In Kentucky, the U.S. Forest Service and state partners are on a mission to find out why streams such as Horse Lick Creek, a tributary of the Cumberland River basin, are no longer able to support freshwater mussels – and they’re using the mussels themselves to solve the mystery.
North America is home to the world’s greatest variety of freshwater mussels, with about 300 currently recognized species; the Cumberland River basin alone historically hosted some 90 mussel species.
Unfortunately, many mussel species are extinct or gravely imperiled. Widespread destruction of river and stream habitat by dams is a major factor in mussel imperilment, but populations nationwide continue to decline for poorly understood reasons.
The Cumberland River basin stands to lose as much as half of its native mussel diversity, and several species are on the verge of extinction. These alarming mussel declines suggest widespread and serious degradation of water quality, which ultimately will affect all users of water resources.
The good news for Kentucky is that the state’s Center for Mollusk Conservation, founded in 2002 by Dr. Monte McGregor of the Kentucky Department Fish and Wildlife Resources (KDFWR), has developed the capacity to propagate juvenile mussels in a hatchery environment. These efforts include culturing the diatoms and other food items needed to grow the salt-grain-sized larvae into young mussels that can be reintroduced into streams they once inhabited.
“The importance of what Monte and the center have achieved cannot be overstated,” says Wendell Haag, Forest Service fisheries research biologist co-located at the center near Frankfort. “The center can raise thousands of young mussels of species such as the golden riffleshell that may be reduced to only a handful of individuals in the wild. The near perfection of these methods has the potential to save many mussel species teetering on the brink of extinction, and it provides a ready supply of juvenile mussels for experimental studies addressing the causes of mussel declines.”
The bad news for Horse Lick Creek is that despite decades of conservation and restoration activities (it was designated a bioreserve in 1992), the mussel populations it supports – some globally important — continue to decline at an alarming rate. Until reasons for the decline are found, Horse Lick Creek is not a good candidate for restoration using propagated mussels.
“The stream, much of which runs through the Daniel Boone National Forest (DBNF), has been the focus of federal and state conservation efforts for some time,” says Haag. “Yet mussel density and species richness have decreased sharply throughout the creek since the early 1980s, and most of the species that inhabited the creek at that time now appear to be gone.”
In general, mussel declines in the Cumberland River system and elsewhere are attributed to habitat destruction, non-point source pollution, and mineral development, but the specific factors responsible for mussel declines in Horse Lick Creek remain unknown. Water quality testing and biological surveys of fish and other aquatic species show the stream to be relatively healthy and, presumably, a good candidate for mussel restoration, yet the mussels continue to disappear.
“This stream should be able to serve as a mussel refuge, and people thought of it that way until recently,” says Haag. “Something happened here in the 1980s and it happened fast. We’re trying to find out what that was. The disappearance of mussels suggests some fundamental problem that we haven’t been able to identify with traditional approaches.”
Haag is approaching this mystery by using as sleuths juvenile mussels propagated by the Center of Mollusk Conservation. It makes sense, really. Measuring water quality and other aspects of the stream’s ecology provides indirect clues to factors affecting mussels, but the mussels themselves may tell the story more directly.
Haag and his team constructed concrete “silos” designed to stay in place at the bottom of the stream. In the center of each silo is a chamber that holds the juvenile mussels and provides them with constant water flow. In a related study in 2015, the team found that mussels in the silos did not grow in streams that are experiencing mussel declines—including Horse Lick Creek—but they grew normally in healthy streams, in some cases more than tripling in size in a few months. This finding suggests that mussel growth can serve as a direct measure of stream impairment.
During the summer of 2016, they placed silos at 12 locations throughout the Horse Lick watershed, in an attempt to identify sources of contamination that may be responsible for reduced growth. Unfortunately, mussels showed growth inhibition at all 12 locations.
In addition to information from the mussel silos, the researchers also collected traditional measures of stream health at each location. These included water quality and fish samples, as well as diatom and bacterial samples collected to assess food resources available to mussels. Analysis of these samples is ongoing, but Haag hopes that these disparate pieces of information will provide clues to the possible sources of water impairment that will in turn guide future measures to address pollution sources. But for now, the reasons for the mussel decline remain a mystery.
“Although much of the basin of Horse Lick Creek is protected within the Daniel Boone National Forest, the headwaters have a more heterogeneous mixture of land ownership,” says Haag. “Possible sources of pollution include illegal dumps, raw sewage, mines, farming, and runoff from small towns. A big factor in Horse Lick Creek is that it is underlain by extensive karst formations and cave systems. These underground conduits can bring groundwater from great distances, sometimes even from beyond the topographical boundaries of the watershed. This really complicates our job of pinpointing the sources of contamination.”
“Horse Lick Creek is just one of many streams across the country that are experiencing enigmatic mussel declines. We hope our method of using juvenile mussels as direct indicators of stream health will be useful across the state and beyond.”
The Horse Lick Creek project is a collaboration involving the U.S. Forest Service Southern Research Station and Daniel Boone National Forest, U.S. Fish and Wildlife Service, Kentucky Department of Fish and Wildlife Resources, Kentucky Division of Water, University of Kentucky, and Kentucky State University.
For more information, email Wendell Haag at firstname.lastname@example.org.