Experimental evidence on prevention of infection by the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae in juvenile chum salmon using ultraviolet disinfection of rearing water

Abstract

In northern Japan, juvenile chum salmon Oncorhynchus keta (Walbaum) are released from hatcheries to enhance the fishery resource. Infections with ectoparasitic protozoans, particularly the flagellate Ichthyobodo salmonis and the ciliate Trichodina truttae, occasionally cause severe mortality among hatchery‐reared juveniles. This study examined the susceptibility of the two parasites to wide‐ranging UV irradiation (experiment 1) and then investigated whether UV disinfection of the rearing water using a commercial device was useful for preventing infections among juveniles in a small‐scale rearing system over a 28‐day period (experiment 2). In experiment 1, parasite mortality reached 100% with UV irradiation doses of ≥9.60 × 105 μW s/cm2 for I. salmonis and ≥8.40 × 105 μW s/cm2 for T. truttae. In experiment 2, disinfection of the rearing water at a UV irradiation dose of 2.2 × 106 μW s/cm2 succeeded in complete prevention of both parasites in the juvenile salmon. These results elucidate the minimum dose of UV irradiation for inactivation of I. salmonis and T. truttae, and demonstrate the usefulness of water disinfection using a commercial UV irradiation device to prevent infections by these parasites in hatchery‐reared juvenile chum salmon.

Sustainable development of factory aquaculture through automation of ultraviolet parasiticide for the prevention and control of cryptocaryoniasis

“…In vitro tests revealed that the minimum lethal UV doses for protomonts and tomonts were 2.0 × 10 6 and 3.5 × 10 6 μWs cm −2 , respectively. These doses were higher than the minimum lethal UV doses for Ichthyobodo salmonis 39 (9.6 × 10 5 μWs cm −2 ), Trichodina truttae 39 (8.4 × 10 5 μWs cm −2 ) and theronts of C. irritans 3 (2.88 × 10 5 μWs cm −2 ). These differences are attributed to species differences or different sensitivities to UV at each stage of the pathogen life cycle 3,38 .…”

“…The UV lamp is a commonly used device whose sterilization effect increases with the increase in UV dose. 3,38,39 The rate of killing for sea lice eggs was 5% at a UV dose of 0.008 J cm −2 , but increased to 95% when the UV dose was 0.09 J cm −2 . 38 Similarly, a UV dose of 2.69 × 10 5 μWs cm −2 did not kill theronts of C. irritans but the UV dose became lethal it was above 2.88 × 10 5 μWs cm −2 .…”

“…The UV lamp is a commonly used device whose sterilization effect increases with the increase in UV dose 3,38,39 . The rate of killing for sea lice eggs was 5% at a UV dose of 0.008 J cm −2 , but increased to 95% when the UV dose was 0.09 J cm −2 38 .…”