Thousands introduced annually: the aquarium pathway for non-indigenous plants to the St Lawrence Seaway

Non-indigenous species are both economically and ecologically costly. Invasions are occurring at an accelerating rate worldwide and therefore present a critical challenge to natural resource managers. The aquarium trade is commonly recognized as a pathway for non-indigenous plants, but few regulations exist to curb such introductions. In addition, very few studies have attempted to quantify the number of propagules introduced through the aquarium trade each year, probably because it is difficult to directly measure the number of propagules introduced. Here, we use a novel approach to quantify propagule numbers by analyzing each step in the path to introduction and synthesizing this information to calculate propagule pressure for each species. We used the aquarium plant trade in Montreal, Quebec, Canada, as our study system and found that thousands of non-indigenous plant propagules are introduced to the St Lawrence Seaway each year, through the Montreal aquarium trade alone. Two known invaders are among those species with the highest measured propagule pressure.