Research Document 2019/050

Candidate Limit Reference Points as a Basis for Choosing Among Alternative Harvest Control Rules for Pacific Herring (Clupea pallasii) in British Columbia

By Cox, S.P., Benson, A.J., Cleary, J.S, and Taylor, N.G.

Abstract

British Columbia's (BC) Pacific Herring (Clupea pallasii) fisheries are managed using a harvest control rule (HCR) that was initially designed in 1986. The HCR includes a stock-specific minimum biomass threshold below which commercial fisheries are not permitted (the "cut-off") and a target harvest rate of 20%. Since adopting the HCR, two of five major herring stocks have remained above the cut-off level each year and continue to support fisheries, while three stocks have recently dropped below cut-off for up to eight consecutive years. Significant increases in estimated natural mortality (M) and decreases in body size have been observed for some stocks the same timeframe. The relative contributions of these factors to stocks falling below cut-offs are currently not well understood.
This paper represents the first step in a management strategy evaluation (MSE) process that develops the analytical framework for future analyses and explores the suitability of candidate conservation objectives (limit reference points) for the five Pacific Herring stocks. The key components of the framework are:

1. operating models that reflect a range of potential future changes in growth and natural mortality;
2. management procedures (MP) comprised of data, stock assessment models, and harvest control rules (HCR) including the current DFO rule and alternatives recommended for forage fish; and,
3. biological limit reference points (LRP) that are used in determining the expected conservation performance of alternative management procedures.

The LRPs explored in this paper include:

1. equilibrium reference points that remain fixed over time;
2. a dynamic reference point that tracks changes in productivity;
3. a historical reference point that defines LRP in terms of lowest observed biomass; and,
4. DFO policy values of 0.4BMSY and FMSY.

This study suggests that future work to identify LRPs for BC herring fisheries should focus on fixed (equilibrium) objectives related to biomass. Fishing mortality-based LRPs were not generally useful for distinguishing between candidate MPs on the basis of conservation performance. Furthermore, our analysis indicates that the current DFO MP performs well only over a narrow range of conditions for particular stocks. Increases in M similar to those estimated over the past several decades revealed relatively poor conservation performance in 4 of 5 stock areas. We therefore recommend exploring alternative MP that can provide good performance across a range of future scenarios for Herring population dynamics.

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