Domestic ballast operations on the Great Lakes: potential importance of Lakers as a vector for introduction and spread of nonindigenous species

Abstract

Ballast water is recognized globally as a major vector of aquatic nonindigenous species (NIS) introductions; domestic ballast water transfers, however, have generally been considered low risk in North America. We characterize ballast operations of domestic ships in the Great Lakes – St. Lawrence River system (Lakers) during 2005–2007 to examine the risk of primary and secondary introductions associated with ballast water transfers over short distances. Results indicate that Lakers transported at least 68 million tonnes of ballast water annually. Approximately 71% of ballast water transfers were interregional, with net movement being from lower to upper lakes. A small proportion of ballast water discharged in the Great Lakes (<1%) originated from ports in the St. Lawrence River that may serve as sources for new NIS. These results indicate that domestic ballast water transfers may contribute to NIS introductions and are likely the most important ballast-mediated pathway of secondary spread within the Great Lakes. Future efforts to reduce invasion impacts should consider both primary and secondary introduction mechanisms.