Identifying trade-offs for alternative rebuilding polices: Insights from simulations and application to tarakihi

Recent legal challenges to fisheries rebuilding plans have highlighted the need for a comprehensive evaluation of alternative rebuilding strategies. This report presents an analysis of different approaches to rebuilding depleted fish stocks, considering both biological and economic factors. We compared timeline-based policies based on different definitions (multiples of minimum rebuilding time T_min and generation times), which set fixed rebuilding periods, with control rule-based policies that adjust fishing pressure based on stock status.

Our analysis found that T_min was similar to generation time at F₄₀ (the fishing mortality that produces a spawning biomass at 40% of unfished levels) for fast and moderate life histories, but considerably faster (10 years) than generation time without fishing (39 years) for slow life history species.

Life history characteristics significantly influenced rebuilding strategies. Short-lived species showed faster rebuilding potential but were more sensitive to recruitment variability, while long-lived species had more stable but longer rebuilding trajectories. For short-lived species subject to environmental fluctuations, responsive control rules may be more effective than specific timelines.

Simulations showed differences between management approaches based on fishing mortality (F) and on catch. F-based management during rebuilding led to larger initial catch reductions, nearly matching reductions imposed by harvest control rules, while catch-based management showed smaller initial reductions but resulted in longer and more variable rebuilding timelines. Adaptive rebuilding scenarios, which allowed for adjustments during the rebuilding period, performed better than fixed policies, especially for F-based management with longer timelines.

In terms of risk, control rule-based policies generally showed lower risk of falling below the hard limit (10% SSB₀) compared to timeline-based policies. While catch stability varied between approaches, control rule-based policies and F-based management typically led to large initial catch reductions but allowed for earlier increases during rebuilding. Timeline-based policies, particularly catch-based ones, showed more gradual initial reductions but were more prone to subsequent large adjustments.

The economic performance analysis revealed that the success of different policies was highly dependent on CPUE (catch-per-unit-effort) hyperstability, and per-unit-effort costs. Under base settings, there was no clear advantage between control-rule and timeline-based policies in terms of net present value (NPV). Nevertheless, with CPUE hyperstability or low per-unit-effort costs, longer rebuilding timelines achieved higher NPV, while the opposite was the case with hyper-depleted CPUE or high per-unit-effort costs. These findings were further illustrated through a case study for east coast tarakihi (Nemadactylus macropterus), which showed similar patterns in rebuilding times and risk.

Control rule policies generally performed better economically when recruitment was below expectations, highlighting the value of continuous fishing mortality adjustments during rebuilding. While no rebuilding policy can be defined as optimal under all circumstances (all policies involve some level of risk), our study suggests that rebuilding plans could be designed to minimise risk through specified and simulation-tested adjustments during rebuilding.