Exceptionally high mortality of adult female salmon: a large-scale pattern and a conservation concern

Abstract

In recent decades, the relative proportion of female sockeye salmon (Oncorhynchus nerka) on spawning grounds of several British Columbia populations has declined. Coincident with the decline has been large changes to oceanic, estuarine, and river migration environments. Over the past 30 years, numerous telemetry tracking and laboratory studies have examined mortality of adult Fraser River sockeye salmon during ocean and freshwater migrations. We reviewed 19 studies that provided 40 situations where male and female mortality could be directly compared. Female mortality averaged 2.1 times greater than that of males and up to eightfold higher. High female mortality was also evident in migrating coho salmon (Oncorhynchus kisutch) and Chinook salmon (Oncorhynchus tshawytscha) and for sockeye salmon in other systems. Female mortality was highest when migration conditions were challenging (e.g., high or turbulent flows, high temperatures, confinement, or handling) and towards end of river migration. We review mechanisms for differential mortality, including energy exhaustion, cardiac performance, physiological stress, and immune factors. Female-specific mortality will become even more pronounced in coming years as ocean and riverine conditions continue to change.