Marine Mortality of Puget Sound Coho Salmon (<i>Oncorhynchus kisutch</i>)

Abstract

A model for natural mortality over the 18 mo of marine life of Puget Sound coho salmon (Oncorhynchus kisutch) is proposed, wherein the natural mortality rate M continuously decreases with increasing weight. Weight closely follows an increasing exponential function of marine age. The model is extended to account for ocean troll and sport fishing mortality and applied to data from marking studies of Puget Sound coho. The survival curve for marine life with only natural mortality declines rapidly for early ocean life and is quite flat over the ages fished. Recent high survival rates for hatchery coho released at a larger than normal size could be partially explained by this mortality model, although additional marking experiments are needed to separate the effects upon survival of size at release and time of release. The model applied to hatchery release-size strategy indicates that 70 g/fish might be optimal.