Pattern of Anguillicoloides crassus infestation in the St. Lawrence River watershed

The non-native swim bladder nematode Anguillicoloides crassus was first documented in wild American eel (Anguilla rostrata) in South Carolina in 1996, and has since spread through rivers and estuaries along the east coast of the United States and Canada. American eel in Canada are a species of conservation concern, primarily due to a severe decline in recruitment within the St. Lawrence River watershed. We report the first occurrence of A. crassus in American eel in the St. Lawrence River watershed in 2010. Prevalence of A. crassus infection remained low through 2014, but has since increased to approximately 30% over the past 3 years. Infection intensity has also increased from only a single nematode up to 2013 to an average of 6.5 nematodes per infected eel in 2018. In outmigrating silver-stage eels sampled in the St. Lawrence estuary, the first occurrence of A. crassus was noted in 2015 and prevalence has fluctuated from a low 0.2% in 2015 to a high of 3.6% in 2017. In 2018, A. crassus was first identified in an eel recruiting to the upper St. Lawrence River. A. crassus was likely inadvertently introduced to the St. Lawrence River watershed during a conservation stocking research project in which glass-stage eels from infected areas were translocated to the region to supplement natural recruitment. It is not clear at this time what harm this additional threat will pose to an already declining contingent of this panmictic species.