Estimating overland transport frequencies of invasive zebra mussels
Zebra mussel invasions of inland lakes in Minnesota are on the rise. In order to develop prevention and control methods, it’s crucial to understand the pathways and mechanisms that are enabling this spread. One suspected source of human-assisted zebra mussel transport is through residual water in recreational boats. The lack of data around this concern (to support it or to rule it out) has led to challenges related to statewide inspection practices and even recreational boat design.
Therefore, the goals of this study were to:
Estimate the relative contributions of different surfaces and compartments on and in recreational boats and trailers to the transport of zebra mussels and their larvae (veligers), focused on measurements of the concentrations of veligers in residual water across a full range of vessel types in Minnesota.
Identify “high-risk” vessel types and “high-risk” areas of watercraft that are likely to transport large volumes of residual water, and evaluate where boat redesign can be targeted to most effectively reduce residual water volumes.
Develop a refined model to assess the risk for residual waters to transport – and thereby spread – live veligers within the state.
This study partnered with the DNR’s watercraft inspection program to collect data on presence and location of adult zebra mussels on seven different watercraft types leaving two popular zebra mussel infested lakes in Minnesota. In addition, a subset of the boats inspected had residual water – that which is left in a boat after a user has attempted to fully drain it – sampled from various compartments of the watercraft, including live wells and bait wells, bilge areas, ballast tanks (if present), motors and any other location that may potentially transport a zebra mussel. The water was analyzed for the presence of veligers. Additionally, lab tests and field experiments determined the ability of veligers to survive in some of these high-risk compartments.
In phase one of this project, DNR watercraft inspectors collected residual water from boats leaving two different Minnesota lakes. Additional engine samples were collected by mechanics at Tonka Bay Marina. Roughly half of these samples contained no veligers and 75% contained five or fewer veligers. Sterndrive engines and ballast tanks ranked first and second for volumes of residual water. Ballast tank samples contained the largest median number of veligers per sample, and sterndrive engines contained the highest maximum number of veligers. Compartments that contained more residual water on average had larger numbers of veligers compared to the compartments with smaller volumes of water and had a greater likelihood of finding one or more veligers. Based on the results of this study, recreational equipment that contains one or more ballast tanks poses the greatest likelihood of moving high numbers of veligers given our findings that ballast tanks were the compartment with the greatest likelihood of containing veligers and had the greatest mean number of veligers per sample. Wakeboard boats are the type most likely to contain ballast tanks, and some boat models can have multiple ballast tanks. Given the amount of veligers ballast tanks may contain, extra steps (such as hot water decontamination) should be taken by boat owners to minimize the likelihood of introducing live veligers to new water bodies via overland transport.
In the second phase, researchers performed experimental mortality trials on larvae in two common boat compartments: live wells and ballast tanks. Both compartments were exposed to various temperatures (20, 27, 32, and 38°C for live wells and 20 and 32°C for ballast tanks). For veligers in the residual water of live wells, > 95% mortality was observed after five hours of exposure to all temperatures. The same level of mortality was reached in ballast tanks at 48 hours. Based on our results, watercraft equipped with ballast tanks are at a greater risk of transporting live veligers to new water bodies over short trips (e.g. 6 hours). This tells us that additional prevention steps (i.e. using hot water) should be taken to reduce the risk of transporting living veligers in residual water.
The results of this study can help resource managers evaluate the potential risks that watercraft in compliance with state laws may pose. Providing on-site or on-call decontamination services to watercraft users can help reduce the risk of transporting aquatic invasive species. These systems are often expensive, however, and not all agencies or organizations have the funding necessary to offer these services. Based on the results found in this study, the nationally recognized suggestion to dry your watercraft for five days or more (Stop Aquatic Hitchhikers!) drastically reduces the risk of transporting living zebra mussel veligers to new water bodies. For boaters planning on moving between water bodies within shorter periods of time, extra precautions such as hot water decontamination could be done to minimize the risk of moving living veligers.