University of California Water Resources Center

Humans play a critical role in the dispersal of exotic invasive species. Estimating pathways for non‐native species by human vectors is a major challenge to invasion biologists, as well as federal, state and regional resource managers. Focusing on dispersal pathways that are available to not just one, but a number of species, allows for the efficient inspection and possible reduction of many exotic species introductions. Transient recreational boating has been used as an estimate of invasion pressure to inland freshwater bodies, and used to predict prior and future species invasions. Specifically, recreational boating traffic is used to predict human-mediated aquatic invasion in the Midwestern United States through the use of spatial interaction models called gravity models. California and Nevada contain some of the largest and most recreationally utilized lakes, rivers and reservoirs in the Western United States. These waterways attract millions of visitor days by boaters not only from within the region, but all over the United States and are currently experiencing increasing numbers of non‐native species introductions from the Midwestern U.S.

This report describes aspects of dispersal of an aquatic invasive plant, Eurasian watermilfoil, both within and between fresh water bodies by recreational boating. This study addresses the question of habitat and/or dispersal limitation for watermilfoil by assessing the movement of recreational boaters within Lake Tahoe, and between Lake Tahoe and other locations, as well as characterizing nearshore habitat locations in highly visited boating destinations. Additionally, this report examines the nature of recreational boater movement data, and the impacts of boater preference as well as the impact of the spatial aspect of data gathering from one versus many locations. Specifically, this report presents the following: 1) an examination of the use of transportation models known as gravity models to describe recreational boater traffic to inland waterways in California and Nevada, 2) an analysis of waterway access point habitat quality as it relates to Eurasian watermilfoil, and 3) the invasion of Eurasian watermilfoil within Lake Tahoe, and how that relates to within-lake boater movement and habitat variables associated with invaded and uninvaded sites within Lake Tahoe.