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

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

Robert A. Clark, Douglas M. Eggers, Andrew R. Munro, Steven J. Fleischman, Brian G. Bue, and James J. Hasbrouck

An Evaluation of the Percentile Approach for Establishing Sustainable Escapement Goals in Lieu of Stock Productivity InformationThis is part of Assessing and Managing Data-Limited Fish Stocks
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Description

Stock-recruitment analysis is the typical method used to establish biological escapement goals (BEGs) that provide the greatest potential for maximum sustainable yield (SMSY) of Pacific salmon stocks in Alaska. For stocks where the necessary stock-specific information is lacking, there are no published methods for estimation of proxies for SMSY to aid in the development of sustainable escapement goals (SEGs). One such proxy for SMSY was developed in an unpublished report by Bue and Hasbrouck in 2001 and is now commonly called the Percentile Approach. We evaluated the Percentile Approach and recommended changes to the approach based on outcomes of our analyses. All of the analyses indicate that the four tiers of the Percentile Approach are likely sub-optimal as proxies for determining a range of escapements around SMSY. The upper bounds of SEGs developed with this approach may actually be unsustainable in that they may specify a spawning escapement that is close to or exceeds the carrying capacity of the stock. The lower bound percentile of SEG Tier 1 (25%) also appears somewhat higher than necessary. Escapements in the lower 60 to 65 percentiles are optimal across a wide range of productivities, serial correlation in escapements, and measurement error in escapements. We recommend that the current four-tier Percentile Approach be replaced with the following three 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) with low to moderate average harvest rates (<0.40), the 20th to 60th percentiles
* Tier 2: high contrast (>8) and low measurement error (weirs, towers) with low to moderate average harvest rates (<0.40), the 15th to 65th percentiles
* Tier 3: low contrast (8 or less) and high or low measurement error with low to moderate average harvest rates (<0.40), the 5th to 65th percentiles

Use of the Percentile Approach is not recommended for the following situations:
* average harvest rates of 0.40 and greater
* very low contrast (4 or less) and high measurement error (aerial or foot surveys)

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