Effects of solar ultraviolet radiation on the behaviour of juvenile coho salmon (Oncorhynchus kisutch): avoidance, feeding, and agonistic interactions

The behaviour of juvenile coho salmon (Oncorhynchus kisutch) was observed in outdoor experimental flumes in which three different spectral regimes were created by plastic covers that selectively blocked all or portions of natural ultraviolet radiation (UVR). The three light treatments were (i) photosynthetically active radiation (PAR, 400–700 nm), (ii) PAR plus longer wavelength ultraviolet (PAR + UVA, 320–700 nm), and (iii) full-spectrum sunlight with both long and short wavelength ultraviolet included (PAR + UVA + UVB, 280–700 nm). Observations made at different times of the day and under both overcast and sunny skies allowed an assessment of PAR intensity on behaviour in addition to that of UVR. We quantified shade-seeking behaviour, feeding strikes, and agonistic interactions (approaches, chases, and nips) between individuals. Under higher PAR intensities, a greater proportion of juvenile coho tended to take cover under rocks. Shade-seeking behaviour increased significantly in the presence of UVR. Feeding and agonistic interactions were partially inhibited at higher PAR intensities and very significantly depressed by UVR. For all behaviours tested, the effects were mediated by UVA with no significant additional impacts from UVB. UVR-mediated effects on behaviour could have ecological consequences through influencing summer densities, density-dependent growth, and size-dependent winter and early marine survivals.