Every summer, king salmon swim 2,000 miles up the Yukon river in Alaska to spawn. For thousands of years, their journey has helped to sustain the native people. But with the effects of climate change and globalisation, the health and numbers of the salmon are  in question, and so is the fate of those who depend on them. Adam Weymouth followed the fish for Kings of the Yukon (Particular Books), which is published this week; he summarised his findings in an essay yesterday for The Observer.