The prevalence and intensity of the trematode, Otodistomum hydrolagi, were determined for the first time in Spotted Ratfish, Hydrolagus colliei. Spotted Ratfish specimens were collected during bottom-trawl surveys in Queen Charlotte Sound (QCS, n = 31) and off the west coast of Vancouver Island (WCVI, n = 242). In all instances, the flukes were found within the abdominal cavity. Most flukes were mature (intrauterine eggs were present) and ranged in length from 9.1 to 61.5 mm, with a mean ± SD length of 28.5 ± 12.1 mm (n = 52). Fluke length was positively correlated with Spotted Ratfish precaudal length (PCL), and no Spotted Ratfish smaller than 247-mm PCL was infected by O. hydrolagi. The overall prevalence (95% confidence interval [CI]) and mean intensity (95% CI) of O. hydrolagi were 27.8% (22.6–33.6) and 1.24 (1.09–1.43), respectively. Neither prevalence nor mean intensity differed between host location or host sex. The majority (86%) of infected hosts harbored a single fluke (range 1–5), but 4 Spotted Ratfish were infected with 3 or more flukes, which accounted for 16% of all observed O. hydrolagi. The fluke population exhibited an aggregated distribution with 80% of individuals found in 21% of the hosts, but among infected hosts there was low variability in the intensity of infection. This is the first ecological study of O. hydrolagi from its Spotted Ratfish host, and the first record of this trematode in Canadian waters, which extends the range of O. hydrolagi to include Canadian coastal Pacific waters.

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