Composition of native crayfish assemblages in southern Ontario rivers affected by rusty crayfish (Orconectes rusticus Girard, 1852) invasions – implications for endangered queensnake recovery

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Orconectes rusticus (rusty crayfish) is an aggressive and prolific aquatic invader. Where introduced, it has caused dramatic ecosystem changes, including the replacement of native crayfishes. In Canada, it was first reported in Lake of the Woods in northwestern Ontario and a small number of south-central Ontario lakes during the 1960s. It has subsequently spread to other regions of the province. Its current expansion into southwestern Ontario rivers presents an unknown risk to the endangered queensnake (Regina septemvittata), an obligate feeder on freshly molted crayfish. We sampled 99 river sites across southern Ontario to: (i) describe crayfish assemblages within river reaches currently occupied by Queensnake; and, (ii) characterize the impact of O. rusticus on native crayfish assemblages. O. rusticus was caught at 41% of sites sampled, and was the only species at 24% of sites. The abundance, richness, and within-site distribution of native crayfishes were all significantly lower at sites with O. rusticus. Within the distribution of queensnake, crayfish assemblages were almost entirely dominated by O. propinquus, with O. rusticus being absent. However, O. rusticus was found along the lower Speed River, where only one dam separates it from Grand River queensnake populations. Given its past spread, O. rusticus will likely be introduced into areas used by queensnake and replace O. propinquus (the primary prey of queensnake). The adaptability of queensnake to prey upon non-native crayfish is unknown and requires investigation.