An Approach to Defining a Sacramento River Fall Chinook Escapement Objective Considering Natural Production, Hatcheries, and Risk Tolerance

Abstract

The escapement objective used to manage fisheries on Sacramento River Fall Chinook (SRFC) was established in 1984. Despite substantial changes to the system and multiple calls to re-evaluate the objective, data and analytical limitations have slowed progress. Synthesizing the available information is further complicated by the different measurement scales employed by relevant studies. Here, I offer a modeling framework for integrating consideration of established hatchery spawning goals, natural-area production or habitat capacities measured at varying spatial scales, and policy decisions about what fraction of potential natural production is desired along with risk tolerance. The model allows evaluating how likely a potential escapement goal (measured at the currently-used scale of fall run adults returning to both hatcheries and natural areas throughout the Sacramento Basin) is to both meet hatchery goals and to produce at least a specified fraction of potential natural production. The framework also incorporates consideration of forecasting and harvest planning error into identifying a pre-season planning target and its probability of resulting in escapement at least as high as the goal. The model indicates that the low end of the current escapement goal range of 122,000-180,000 adults, if achieved, would be more likely than not to achieve hatchery goals while achieving around 50% of potential natural production. Realized escapement equal to the high end is modeled to be highly likely to achieve hatchery goals and likely to achieve around 75% of potential basin-wide natural production or around 60% of Upper Sacramento potential production. The model indicates diminishing returns from total adult SRFC escapements higher than about 300,000 adults. However, past performance of forecast and harvest planning models suggest that a preseason target higher than the ultimate escapement goal is needed to have even a 50% chance of achieving the escapement goal.