Stock assessment and spawning potential ratio-based management procedures for pāua (Haliotis iris) fisheries in PAU 3A

In November 2016, a strong earthquake struck the core area of the PAU 3 pāua (Haliotis iris) fishery,particularly impacting areas surrounding Kaikōura peninsula. The earthquake caused coastal uplift of up to 6 metres, and widespread mortality across various marine organisms, including pāua. For this reason, a fishery closure was imposed on the northern part of PAU 3 for five years, whereas the southern section remained open with a reduced Total Allowable Commercial Catch (TACC).

To manage the earthquake-affected area separately, quota management area PAU 3 was divided into PAU 3A (Kaikōura) and PAU 3B (Canterbury coast and Banks Peninsula). Dive surveys post-earthquake revealed significant recovery and new recruitment in PAU 3A, leading to the reopening of the fishery in December 2021, albeit with cautious restrictions and monitoring measures.

Before the earthquake, stock assessments in PAU 3 relied on length-based statistical methods, which posed challenges due to limited biological data. The current project aimed to develop a new stock assessment model based on updated fisheries, biological and survey data, and to evaluate management strategies that enable sustainable harvest post-earthquake. While acknowledging the complexities of earthquake impacts, the focus of this project was on developing parsimonious assessment models and robust harvest control rules for the commercial fishery to ensure the fishery’s long-term sustainability.

Assessment models were able to fit both pre- and post-earthquake data well, including survey indices, which provided evidence of the strong rebuild post-earthquake. The selected base-case model did not appear to require strong assumptions about earthquake impacts to fit these data, suggesting that the assessment model is inherently flexible enough to represent dynamics pre- and post-earthquake in a relatively straightforward way. Despite some remaining uncertainties, including uncertainties regarding recreational harvest levels and survey representation, the base-case model consistently estimated that the stock was at or above default target levels in 2023.

New harvest control rules were based on a target spawning potential ratio (SPR) of 50% (i.e., the ratio of reproductive output estimated from fishery length data relative to unfished reproductive output), and used CPUE to gauge the “direction of travel” when the stock is outside of the SPR target zone to adjust catches and return the stock spawning potential towards the target level. This semi-empirical control rule was tested against the base-case assessment and a range of models and management options for immediate increases in total catch. Proposed harvest control rules based on spawning potential ratio targets appeared to maintain the fishery in a healthy state.