This paper explores the integration of traditional ecological knowledge (TEK) and scientific methods for monitoring wildlife populations to support sustainable customary harvests by indigenous peoples. It highlights case studies from Canada and New Zealand, where TEK, despite being imprecise and qualitative, is valuable due to its long-term observations, large sample sizes, and cost-effectiveness. TEK methods such as catch per unit effort (CPUE) and body condition are discussed, showing how they can complement scientific approaches. Science can provide more precise data on population changes, but it is often expensive and may not be trusted by customary users. Combining TEK with scientific methods can build partnerships, foster community consensus, and allow indigenous users to critically evaluate scientific predictions. The paper argues that sustainable management requires the complementary use of local and scientific knowledge for co-management. It emphasizes the importance of reliable monitoring methods to assess sustainability and promote adaptive management. Traditional ecological knowledge is seen as similar to adaptive management, as both involve learning by doing. The paper also discusses the limitations of traditional monitoring methods, such as potential biases in body condition assessments and the curvilinear relationship between CPUE and population abundance. It concludes that integrating TEK and science can enhance population monitoring, provide a more holistic understanding of ecological systems, and support sustainable resource management. The paper advocates for collaborative approaches that respect both scientific and traditional knowledge systems.This paper explores the integration of traditional ecological knowledge (TEK) and scientific methods for monitoring wildlife populations to support sustainable customary harvests by indigenous peoples. It highlights case studies from Canada and New Zealand, where TEK, despite being imprecise and qualitative, is valuable due to its long-term observations, large sample sizes, and cost-effectiveness. TEK methods such as catch per unit effort (CPUE) and body condition are discussed, showing how they can complement scientific approaches. Science can provide more precise data on population changes, but it is often expensive and may not be trusted by customary users. Combining TEK with scientific methods can build partnerships, foster community consensus, and allow indigenous users to critically evaluate scientific predictions. The paper argues that sustainable management requires the complementary use of local and scientific knowledge for co-management. It emphasizes the importance of reliable monitoring methods to assess sustainability and promote adaptive management. Traditional ecological knowledge is seen as similar to adaptive management, as both involve learning by doing. The paper also discusses the limitations of traditional monitoring methods, such as potential biases in body condition assessments and the curvilinear relationship between CPUE and population abundance. It concludes that integrating TEK and science can enhance population monitoring, provide a more holistic understanding of ecological systems, and support sustainable resource management. The paper advocates for collaborative approaches that respect both scientific and traditional knowledge systems.