On signals of phase transitions in salmon population dynamics
Critical slowing down (CSD) reflects the decline in resilience of equilibria near a bifurcation and may reveal early warning signals (EWS) of ecological phase transitions. We studied CSD in the recruitment dynamics of 120 stocks of three Pacific salmon ( Oncorhynchus spp.) species in relation to critical transitions in fishery models. Pink salmon ( Oncorhynchus gorbuscha ) exhibited increased variability and autocorrelation in populations that had a growth parameter, r , close to zero, consistent with EWS of extinction. However, models and data for sockeye salmon ( Oncorhynchus nerka ) indicate that portfolio effects from heterogeneity in age-at-maturity may obscure EWS. Chum salmon ( Oncorhynchus keta ) show intermediate results. The data do not reveal EWS of Ricker-type bifurcations that cause oscillations and chaos at high r . These results not only provide empirical support for CSD in some ecological systems, but also indicate that portfolio effects of age structure may conceal EWS of some critical transitions.