Fish/Shellfish Research and Management - Fish/Shellfish Research
Date Published: February 2011
Number of Pages: 37
Author(s): Mara Zimmerman
Run size forecasts for wild coho stocks are an important part of the pre-season planning process for Washington State salmon fisheries. Accurate forecasts on a management unit level are needed to ensure adequate spawning escapements, realize harvest benefits, and achieve harvest allocation goals.
Ocean recruits of wild coho have been predicted using various approaches across Washington’s coho producing systems. Methods that rely on the relationship between adult escapement and resulting run sizes are problematic due to inaccurate escapement estimates and difficulty allocating fishery catches by stock. Furthermore, escapement-based coho forecasts often have no predictive value because watersheds become fully seeded at low spawner abundances (Bradford et al. 2000). The accuracy of coho run size forecasts are improved by partitioning recruitment into freshwater production and marine survival.
Freshwater production, or smolt abundance, is measured as the number of coho smolts leaving freshwater at the conclusion of the freshwater life stage. The Washington Department of Fish and Wildlife (WDFW) and tribal natural resource departments have made substantial investments in monitoring smolt populations in order to assess escapement goals and improve run size forecasts.
Marine survival is survival from ocean entry through the ocean rearing phase to the point that harvest begins. Marine survival is measured by summing coho harvest and escapement for a given stock. Marine survival rates for wild coho stocks have been measured at four stations in Puget Sound and at one station in the Grays Harbor system. Survival-to-return at these stations is considered accurate and unbiased because the tag groups are enumerated at upstream trapping structures. Data from these monitoring stations describe patterns in survival among years and watersheds.
Adult recruits are the product of smolt production and marine survival and can be expressed in a matrix that combines these two components. This approach is similar to that used to predict hatchery returns where the starting population (number of smolts released) is known. For stocks where smolt abundance or marine survival is not measured, adult ocean recruits can be forecasted by extrapolating information from neighboring or comparable watersheds. Long-term studies on wild coho populations have been used to identify environmental variables contributing to freshwater production and to regional patterns in marine survival.
The Wild Salmon Production Evaluation (WSPE) Unit within the WDFW Fish Program Science Division has developed forecasts of wild coho run size for the last seventeen years. Beginning in 1996, a wild coho forecast was developed for all primary and most secondary management units in Puget Sound and the Washington coast (Seiler 1996). A forecast for Lower Columbia wild coho was added in 2000 (Seiler 2000). Forecast methodology for the Lower Columbia continues to evolve (Volkhardt et al. 2007) in response to listing of Lower Columbia coho under the Endangered Species Act in 2005. Forecasts for Lower Columbia natural-origin coho are increasingly important for the management of Columbia River fisheries because harvest impacts on Lower Columbia coho are restricted in order to rebuild this Evolutionary Significant Unit (ESU).
Table 1 summarizes the 2011 run-size forecasts for wild coho for Puget Sound, Washington Coast, and Lower Columbia River systems. Estimates of three-year old ocean recruits were adjusted to January age-3 recruits in order to provide appropriate inputs for coho management models (expansion factor = 1.23). December age-2 recruits, which have been included in this table in previous years, are not provided as they are no longer used by fisheries managers. The following sections describe the approach used to derive smolt production and marine survival estimates.