A Cross-Species Spawner-Recruit Relationship:Pink and Coho Salmon in Ford Arm Creek, Chichagof Island, Alaska

 The coho salmon population in Ford Arm Creek in Southeast Alaska was studied as an indicator stock for fishery management during 1980−2009. A doubling of the average adult return between the 1982−1991 and 1992−2009 periods resulted from a 48% increase in average presmolt production and a 37% increase in average presmolt-adult survival. The increase in freshwater production occurred concurrent with a quadrupling of both average pink salmon spawner abundance and average all-species carcass biomass. We fit several stock-recruit curves to the adult coho production series. Surprisingly, some of the best fits were found by predicting coho recruitment using pink salmon escapement (as independent variables) with a logistic hockey stick model, either with or without the addition of a coho escapement series into the model. The models that used pink salmon as an indicator fit much better that the usual Beverton-Holt and Ricker models, as judged by the Akaike information criterion. The pink salmon escapement smoothed by averaging over the brood year and the following year (the years that will potentially have the greatest potential to affect juvenile coho salmon) in a hockey stick model explained 37% of variation in the survival-adjusted brood year return of coho salmon, while a combined model incorporating coho salmon brood year escapement explained 57% of variation on an arithmetic (non-logarithmic) scale. In contrast, the Beverton-Holt model explained less than 20%. At a constant average coho salmon escapement of 3,275 spawners, the model predicts more than doubling of the coho salmon return as pink salmon escapement increases from near zero through an inflection point at an annual peak count of 78 thousand spawners to a nominal saturation point at 100 thousand spawners, above which there was no additional response. The nominal saturation point falls near the mid-point of the current pink salmon escapement goal of 48−156 thousand spawners, established using single-species yield analysis. On an area-density basis, the relationship between MDN and coho salmon production in Ford Arm Creek was consistent with the observed growth response by coho salmon fry to the addition of pink salmon carcasses reported from other research based on a controlled experiment in an artificial stream. These observations further support inter-species relationships and the response to MDN as important considerations in setting escapement goals for salmon.