Plain pocketbook mussel (Lampsilis cardium) showing the lure it uses to draw fish close enough to deposit larvae in their gills as part of its unique reproductive cycle. Photo by Wendell Haag.
North America’s freshwater mussels are in grave danger of disappearing. Though there’s been progress in learning about freshwater mussel biology and effective techniques developed to propagate mussel species, conservation efforts should focus more directly on bold and aggressive habitat restoration, according to a recently published review paper by U.S. Forest Service researcher Wendell Haag.
In the article, Haag, fisheries research biologist with the Southern Research Station Center for Bottomland Hardwoods Research, and co-author J.D. Williams from the University of Florida Museum of Natural History assess and revisit the goals put forward in the National Strategy for the Conservation of Native Mussels developed in 1997 by representatives from federal, state, and local natural resource agencies, conservation groups, and commercial groups.
North America is home to the world’s greatest variety of freshwater mussel species. About 300 species are currently recognized, but this number is likely to increase as genetic methods allow more precise species identification. Many of these species are gone already or are in danger of going extinct due to the massive loss, deterioration, and fragmentation of their river habitats.
“To put it into perspective, the Mississippi River Basin alone supports three to four times as many mussel species as either the Amazon or Congo River basins, which support the world’s richest freshwater fish faunas,” says Haag. “At the same time, North American freshwater mussels have the highest extinction and imperilment rate of any group of organisms on the planet.”
The widespread destruction of riverine habitat by dams and channelization from the 1920s to the early 1980s directly caused the majority of mussel extinctions to date; most of these species were restricted to the large mainstream rivers that were dammed most extensively. Equally as troubling are the enigmatic declines and crashes of mussel populations that have occurred in the last 30 years in streams that remained unimpounded and continue to support other aquatic life.
“What’s causing these enigmatic declines remains unknown, but their characteristic and rapid effects suggest an extremely virulent and widespread factor mostly specific to mussels,” says Haag. “Although we don’t know the cause of these declines, a growing body of evidence shows that mussels are more sensitive to pesticides and ammonia than other stream organisms.”
In revisiting the goals of the national strategy, the researchers recommended more emphasis on identifying causes of the enigmatic mussel declines that have affected streams across the U.S. and Canada. They also recommended that the ability of individual streams to support mussels could be evaluated directly with in situ survival trials using juvenile mussels propagated in captivity.
Recent advances in captive propagation techniques allow many species to be produced in large numbers in a hatchery environment. 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. However, Haag cautions that this tool should be used only with careful planning and primarily to reestablish extirpated populations. The widespread use of propagation to augment existing populations poses potentially serious genetic and ecological risks, and Haag and Williams recommend that this approach be used with particular caution and mainly when a species’ global extinction is imminent.
“The primary emphasis of conservation efforts should be on habitat and mussel community restoration,” says Haag. “A critical first step in restoration is the development of a prioritized list of candidate stream reaches. Such a list should be realistic but bold. For example, removing navigation dams on large rivers such as the Mississippi is unlikely, but specific recommendations can be made on how the operation of these dams can be modified to support aquatic ecosystem values.”
High priority streams for restoration would include those affected by non-functional, unsound, or aging dams, and stream reaches affected by enigmatic declines but with otherwise intact habitat. Of particular importance are stream reaches that have potential to serve as dispersal corridors between existing mussel populations.
“What’s needed is a bold vision that’s not limited to what is perceived to be feasible in the short run, but rather one that allows for unanticipated opportunities in the future,” adds Haag. “The mussel conservation community should be ready to capitalize on the growing momentum and necessity for ecological restoration.”
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