This study examines the price elasticity of demand for mackerel in the Busan Cooperative Fish Market, the largest wholesale fish market in South Korea. Using a two-stage least squares (2SLS) approach, the analysis addresses endogeneity in pricing by incorporating exogenous environmental variables, such as sea surface temperature (SST) and wind speed. The study estimates demand elasticity for three size categories of mackerel-large, medium, and small-and reveals significant differences across these categories. Large-size mackerel exhibits inelastic demand (-0.875), reflecting its status as a staple product with relatively stable consumer demand. Medium-size mackerel shows highly elastic demand (-2.450), likely due to its role as a substitute for both large and small mackerel. Small-size mackerel also demonstrates high elasticity (-3.444), attributed to its primary use in feed and processing, where demand is particularly sensitive to price changes. Diagnostic tests confirm the validity and relevance of the instrumental variables, with SST and wind speed strongly correlated with price but uncorrelated with consumer demand. These findings highlight the critical role of size-specific market characteristics in shaping price elasticity and provide valuable insights for policymakers and industry stakeholders to better manage mackerel supply and ensure pricing stability.