Distribution, impact, and life histories of round gobies in the Laurentian Great Lakes and their tributaries: Lessons for invasion biology

The round goby (Neogobius melonostomus) is among the most wide-ranging invasive fish on earth, with substantial, rapidly expanding introduced populations in the Laurentian Great Lakes watershed, the Baltic Sea, and several major European rivers. This dissertation focused on the round goby's secondary invasion of inland systems, drawing comparisons with the Great Lakes. First, I provided a comprehensive review on round gobies, providing range maps and meta-analyses of round goby diets, competitors, predators, and growth rates from invasive populations across the globe. Second, I used round gobies to explore general relationships between invader density and impact. Typically, invader impact is a function of density; as density increases, however, the potential for self-limiting intra-specific interactions rises. We found native fishes experienced decreased growth rates at low round goby densities, but growth rates were similar between goby-free and high-goby density treatments due to increased interference competition between individual round gobies at high densities. Third, we compared fish communities in streams with and without round gobies, allowing for an assessment of ecological impact. Round gobies were associated with warm water fish communities and invaded communities had significantly greater species diversity. We also examined trends in the abundances of 5 native stream benthic fishes that had experienced round goby related declines in the Great Lakes. Contrary to expectation, none of the 5 species declined over the four year study period, suggesting round goby impacts on stream fishes will remain low unless round goby abundance increases. Lastly, I related differences in round goby density and impact between the Great Lakes and their tributaries to divergent life histories associated with these distinct environments. Round gobies grew much faster in streams (25.75 vs. 3.90 grams at Age 2+). Stream gobies also reach sexual maturity 1 year sooner (1+ vs. 2+), allocate a greater percentage of energy to gonadal growth at ages 1+ and 2+, and have a lower maximum age (2+ vs. 5+ in streams). These results support the theory that high mortality/short life expectancy leads to greater reproductive effort at earlier ages and demonstrate that environmental differences between lakes and streams can dramatically affect fish life histories.