An Evaluation of the Percentile Approach for Establishing Sustainable Escapement Goals In Lieu of Stock Productivity Information

Stock-recruitment analysis is the typical method to establish escapement goals that provide the greatest potential for MSY of Pacific salmon stocks. For stocks where productivity information is lacking, there are no published methods for proxies of S_MSY to aid in the development of an escapement goal. One such proxy for S_MSY was developed in Alaska by Bue and Hasbrouck (Unpublished) in 2001 and is also called the Percentile Approach. We evaluated the Percentile Approach using three analytical methods. The analyses indicate that the four tiers of the Percentile Approach are likely sub-optimal as proxies for determining a range of escapements around S_MSY. Escapements in the lower 60 to 65 percentiles are optimal across a wide range of productivities, serial correlation, and measurement error in escapements. We recommend that the current 4 tier approach be replaced with 3 tiers for stocks with low to moderate (less than 0.40) average harvest rates:

• Tier 1 – high contrast (>8) and high measurement error (aerial and foot surveys), the 20^th to 60^th percentiles
• Tier 2 – high contrast (>8) and low measurement error (weirs, towers), the 15^th to 65^th percentiles
• Tier 3 – low contrast (8 or less), the 5^th to 65^th percentiles