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Catch data of the yellowfin tuna (YFT) fishery were used to study the relationship between the seasonal and spatial variations of YFT with or without the presence of mesoscale structures at the entrance to the Gulf of California (19°-24° N, 104°-112° W). The YFT catches were distributed mainly over the continental shelf and around islands and seamounts. High concentration sets of YFT occurred throughout the year around the Marías islands and Cabo Corrientes, particularly in March and June. A warm (>28°C) coastal jet with a width of 1.5 Rossby radius (∼20.6 km) was observed off Cabo Corrientes during February and April, covering all the continental shelf off Cabo Corrientes. The jet moved at an average speed of 0.31 m s-1 and extended northwards to the Roca Corbetera seamount. The interaction with cooler water generated a frontal zone around which the YFT catches were concentrated. In contrast, during March the wind field was more favorable for the development of coastal upwelling off Cabo Corrientes. We observed an upwelling region with sea surface temperature of 18°C and an offshore width of about a 3.7 Rossby radius of cold water that covered some 2600 km². The YFT catches occurred in warmer water north of the upwelling zone. [ABSTRACT FROM AUTHOR]