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Character of temporal variability in stock productivity influences the utility of dynamic reference points

Berger, Aaron M.
Fisheries research 2019 v.217 pp. 185-197
biomass, coasts, demersal fish, ecosystems, fisheries management, life history, mortality, temporal variation, uncertainty, United States
Reference points identify benchmarks, thresholds, or decision points for fisheries management, and are commonly defined by stock status indicators that presume equilibrium population conditions in the absence of fishing (e.g., equilibrium biomass, B0). However, equilibrium population biomass may be an inappropriate reference level when stock productivity is influenced by environmental change, predator-prey dynamics, ecosystem thresholds, and myriad other factors. Simulations were conducted to compare equilibrium-based (static B0) and non-equilibrium based (dynamic B0) indicators of stock status under alternative states of nature driven by time-varying recruitment dynamics (productivity regime), fishing dynamics (mortality regime), and species life history. Using dynamic B0 often implied a different state of the stock under directional productivity regime shifts, but was more similar to static B0 reference points under cyclic or white noise productivity scenarios. Uncertainty in stock status arising from incorrectly identifying changes in system productivity generally outweighed the uncertainty associated with initial equilibrium conditions. Empirical results across 18 U.S. West Coast groundfish stock assessments indicated predominantly small differences (<10%) between static B0 and dynamic B0 indicators of stock status, although in some cases differences were large (up to 72%) or spanned reference points that trigger management action. Although caution is warranted when considering dynamic reference points, this paper shows these approaches are likely to be most useful when stock productivity shifts directionally, if that productivity signal can be correctly ascertained.