Chance fluctuations and the survival of small salmon stocks

In this paper, we use a Markovian approach to demonstrate that stocks with modest growth potential at low abundance can be driven quite rapidly to extinction. Under relatively benign conditions, stock survival rates may be high, but even a modest increase to the amount of chance variation in recruitment can reduce survival rates dramatically. Survival rates also depend on the form of the stock-recruitment curve, including depensation that takes hold only at extremely low abundances. Fortunately, small amounts of straying among adjacent populations can push the survival rate substantially toward the value for a combined, homogeneous unit. Our analysis also suggests modifications to existing approaches of establishing minimum target levels for spawning densities and the near impossibility of reliably estimating extinction rates.