Apr. 22, 2017: Swimming 3200 km to get home
We have all seen video of salmon leaping natural obstacles and dodging bears. We have also heard that the spawning runs are faltering in many places. Today Nick Lapointe explained that Chinook Salmon spawning in the upper Yukon River are a shadow of their former abundance. Why this should be is a puzzling problem to fish biologists like him, because the Yukon River is largely free of trouble.
In its 3200-km course, there is only one dam, built in Whitehorse in 1958, and salmon have always been able to bypass it. (Nick explained in detail the factors that had to be taken into account to make the fish ladder work.) Running through the Yukon and Alaskan wilderness, the river water is also clean. Being in such a rigorous northern environment, it is largely free of invasive species. There are commercial fisheries along the river’s length, but they appear to be actively managed. The destruction of salmon in the open ocean of the Bering Sea has been curtailed. Yet since 1961 the Chinook population upstream of Whitehorse has been only a tenth of what it had been.
Nick is employed by the Canadian Wildlife Federation to investigate more closely. He carries out field research on the river, and studies existing reports for clues that have eluded others. He told us that the large number of fish people see going up the fish ladder at Whitehorse breaks up into many smaller sub-populations that spawn separately on small tributaries of the Yukon River. Each of these might be made up of no more than a few hundred individual fish, and any one of them could be wiped out through either some local natural disaster or unevenness of the commercial catch along the course of the lower river. Such losses would eat holes in the overall population. Nick also highlighted for us evidence that the age structure of the population has been altered so as to favour younger fish, which are far less prolific than the much bigger, old fish that used to be present. It is possible that the successful management of the fish as a single population along the whole Yukon River has not been able to protect the sub-populations in its farthest reaches.