18. Natural mortality (M) includes disease, starvation, cannibalism, old age, and predation by other species of fish and marine mammals. With no realistic way of measuring M, it is incorporated as an assumed constant with a value of 20 percent. Given that M must also vary due to the same complex interactive factors driving the extreme observed variability in growth and recruitment, the choice of any constant value to represent M in the model must be considered to be highly arbitrary and speculative.