Abstract
Marine survival rates of many juvenile salmon populations have declined in recent decades. Although several potential causes have been proposed, there has been little conclusive evidence for which factors are responsible or not responsible for these declines. We experimen- tally addressed the hypothesis that exposure of coho salmon Oncorhynchus kisutch or sockeye salmon O. nerka to solar ultraviolet-B radiation (UVB) during freshwater rearing of fry and parr life-history stages increases mortality at the time of smoltification and ocean entry. Juvenile coho and sockeye salmon were reared in outdoor hatchery tanks either exposed to full spectrum sun- light or shielded from UVB radiation by plastic screens for up to 9 mo prior to release. Smolts were tagged with acoustic transmitters and detected with hydrophone receivers during their downriver and early ocean migration. Survival of treatment groups was compared using Cormack-Jolly- Seber and Burnham mark-recapture models. While exposure to UVB resulted in decreased growth of juvenile coho salmon, survivorship during the early marine period was unaffected by the UVB treatment for both populations. This first attempt to experimentally address the hypo - thesis of impaired survival resulting from solar UVB radiation has shown that other factors are more likely responsible for observed declines in salmon marine survival rates.
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