Research Document 2017/030
Ecological Risk Assessment of Recreational Boating as a Pathway for the Secondary Spread of Aquatic Invasive Species in the Great Lakes Basin
By Drake, D.A.R., Bailey, S.A., and Mandrak, N.E.
Abstract
A model-based assessment was conducted to estimate the ecological risk of recreational boating as a pathway for the secondary spread of aquatic invasive species (AIS) in the Great Lakes basin (GLB). Boater-mediated spread was quantified based on the number, type, and spatial distribution of recreational boating trips in the GLB, the infestation of boats with functional groups of fouling AIS (plankton, molluscs, and aquatic macrophytes), and the likelihood that organisms would establish reproducing populations based on demographic factors and transported population sizes. Boater-mediated spread timelines were estimated probabilistically up to a maximum period of ten years, allowing the expected timeline of spread to be compared with baseline estimates of natural dispersal. The model revealed a total of 11.8 M yearly recreational boating events in the GLB (3.8 M originating at Canadian recreational access sites, 8.0 M originating at U.S. recreational access sites). The large number of boating events, combined with relatively high densities of transported organisms, was sufficient to overcome demographic constraints to establishment in many cases. Boater-mediated spread varied across functional groups and was fastest for invasive phytoplankton, with among-lake spread expected in as little as a single year in some scenarios. A relatively robust spatial pattern emerged with the highest rate of spread between neighbouring lakes; however, upstream movement through GLB lock structures and among multiple lake basins was possible with sufficient time (e.g., average modeled estimate of eight years for invasive phytoplankton to spread from the St. Lawrence River to Lake Superior). Two ecological consequences emerged. Boating activity was mostly unlikely to surpass downstream rates of natural dispersal, though in some cases could be exceeded by a period of up to four years. However, for all functional groups of AIS, boating activity was far more likely to lead to new upstream pathways of secondary spread that would otherwise be unlikely to occur through natural dispersal at short timescales (10–20 years). The overall risk of secondary spread was highest for Lake Superior due to the frequent development of upstream pathways. Risk was usually moderate for Lake Michigan, Lake Huron, and Lake Erie, while risk was generally low for Lake Ontario and the St. Lawrence River because boater-mediated rates of spread were consistent with expectations under natural dispersal. Results were sensitive to background estimates of natural dispersal, indicating that the risk of secondary spread would be higher if natural dispersal progressed more slowly than expected. These findings indicate that for certain geographic routes and most functional groups of AIS, increased attention to in-water recreational boating as a pathway of the secondary spread of AIS is warranted.
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