Occurrence and effects of the parasitic copepod Salmincola carpionis on salmonids in the Nikko District, central Japan

Salmincola carpionis (Krøyer, 1837) occurred on brook trout (Salvelinus fontinalis) and whitespotted charr (Salvelinus leucomaenis) reared at a fisheries research institute in the Nikko District, central Japan, and also on a wild population of the latter species that had returned for spawning from a lake to streams near the institute. Its infection levels on these salmonids were associated with age and size of fish and the location of rearing ponds, older (larger) fish reared in the lower-located ponds being more frequently and heavily infected than smaller (younger) fish from the upper-located ponds. Swellings were observed at sites where the bulla of S. carpionis was deeply implanted. The condition factor of heavily infected (>50 copepods) fish was lower than those of lightly infected fish (1–18 copepods). Salmincola carpionis did not occur on lake trout (Salvelinus namaycush) which belong to the subgenus Cristivomer, as well as on salmonids of the genera Oncorhynchus and Salmo, possibly due strict host specificity.