First appearing in North American waters in 1988, the Eurasian zebra mussel (Dreissena polymorpha) has rapidly colonized freshwater systems and can now be found in all of the Great Lakes as well as rivers large and small across the United States and Canada. Halting its spread has proven to be a significant undertaking, with researchers and policymakers fanning out to try and contain the harmful nonnative species. Researchers have long assumed that transient recreational boating provides a conduit for zebra mussels to be transported to inland lakes. A new study published in the December issue of Ecological Applications provides insight into the ecological risks posed by this popular recreational activity.
The zebra mussel is damaging to both human infrastructure and natural ecosystems. Large groupings of the mussels clog pipes of power plants, water, and sewage treatment sites. The weight of mussels has even been known to sink buoys and navigational aids. Ecologically the mussels are adversely affecting native mussels by interfering with their feeding, growth, movement, respiration, and reproduction. For example, scientists have found that where zebra mussel densities are highest, the number of native mussels is dramatically lower than normal levels.
With many rivers and lakes already playing host to the zebra mussel, it is important to assess the risk of spreading the interloping species to new water bodies. To that end, Ladd Johnson of Laval University in Canada, Anthony Riccardi of McGill University, and James Carlton of Williams College, examined the destinations of recreational boats and their ability to transport the adult and larval stages of this invader. Their findings are surprising.
It had been assumed that mussel dispersal from boat launches occurred by direct attachment to transient boats. The researchers found that this is not the case; "During the course of the study it became apparent that the conventional wisdom was wrong, and a previously unrecognized mechanism for the overland transport of zebra mussels existed: the transport of mussels attached to aquatic weeds called macrophytes that become entangled on boats or on boat trailers," Johnson said.
For their study a total of 920 boaters were interviewed as they pulled their boats from the water at three popular public launches on Lake St. Clair, near Detroit, Michigan. The boaters were asked questions pertaining to the duration of their outing, the activities and devices they used for fishing, whether they dropped anchor, and the anticipated time and location of the boat's next use. The authors identified seven mechanisms of potential transport of zebra mussels: Attachment to the exterior of the boat, anchor, entangled aquatic weeds, engine cooling system, bilge water, live wells, and bait buckets. These latter four mechanisms have the potential to transport water containing the microscopic larval stage of the zebra mussel to other bodies of water.
They found that zebra mussels were transported by all of these mechanisms except for the direct attachment to the boat's exterior. This may have been due to the fact that most boats encountered in the study were in the water for one week or less, and it is usually when a boat is left for longer that the direct attachment of zebra mussels may occur. However, even if zebra mussels were to attach themselves to the boat's external surfaces, the authors concluded that they probably would not survive exposure to the air during transit.
The researchers also found zebra mussel larvae in every area where water is stored on a boat, including the engine cooling system, bilge water, and bait buckets used in fishing. However, because of the low probability of establishing new populations from introductions of larvae, this is not thought to pose a significant risk.
From their data and information on out-of water survival, the authors developed a simple probabilistic model to estimate that a maximum of 12 of every 10,000 boaters who used the sampled boat launches transported live (juvenile and adult) zebra mussels on entangled aquatic plants to inland waters. While this may not seem significant at first glance, when the large volume of transient boat traffic at these boat launches is taken into account it becomes clear that there is the potential for these boats to transport a live zebra mussel into inland waters. Indeed, their model predicted over 170 dispersal events from just one launch during the course of the summer.
Johnson says that all other factors being equal, it is not the overall use of recreational boats on a given lake, but rather the frequency of "promiscuous" behavior (i.e. the sequential use of multiple water bodies by boaters) that will determine the likelihood of invasion.
"The results of this study clearly demonstrate the potential for recreational boating to transport zebra mussels away from infested waters," Johnson said. "Since the zebra mussel is already well-established in many North American waterways, the key to containing the species is to limit secondary spread. This study will enable policymakers and boaters to make informed decisions about their actions in order to reduce their contribution to a costly environmental problem," he continued.
In the past, boaters have not been told to remove aquatic plants from their boats and trailers, instead they were directed to sterilize the exterior surfaces of boats with chlorine solutions, hot water, or long periods out of the water. The results of this study permit the development of better educational policies that can focus more precisely on the mechanisms of highest risk, i.e., entangled macrophytes. The results of this study are applicable to the dispersal of other aquatic species, including the past, present and future invaders of North American inland waters, and demonstrate the need for further investigation into the complex dynamics of nonnative invasive species.
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