Spatial, seasonal, and annual fluctuations in relative abundance of yellowfin tuna in the eastern Pacific Ocean during 1984-1990 based on fishery cpue analysis

Spatial and seasonal fluctuations of the relative abundance of yellowfin tuna in the eastern Pacific Ocean was analyzed using the logbook records of the Mexican tuna purse-seine fleet that operated during 1984-1990. We used fishing day as the unit of effort, standardized through the principal components analysis. The first three components in the model were significant. The catch-per-unit-effort monthly average as an index of abundance was estimated for one-degree area quadrants for the total fleet operation area and classified into five levels of abundance for graphic presentation. The variation of seasonal relative abundance was meaningful, with its maximum during the second quarter. Variation of annual relative abundance was also significant with a minimum during 1984 and a maximum in 1986. This led us to believe the El Niño (1986-1987) had no relevant effect on the abundance of the resource. The lowest relative abundance (12 t per fishing day) coincided with the greatest percentage of unsuccessful fishing, and it was found in northwest Mexico. The greatest abundance was found in the oceanic area (west of 120°W; 18.5 t per day fishing) coinciding with the area of low-percentage unsuccessful fishing. No significant differences were found in the average catch-per-standard-day-fishing distribution and value between the pre-1988 period, when operations in the area were dominated by the U.S. fleet and those after 1988 when the Mexican fleet took its place.