High Prevalence of the Copepod in Steelhead Trout in Lake Ontario Following its Recent Invasion.

(Dana, 1853) (Subclass Copepoda: Family Lernaeopodidae) is known to parasitize salmonids of the genus including (rainbow trout), (chinook salmon), and (coho salmon). These 3 salmonids have been introduced to the Great Lakes intermittently since the mid-1800s. As we demonstrate here, the introduction of these salmonids to the Great Lakes was followed, at some point, by the introduction of their parasitic gill copepod, . Given anecdotal accounts of in introduced salmonids in Lake Ontario since 2012, we chose to conduct a survey to formally document the occurrence of this introduced species. Our survey took place during spring, summer, and fall of 2018 and during spring of 2019 at the south-eastern side of Lake Ontario. Prevalence of was 69, with a mean intensity of 2.7 in 61 rainbow trout examined in 2018. In 2019, prevalence of was 71, with a mean intensity of 3.6 in 59 rainbow trout examined. The prevalence of was 39, with a mean intensity of 1.6 in 223 chinook salmon examined in 2018. No specimens of were found in the 100 coho salmon examined in 2018. The prevalence of in rainbow trout is of great concern considering that it is double that found in rainbow trout in the native range (69 [in 2018] and 71 [in 2019] vs. 35). This is the first formal documentation of the invasion of in Lake Ontario. Future fisheries management decisions in Lake Ontario and its tributaries should take into account these data.