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What Small Smallies Can Tell Us About Water

Male smallmouth bass defending the nest.

Male smallmouth bass defending the nest.

In the wee hours of a sunny day in June, a party of three took off to go bass fishing on a modest lake set in the forests of northeastern Minnesota. Despite the lake whitefish and walleye that live here, the trio was most definitely after bass. Little bass. So little, in fact, that if they didn't have fins you might mistake them for a grain of wild rice.

Sarah Kadlec, Ph.D. Candidate in the Integrated Biosciences Graduate Program at the University of Minnesota Duluth, particularly wanted these small smallies. Kadlec seeks to provide a scientific context for a troubling phenomena: researchers have been finding bass with testicular oocytes in other words, intersex fish and are unsure why.

The little fish Kadlec sought were to shed light on the correlation between endocrine disruption and estrogen in natural waters. While the science community has long suspected that estrogenic compounds in the effluent flowing from wastewater treatment facilities can alter the development of fish living nearby, baseline rates of reproductive abnormalities in fish inhabiting relatively unpolluted waters are scarce. Smallmouth bass seem to exhibit testicular oocytes with much more frequency than other species of freshwater fish.

Kadlec is conducting this study along with Patrick Schoff, Research Associate at the Natural Resources Research Institute, and with researchers at the Mid-Continent Ecology Division of the Environmental Protection Agency. Prior to this day in June, Kadlec, Schoff and Fond du Lac fisheries biologist Brian Borkholder had scoured the lake for smallmouth bass nests a difficult task, given spring's temperature fluctuations. Despite the reality that 2015 could have been a failed spawning year for bass in this lake, the team optimistically set out in search of nests with offspring in the "pre-swim-up" phase.

Borkholder captained the boat to potential spawning areas that were programmed into a handheld GPS unit. When the team reached these spots, Borkholder and Schoff, both wearing wetsuits, engaged in the difficult task of spotting smallmouth bass protecting their nests. A few times they found nests but no offspring. Other nests had fry past the swim-up phase where they would be too difficult
to collect.

Close-up of smallmouth bass at the pre-swim-up, yolk-sac stage.

Close-up of smallmouth bass at the pre-swim-up, yolk-sac stage.

After a challenging afternoon search in windy conditions, the team, with Borkholder and Schoff donning SCUBA tanks and armed with handheld suction tubes, were finally able to collect about 1,500 smallmouth bass yolk-sac larvae. The larvae were carefully packaged for the two-and-a-half hour ride back to Duluth. There, the fry were transferred to separate holding tanks outfitted with specialized pumps that delivered extremely low concentrations of estrogen.

Over the course of the next 90 days, sample groups were periodically removed and studied histologically, partially aided by Minnesota Sea Grant's former student assistant Rachel Kuntz. With the experimental phase of the study complete, Kadlec is now analyzing data.

This particular experiment marks the end of a four-year project studying the potential for estrogenic compounds to disrupt the reproductive systems of developing smallmouth bass.

In addition to rearing bass in a controlled environment, Kadlec also examined tissue samples from specimens originating in the St. Louis River, its estuary, and several inland lakes in Minnesota. She is comparing testicular oocyte rates in wild fish relative to human influence and to the laboratory-reared fish. Kadlec, the 2015 Lake Superior Chapter of Muskies, Inc./Minnesota Sea Grant scholarship winner, presented her research to chapter members this autumn.

"What is surprising is that male smallmouth bass from lakes with no known estrogenic sources exhibit testicular oocytes at a rate of 7 43 percent," said Kadlec. She reports that bass taken from near wastewater treatment plants ... from waters that presumably contain more endocrine disrupting compounds than average ... show testicular oocytes at a rate of around 55 percent. "Toxicological studies with smallmouth really place the issues of water contamination in a context that's immediately important to many Minnesotans," said Kadlec.

Kadlec's emerging results and interest in communicating them are helping water resource managers, wastewater treatment facility operators and fishermen understand the nuances of estrogenic chemicals and their effect on smallmouth bass. Kadlec is not able to explain the among-lake variation she detected in male smallmouth bass but she is interested in pursuing this line of inquiry and the members of the Lake Superior Chapter of Muskies, Inc. are cheering her on.


By Ryan Strother with Rachel Kuntz and Sharon Moen
December 2015

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