicides and industrial chemicals called PCBs among the possible suspects – because they were also found upstream can also chemically mimic estrogen.
But these suspects are also difficult to pin down. It’s not clear that these or any chemicals – or even the sexual abnormalities in the smallmouth bass – have been hurting the ability of the fish to reproduce.
“Actually they are doing quite well,” Mullican said of smallmouth bass. “Our catch rates for what we call stock-size smallmouth….are the highest that we’ve seen in the last 20 years of monitoring.” This could either mean that the intersex bass are still perfectly male enough to produce plenty of offspring. Or the harm to their reproduction could be subtle and long-term, and masked thelast few years by good weather conditions that have boosted the hatching of eggs and survival of young fish, Mullican said. Perhaps a combination of multiple chemicals is having an effect on the fish that is unknown now, but will surface in future generations. Which ever direction the case takes, the investigation is still open and continuing. There’s no question that something killed all those fish five years ago – and that male fish growing eggs is a hint of a natural world flipped upside down and perhaps a chemical assault on aquatic life. But who done it? “It’s definitely a mystery, yes, still,” Dr. Blazer said. John Mullican, a biologist with the Maryland Department of Natural Resources (pictured on right in photo), said that the abundance of smallmouth bass – as measured by how easy it is to catch them -- appears to have only increased during the years since Dr. Blazer discovered their sexual issues.
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