ABSTRACT
Use of the deterministic maximum sustainable yield (MSY) concept has led to unrealistic expectations for harvest and escapement of salmon populations. Fits of stock-recruitment data to a Ricker curve for Pacific salmon typically show that most variation in recruitment is environmentally driven. Frequency distributions for the number of recruits produced per spawner from several coho salmon Oncorhynchus kisutch and chinook salmon O. tshawytscha populations show that recruitment varies among years by more than an order of magnitude, and that this variation is strongly skewed with a preponderance of below-average recruitment rates. The same is true for smolt-to-adult survival of hatchery coho and chinook populations, which indicates that a large share of the variability in survival is generated while salmon are in the ocean. Stochastic simulations, based on Ricker parameter values estimated from naturally produced coho in the Nehalem River, Oregon, were used to illustrate how environmental variation affects the sustainability of catch and escapement. This chapter does not develop new stock-recruitment theory but instead illustrates often-neglected aspects of its application to harvest management. The skewed, or log-normal, nature of environmental variation causes catch and escapement to be less than the deterministically predicted equilibrium in about 60% of all years. Options for adjusting harvest rate annually to reflect variations in survival for a specific stock are few, because recruitment varies asynchronously among populations harvested predominantly in mixed stock fisheries. By reducing the harvest rates on Oregon coho to about two thirds of the theoretical MSY level, total catch over the long term would remain near the maximum, but spawning escapement would double. Increased escapement would reduce genetic risks and sustain greater marine nutrient supply to rearing areas for juvenile salmonids.
