Scale mismatches limit the efficacy of customary management

1. Indigenous lands are some of the most biodiverse areas in the world, providing protection for many species and spillover benefits for wider communities. However, these areas face increasing threats. Indigenous communities face many challenges in protecting and managing these lands, particularly in the form of power imbalances and spatial, temporal, and functional-conceptual mismatches. 2. Here, we explore these issues in the context of a wild tuna|eel (predominantly Anguilla australis) fishery at Te Waihora|Lake Ellesmere in Aotearoa|New Zealand. This is the largest commercial eel fishery in New Zealand and is a key customary fishery for Ngāi Tahu (southern-most tribe in New Zealand) communities. To protect the customary fishery, a Kōhanga|Indigenous reserve area was established in 2005. 3. We combined mātauranga (traditional knowledge) from a Ngāi Tahu elder (a customary fisherman) with 16 months of sampling data to determine whether the Kōhanga currently supports a sufficient population of customary-sized eels. We then developed a matrix population model to simulate the population and size structure of eels inside and outside of the Kōhanga. Informed by the sampling data, we compared the potential of different management scenarios to improve customary-sized eel populations. 4. We find that the abundance of customary-sized eels (>1000g) has plummeted in recent years and customary fishing is no longer viable. The Kōhanga is too small to provide adequate protection, and the benefit it does provide is not fully realized by local Ngāi Tahu communities because eels move from the Kōhanga to the commercially fished part of the lake. Our simulations show that increasing the size of the Kōhanga would help grow the abundance of customary-sized eels, but ultimately growth rate is the most important factor limiting their populations. Improving growth rate would require catchment-scale restoration of water quality and the food chain in Te Waihora. 5. Overall, customary management of the eel population is severely limited by a mismatch between the scale of the customary protected area and the social-ecological processes driving eel populations. An effective solution will call for a recognition of the wide spatial and temporal scales relevant at Te Waihora and the core values, world views and aspirations of Ngāi Tahu and diverse stakeholder groups.