The Epidemiology of the Trichodinid Ciliate Trichodina truttae on Hatchery-reared and Wild Salmonid Fish in Hokkaido

Abstract

Trichodinids are ectoparasitic ciliates that infect the body surface of salmonid fish and occasionally cause high mortality in juvenile chum salmon Oncorhynchus keta reared at hatcheries in northern Japan. The present study examined the occurrence of trichodinid ciliates on wild fish and hatchery-reared juvenile chum salmon in Hokkaido to determine the epidemiology of these parasites. Trichodinids were observed on juvenile chum salmon reared at 28 out of the 87 examined hatcheries and on wild fish, including masu salmon O. masou, rainbow trout O. mykiss, adult chum salmon, white-spotted char Salvelinus leucomaenis, and Dolly Varden S. malma in rivers. All isolates of trichodinid ciliates had the same nucleotide sequences of small subunit ribosomal RNA gene and were identified morphologically as T. truttae. A transmission experiment showed that T. truttae was transmitted horizontally from wild masu salmon to hatchery-reared juvenile chum salmon. Our results demonstrate that a dominant species of trichodinid ciliate infecting salmonid fish in Hokkaido is T. truttae and that wild salmonids can be an infection source of hatchery-reared juvenile chum salmon.