Biological Reference Points for the Conservation and Management of Steelhead, Oncorhynchus mykiss
Abstract
We derive two simple biological reference points from a Beverton-Holt stock-recruitment relationship. We then compare the performance of several biological reference points as thresholds in single threshold and dual threshold harvest control rules, using an age-structured population model based on Keogh River steelhead that incorporates realistic levels of process error and implementation error.
Simple harvest control rules that use abundance thresholds to initiate reductions in harvest rates below the threshold can considerably reduce the risk of quasi-extinction for small populations of steelhead at low abundance compared to constant exploitation rate (CER) harvesting policies. Threshold harvesting policies can also reduce recovery time and increase both catch and escapement compared to a CER policy. A CER policy will maximize ln(catch) and reduce the frequency of fishing closures, but it will also increase the risk of quasi-extinction for low productivity stocks compared to threshold harvesting policies. “Constant” exploitation rate control rules that use abundance thresholds can perform similarly to proportional threshold harvesting under conditions of strongly autocorrelated environmental variability and realistic levels of prediction error and implementation error. Harvest control rules that have both an upper precautionary threshold and a limit reference point reduce extinction risk at low stock productivity. The thresholds and harvest rates that maximize catch change with stock productivity, variance in smolt-to-adult survival, temporal autocorrelation in survival deviations, maximum population size, uncertainty in the asymptotic maximum recruitment, and the form of the spawner-smolt recruitment function, but the qualitative results hold true. NLRP, the abundance threshold from which a population can recover to 0.25·B in one generation in the absence of harvesting, increased escapement and lowered extinction risk at low stock productivity with only small reductions to maximum catches. Empirical estimates of the catch-maximizing LRP for the Keogh River steelhead were about 0.17·B to 0.18·B for both Beverton-Holt and rectilinear “hockey stick” stock-recruitment functions.