By EurekAlert
Monday, September 14, 2009
Intersex in smallmouth and largemouth basses is widespread in
numerous river basins throughout the United States is the major
finding of the most comprehensive and large-scale evaluation of the
condition, according to U.S. Geological Survey (USGS) research
published online in Aquatic Toxicology.
Of the 16 fish species researchers examined from 1995 to 2004,
the condition was most common by far in smallmouth and largemouth
bass: a third of all male smallmouth bass and a fifth of all male
largemouth bass were intersex. This condition is primarily revealed
in male fish that have immature female egg cells in their testes,
but occasionally female fish will have male characteristics as
well.
Scientists found intersex fish in about a third of all sites
examined from the Apalachicola, Colorado, Columbia, Mobile,
Mississippi, Pee Dee, Rio Grande, Savannah, and Yukon River basins.
The Yukon River basin was the only one where researchers did not
find at least one intersex fish.
Although intersex occurrence differed among species and basin,
it was more prevalent in largemouth bass in southeastern U.S.,
where it occurred at all sites in the Apalachicola, Savannah, and
Pee Dee river basins, said Jo Ellen Hinck, the lead author of the
paper and a biologist at the USGS Columbia Environmental Research
Center. The researchers also documented intersex in channel catfish
for the first time.
"Although the USGS has already documented the severity of
intersex in individual basins such as the Potomac, this study
reveals the prevalence of intersex is more widespread than anyone
anticipated, said Sue Haseltine, associate director for biology at
the U.S. Geological Survey. "This research sends the clear message
that we need to learn more about the hormonal and environmental
factors that cause this condition in fish, as well as the number of
fish afflicted with this condition."
The study, said Hinck, presents the observed occurrence of
intersex in a variety of freshwater fish species, but not potential
causes. "This study adds a lot to our knowledge of this phenomena,
but we still don't know why certain species seem more prone to this
condition or exactly what is causing it. In fact, the causes for
intersex may vary by location, and we suspect it will be unlikely
that a single human activity or kind of contaminant will explain
intersex in all species or regions," she said.
For example, said Hinck, at least one of their sites with a high
prevalence of intersex - the Yampa River at Lay, Colo. - did not
have obvious sources of endocrine-active compounds, which have been
associated with intersex in fish. Such compounds are chemical
stressors that have the ability to affect the endocrine system and
include pesticides, PCBs, heavy metals, household compounds such as
laundry detergent and shampoo, and many pharmaceuticals. Yet other
study sites with high occurrence of intersex were on rivers with
dense human populations or industrial and agricultural activities,
which are more generally associated with endocrine-active
compounds.
"We know that endocrine-active compounds have been associated
with intersex in fish, but we lack information on which fish
species are most sensitive to such compounds, the way that these
compounds interact to cause intersex, and the importance of
environmental factors," Hinck said. "Proper diagnosis of this
condition in wild fish is essential because if the primary causes
are compounds that disrupt the endocrine system, then the
widespread occurrence of intersex in fish would be a critical
environmental concern."
Specific river basin results include:
- Intersex smallmouth bass were found in a third of male bass at
almost half of the sites examined in the Columbia, Colorado, and
Mississippi River basins. The percentage of intersex smallmouth
bass ranged from 14 to 73 percent at different sites. It was
highest (73 percent) in the Mississippi River at Lake City, Minn.,
Yampa River at Lay, Colo. (70 percent), Salmon River at Riggins,
Idaho (43 percent), and the Columbia River at Warrendale, Oreg. (67
percent).
- Intersex largemouth bass were found in nearly a fifth of the
fish examined from the Colorado, Rio Grande, Mississippi, Mobile,
Apalachicola, Savannah, and Pee Dee River basins; intersex was not
observed in male largemouth bass from the Columbia River Basin. The
percentage of intersex largemouth bass per site ranged from 8 to 91
percent and was most prevalent in the southeastern United States.
The Pee Dee River at Bucksport, S.C., contained the highest
percentage of intersex fish (91 percent), with high percentages
occurring elsewhere on the Pee Dee too. Sixty percent of male bass
examined at the Apalachicola River at Blountstown, Fla., were
intersex, 50 percent in the Savannah River at Port Wentworth and
Sylvania, Ga, 43 percent in the Savannah River at Augusta, Ga., and
30 percent in the Chattahoochee River at Omaha, Ga., and the Flint
River at Albany, Ga. Lower percent intersex (10-25 percent) were
found in bass from sites in the Mobile River in Alabama.
- In addition, relatively high proportions of intersex largemouth
bass were observed at three sites in the lower Rio Grande Basin
including Rio Grande at Brownsville, Texas (50 percent), Rio Grande
at Falcon Dam, Texas (44 percent), and Rio Grande at Mission, Texas
(20 percent). In addition, 40 percent of male largemouth bass from
the Colorado River at Imperial Dam, Ariz. and at the Gila River at
Hayden, Ariz., in the Colorado River Basin were intersex.
SOURCE