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 ext inction. Under relatively benign conditions, stock survival rates may be hi gh, but even a modest increase to the amount of chance variation in recruit ment can reduce survival rates dramatically. Survival rates also depend on the form of the stock-recruitment curve, including depensation that takes h old only at extremely low abundances. Fortunately, small amounts of strayin g among adjacent populations can push the survival rate substantially towar d the value for a combined, homogeneous unit. Our analysis also suggests mo difications to existing approaches of establishing minimum target levels fo r spawning densities and the near impossibility of reliably estimating exti nction rates.