Abstract
This study compares landings, revenues, and price structures between two major Andalusian first-sale auctions in the Gulf of Cadiz (SW Spain): Bonanza (Sanlucar de Barrameda) and Isla Cristina. Using 2024 auction records aggregated at the vessel–species level, we analyse how outcomes differ across ports and whether contrasts are driven by species composition, within-species pricing, or scale (lot-size) effects. Descriptive statistical indicators, non-parametric inference (Mann–Whitney tests) and effect-size measures are complemented with alternative price models (log-OLS, Gamma-GLM and GAM) to assess the robustness of the price–quantity relationship. Results indicate that Bonanza tends to exhibit stronger performance for the typical vessel (higher landings, higher revenues, and greater species diversification), whereas Isla Cristina concentrates a larger share of total activity, consistent with a more scale-oriented structure dominated by a subset of high-throughput vessels. Differences in average unit values between ports are largely explained by species mix and volume weights (i.e., the relative importance of high- versus low-value taxa), while composition-controlled models point to a smaller within-species port effect for Isla Cristina that varies with landed quantity. Overall, routinely collected auction data provide a consistent basis for comparing port-level market outcomes and for distinguishing whether cross-port differences arise primarily from composition, pricing, or both.