Invasion note Popularity and propagule pressure: determinants of introduction and establishment of aquarium fish

Propagule pressure is frequently cited as an important determinant of invasion success for terrestrial taxa, but its importance for aquatic species is unclear. Using data on aquarium fishes in stores and his-torical records of fish introduced and established in Canadian and United States waters, we show clear relationships exist between frequency of occurrence in shops and likelihood of introduction and of estab-lishment. Introduced and established taxa are also typically larger than those available from stores, con-sistent with the propagule pressure hypothesis in that larger fish may be released more frequently due to outgrowing their aquaria. Attempts to reduce the numbers of introductions may be the most practical mechanism to reduce the number of new successful invasions