Considering ecosystem aspects when setting management targets for New Zealand’s fisheries: Insights from simulations and application to snapper and gurnard in FMA 7

Fisheries management in Aotearoa/New Zealand has historically focused on single-species models, supplemented by considerations about changes in the ecosystem affecting stocks, as well as bycatch and habitat impact considerations. The present project explored the incorporation of ecosystem considerations into fisheries management for single species in New Zealan’s fisheries, with a case-study focusing on snapper (SNA; Pagrus auratus) and gurnard (GUR; Cheliodonichthys kumu) in the Tasman Bay Golden Bay region (Fisheries Management area (FMA) 7). Using simulation models and empirical analyses, we evaluated implications of setting management targets above traditional Maximum Sustainable Yield (SSB_MSY) proxies, and utilising dynamic reference points to account for ecosystem dynamics. For dynamic reference points, we explored the utility of varying target reference points, while maintaining fixed limit reference points.

Two simulation models were developed: a single-species framework evaluating management at or above default targets (40%, 55%, and 70% of unfished spawning stock biomass, SSB₀) and a multispecies case study for FMA 7 focusing on higher catch limits for snapper (SNA 7) and the indirect effects of setting higher snapper catch limits for gurnard fishery performance. Key performance metrics across simulations included catch, catch-per-unit-effort (CPUE), stock status, economic value (Net Present Value), and age/length composition of stocks. Fishing mortality-based harvest control rules were applied in all scenarios.

Simulations showed significant trade-offs in managing stocks above traditional SSB_MSY targets. Higher targets (e.g., 55% and 70% of SSB₀) resulted in improved CPUE and increased numbers of larger and older fish, reflecting potential ecosystem and conservation benefits. However, catches decreased by 20% to 45% at these higher targets, potentially impacting economic returns. The economic performance of higher targets depended on operational costs, with high-cost scenarios benefiting from higher biomass levels requiring lower levels of effort to catch the available quota, whereas relatively low-cost fisheries had diminished profitability due to the lower catches associated with higher biomass targets.

While dynamic target reference points allowed for additional responsiveness to environmental changes (e.g., recruitment shifts) over the application of a static harvest control rule, our simulations showed little actual benefit in adopting dynamic targets while maintaining static limit reference points in view of regime shifts. For stocks with declining productivity, under static limit reference points, a higher average catch than under static target reference points was balanced by periodic reductions in catch due to breaching limit reference points, necessitating more frequent reductions in catch to balance higher average catches. In contrast, for stocks with increasing productivity, dynamic and fixed targets resulted in comparable outcomes, as the simulated harvest control rule effectively adjusted fishing mortality to reflect higher stock biomass regardless of the dynamic nature of the target reference point.

The FMA 7 multispecies case study revealed that recent high recruitment for snapper and gurnard has moved biomass above SSB_MSY. Projecting stock dynamics under higher targets highlighted notable differences in response times and length compositions, with snapper showing more substantial increases in larger fish numbers at higher targets. Economic outcomes for both species reflected broader trends, with low-cost fisheries benefiting from lower biomass targets, while fisheries with higher costs benefit from increased CPUE under higher biomass targets.

Balancing ecological and economic outcomes requires nuanced approaches. Higher biomass targets enhance ecosystem resilience and conservation outcomes but reduce short-term catch volumes. In a multispecies management context, management decisions for any single species will impact on associated fisheries. Our multispecies characterisation focused on the spatial and seasonal overlap of species in Tasman Bay Golden Bay, with a focus on overlap of snapper and gurnard. Snapper was found more frequently in shallower waters during summer, while gurnard was more evenly distributed year-round. Nevertheless, the multispecies characterisation identified significant overlaps in snapper and gurnard fisheries. Approximately 58% of snapper catch has been associated with gurnard-targeted tows in recent years, highlighting the interdependence of these stocks. Managing snapper at higher targets would likely reduce gurnard catches, but increase CPUE and biomass for both species.

Incorporating ecosystem considerations into management targets can offer tangible benefits for stock sustainability and biodiversity conservation (e.g., maintaining a more diverse age structure, and lower effort leading to reduced indirect impacts), but requires careful trade-off analyses in terms of its potential socio-economic impacts on the fisheries sector. Given the difficulty of predicting, quantitatively, the outcome of these single-species management decisions in a multispecies context, multispecies characterisations such as provided for this project can help establish scenarios based on current fishing practice, which then can be discussed with stakeholders, and used to guide management decisions in an ecosystem context.