Research Document 2018/011

Evaluating Benchmarks of Biological Status for Data-limited Conservation Units of Pacific Salmon, Focusing on Chum Salmon in Southern BC

By: Holt, C.A., Davis, B., Dobson, D., Godbout, L., Luedke, W., Tadey, J., et Van Will, P.

Abstract

Status assessments for Chum Salmon (Oncorhynchus keta) under the Wild Salmon Policy (WSP) have been limited, in part because recruitment time-series required to calculate stock-recruitment based benchmarks are not consistently available. Alternative benchmarks have been proposed for data-limited Conservation Units (CUs) using percentiles of the observed spawner abundance time-series.  However, these benchmarks have not been evaluated against stock-recruitment benchmarks currently used to assess status on abundances for data-rich CUs.  Our goals were to evaluate percentile-based benchmarks against stock-recruitment based benchmarks accounting for high uncertainties and possible biases in spawner abundances, catches, recruitment estimates, and age-at-maturity.  We used two approaches to evaluate benchmarks based on a retrospective comparison through the historical record and a prospective simulation model under numerous hypothetical future scenarios. We demonstrate an approach for providing assessments that accounts for uncertainties in benchmarks, and provide advice on the applicability of percentile-based benchmarks for data-limited CUs of Chum Salmon relative to stock-recruitment benchmarks used for data-rich CUs. In general, our results support the application of percentile-based benchmarks for data-limited CUs of Chum Salmon when productivity is moderate to high (>2.5 recruits/spawner) and harvest rates are low to moderate (≤40%). However, we suggest further evaluation of percentile benchmarks (and the consideration of alternatives) when productivity is expected to be low and/or harvest rates high. Under these conditions, concurrent declines in abundances and percentile benchmarks can results in status assessments that are more optimistic than those from stock-recruitment benchmarks due to a shifting baseline.

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