Size Selective Mortality and Early Marine Growth: Potential Mechanisms Regulating Salmon

Our goal is to address the Fraser River Panel Priority #2, the examination of mechanisms affecting early survival of Fraser River Sockeye Salmon, as early marine life is expected to set recruitment dynamic patterns of Pacific salmon. In this two-year project, we propose to examine the factors influencing the size of Fraser Sockeye during their first year at sea, a hypothesized critical time in determining their survival, given that fish must be large enough to avoid predators and have sufficient reserves to survive their first winter (Beamish and Mahnken 2001). Currently, there is no consensus on the importance of size-selective mortality and compensatory growth as mechanisms regulating the mortality and abundance of Fraser River Sockeye Salmon.

We propose retrospective and comparative analyses of growth characteristics derived from the scales of two Fraser River stocks demonstrating contrasting patterns of freshwater and marine residence and, importantly, opposite trends in adult returns. Over the last decade Harrison (sea type) has had a high number of adult returns contrary to Chilko (lake type) Sockeye Salmon returns, which have been low. These stocks differ in their life-history in that Harrison fry outmigrate to sea ~ 2 months after Chilko smolts (Beamish et al. 2016).

Our goals are to: (a) evaluate the extent of size-selective mortality, (b) determine the factors influencing juvenile length at the end of their first year at sea, and (c) determine the factors influencing the number of returning adults.