Unfortunately, spawning Chinook salmon are increasingly younger and concentrated within fewer age groups, with the oldest age classes of spawners rarely seen in recent years. The researchers focused on Sacramento River fall-run Chinook salmon, which contribute heavily to the salmon fisheries of California and southern Oregon. If most of the salmon return to spawn at the same age, one bad year could be devastating for the overall population. The model allowed the researchers to assess the effects of different mechanisms that can affect the age structure of the population. But fishing pressure is not the only factor driving changes in the age structure of the salmon population.
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