Illinois Natural History Survey - University of Illinois

INHS Reports November-December 1998

Zebra Mussel/Unionid Interaction: Remediation Strategies

The zebra mussel, Dreissena polymorpha, is a small (up to 40 mm total valve length) bivalved mollusk native to rivers near the Caspian and Ural seas. Its life cycle includes a free-floating larval stage followed by juvenile and adult stages where the mussel is attached by byssal threads to firm substrata. These two life-history traits contribute to the success of zebra mussels in colonizing much of central Europe and North America.

In North America, the zebra mussel was first reported from the Great Lakes in 1988 and likely was introduced during 1985-1986. Initial concern about the negative effects of zebra mussel colonization on unionid bivalves native to North America has been largely born out in the Great Lakes region. Similarly, colonization of the Mississippi River drainage will also likely lead to dramatic changes in unionid demography in that system.

Zebra mussel specimens in INHS Mussel Collection.

A number of remediation strategies have been proposed in anticipation of the detrimental impact of zebra mussels on unionids. Suggested remediation strategies include enhancing predator/prey interactions, translocations of unionid stocks, manually cleaning zebra mussels from the shells of individual unionids, and natural refugia. A review of each strategy follows.

A number of predators include zebra mussels in their diets. For instance, several species of crayfish will prey on zebra mussels in the field and in the laboratory, and a map turtle (Graptemys geographica) also consumed zebra mussels in the laboratory. Several North American fish are likely to prey on zebra mussels; however, field studies have been conducted only on the freshwater drum, Aplodinotus grunniens, and the common carp, Cyprinus carpio. Regardless, the impact of fish predation on zebra mussel density and demography is unknown because few experimental studies have been conducted.

In contrast, the effects of diving-duck predation on zebra mussels have been studied experimentally. Generally, these studies find that duck predation has important but usually temporary effects on zebra mussels. Zebra mussel density may be reduced when ducks are present, but then mussel numbers rebound after the ducks migrate out of the area.

Although predation may have an impact on zebra mussels given the large number of potential predators, predation has not prevented zebra mussel impacts on native unionids in areas extensively colonized by zebra mussels. In any case, resource managers should act to prevent declines of and where appropriate encourage increases in species known to be zebra mussel predators as a remediation strategy.

Unionid translocation entails moving individual unionids from waters colonized by zebra mussels to areas where zebra mussels would not be expected to colonize, such as artificial ponds, captive holding facilities (e.g., fish hatcheries), or other natural waters. Translocation of unionids has a long history but recently it has been used to shelter selected rare species from the effects of pollution.

Translocation would be of little use in maintaining unionid species diversity unless suites of species can be sheltered. Thus, application of translocation strategies is complicated by the lack of knowledge about species-specific requirements for survival and propagation. Moreover, survival of some species may be low in translocations.

Research suggested that periodic removal of zebra mussels from unionids would allow unionids to survive in habitats where survival and reproduction are successful despite heavy zebra mussel densities. Moreover, cleaning can be applied to all members of the fauna, thereby preserving diversity. Also in this study, experimental evidence demonstrated improved survival for cleaned unionids compared to those left uncleaned. However, the potential costs in human effort and in habitat disturbance needed to use this strategy were not addressed.

Ponds and other artificial habitats have been used to shelter unionid mussels from the adverse effects of zebra mussel colonization; however, the efforts required to use them are significant.

A recent study demonstrated that some local habitats, such as river backwaters, may act as natural refugia for unionid mussels. Researchers defined such refugia as sites where unionids continue to live in the presence of zebra mussel colonization. They further noted that such refugia would be needed only in a few areas when protection of unique unionid species is the primary objective.

Identification of natural refugia has been haphazard. Natural refugia for unionids have now been reported in an Illinois River backwater, in nearshore firm compacted sand habitat in Lake Erie, and in a Lake Erie wetland. Ideally, a model to identify potential natural refugia should be developed, and this model development should be a high priority because protection of potential natural refugia requires little additional effort.

It seems likely that discovery, study, and protection of natural refugia will be the best way to maximize conservation of unionid diversity. Nonetheless, the brief overview of potential mitigation strategies highlights the need for continued research. In particular, an understanding of the impact of fish predation on zebra mussels is needed. Likewise, development of a model to help identify natural refugia on a local scale is extremely important. Until these research needs are realized, any action that delays or reduces unionid mortality should be investigated.

John Tucker and Timothy Mihuc, Center for Aquatic Ecology

Charlie Warwick, editor