Operating model and management procedure evaluation for pāua (Haliotis iris) fisheries in PAU 3A

The pāua (Haliotis iris) stock in quota management area (QMA) PAU 3 was affected by a significant earthquake in November 2016, with coastal uplift of up to 6~m causing widespread mortality of all life stages of p=aua and other intertidal and subtidal organisms. The northern part of the fishery (now PAU 3A) was subsequently closed for 5 years until 2021. It was also monitored using surveys to document the rebuild post-earthquake and develop indicators that could signal that re-opening the fishery would not negatively affect it long term.

Prior to the earthquake, the PAU 3 QMA was assessed on the basis of length-based statistical stock assessments up to 2016. The stock was considered to be in healthy condition, although considerable uncertainties about stock status remained, due to insufficient biological information to provide an understanding of local stock productivity. Dive surveys during the post-earthquake closure showed strong rebuilding of pāua numbers, and presence of all size classes. On the basis of this information, the fishery was reopened in December 2021 with a total allowable commercial catch (TACC) of 23 t. This catch weight was considered to be precautionary (catch prior to the earthquake was regularly in excess of 50 t). Nevertheless, monitoring of recreational catch suggested a take of 40 t for the first open season since the earthquake, owing to a high number of pāua that were easily accessible.

A fisheries plan for PAU 3 was developed following the earthquake, stipulating management strategies for re-opening and post re-opening. After the subdivision of the PAU 3 fishery into PAU 3A, the earthquake-affected area, and PAU 3B, the relatively unaffected area south of the Conway River, the fisheries plan suggested that both areas should be managed on the basis of harvest control rules for commercial catch. The present project attempted to develop models and test management options for PAU 3A as the fishery rebuilds. We focused on key uncertainties that remain for management of the fishery long term: modelling of earthquake impacts and recovery, and the developments of recreational catch over time.

Models used a range of simple assumptions about plausible earthquake impacts, and attempted to integrate survey data directly into stock assessments. While available data do not provide a basis for determining the likelihood of earthquake impact scenarios, a range of scenarios were tested in terms of their implications for management. Similarly, recreational catch cannot be precisely controlled, and alternative recreational catch scenarios were explored to understand the trade-off between long-term recreational catch and the commercial fishery in the long term.

The models were able to fit the fishery and aspects of survey data (indices), suggesting that models are able to represent the fishery development despite the earthquake. Although the simple assumptions were not able to capture more subtle trends in survey indices, and provided relatively poor fits to survey length-frequency data, they provided a starting point to modelling earthquake impacts in future assessments.

Depending on the model fitting strategy and earthquake impacts, tested commercial harvest strategies, combined with recreational harvest scenarios, led to relatively stable, or declining fisheries. The commercial fishery was especially affected by the trade-off with recreational take when recreational catch was not markedly reduced, leading to long-term reductions in commercial catch from re-opening levels. While differences between scenarios were minor in the short term, uncertainties were substantial over long periods, leading to non-negligible risk in some models. Given uncertainties about stock trajectories and risks, we recommend ongoing monitoring of commercial and recreational catch. Other recommendations include a review of management options in 2 to 3 years’ time once trends and levels of recreational catch have stabilised and key uncertainties can be re-assessed to ensure fishery performance in the long term as the fishery develops post-closure.