Abstract

Chum salmon (Oncorhynchus keta) eggs are eaten in large numbers by Japanese dace (Tribolodon hakonensis) in the spawning streams in Hokkaido. One way to reduce predation on salmon eggs might be to use flickering near-ultraviolet (UV) radiation at 386 nm, to which Japanese dace is known to react negatively. The effect of flickering near-UV on chum salmon was determined in this paper. Sexually mature chum salmon were examined for (i) retinal photoreceptors (corner single cones) sensitive to near-UV; (ii) behavioural responses to flickering radiation at near-UV (λmax 386 nm), green (λmax 524 nm), and red (λmax 618 nm); and (iii) near-UV transmittance through the cornea and lens. The chum salmon had complete square cone mosaics (four double cones around a central single cone and four single cones at the corners of the square) at the central sector of the retina. The corner single cones are the photoreceptors sensitive to near-UV. About 300 mature chum salmon did not respond to flickering near-UV radiation from light-emitting diodes in an indoor tank, but avoided flickering green light and red light. The eye lens of chum salmon almost totally blocked short wavelengths below 390 nm. Thus, despite having UV-sensitive photoreceptors in the retina, mature chum salmon can not respond to near-UV radiation because of the UV-blocking lens. We conclude that flickering near-UV radiation can be a feasible means to scare away the Japanese dace from the spawning streams without any influence on the chum salmon spawners in daytime as well as at night.

Key words: Chum salmon, ocular transmission, near-ultraviolet, corner single cones, egg predation, Japanese dace.