Understanding spatial and temporal variation in the site of attachment by the parasitic copepod Salmincola californiensis on rainbow trout

Background/Question/Methods

Salmincola californiensis is a parasitic copepod of rainbow trout. This copepod is native to the western U.S., but was discovered at Maramec Spring Trout Hatchery, St. James, MO, in September 1992. This introduction has resulted in a decline in fish health, survivorship and aesthetic quality, as well as quarantine on the transport of trout raised in the Maramec hatchery.  As part of a broader effort to address this parasite-host interaction, we examined the spatial and temporal variation in the site of copepod attachment on hatchery-reared rainbow trout. We placed 235 tagged, uninfected 6mo old fish into cages which were in turn placed in pools inhabited by infected trout. We regularly reexamined each fish to document the timing and site (left gill, right gill, mouth) of infection. We then analyzed the first two months of the dataset to assess whether copepods preferentially infected one site, the correlations between intensity of infection of the three sites, and whether infection of a site increased the likelihood of subsequent infection of the site.

Results/Conclusions

Parasitism was first observed 2 weeks post-exposure. At that time, prevalence was 21%, with no difference between left or right sides (prevalence for both was 9%). Seven weeks post-exposure, prevalence was 76% and mean intensity was 1.8 parasites per host (range = 0 - 10). There was again no evidence of asymmetrical preference (prevalence and intensity: left = 51%; 0.77; right = 54%; 0.76).  Thus, there was no evidence of copepod preference for left or right gills. Prevalence in the mouth was 17%, but parasitism of the mouth without concurrent gill parasitism was rare (2%). While the intensity of parasitism of left and right gill was significantly correlated (p < 0.0001), as was the correlation between intensity of parasitism of the mouth and both gills combined (p <.001), correlation coefficients were weak (R = 0.22 - 0.38) indicating that counts on one body region do not aptly indicate the total intensity of parasitism of an individual.