Ocean variations associated with fishing conditions for yellowfin tuna (Thunnus albacares) in the equatorial Atlantic Ocean

In this study, the Taiwanese longline (LL) fishery data were divided into two types: regular LL and deep LL. Furthermore, we collected environmental variables, such as sea surface temperature (SST), subsurface temperature, chlorophyll a concentration, net primary productivity, windspeed, and the north tropical Atlantic SST index (NTA) during the period 1998-2007 to investigate the relationship between LL catch data and oceanic environmental factors using principal component analysis (PCA). After the daily LL was separated into two types of LL, the results indicated that the deep LL was the major fishery catching yellowfin tuna (YFT) in the equatorial Atlantic Ocean. In 2003-2005, especially in 2005, the monthly catch by deep LL was double those of other years. The spatial distribution of the nominal catch per unit effort (cpue) by deep LL showed the maximum aggregation of YFT in waters with temperature above 24-25 degrees C. The YFT mainly aggregated in the equatorial Atlantic, extending east in the first and second quarters of the year. In the third quarter of the year, the SST decreased off West Africa and the YFT migrated westwards to 15 degrees W. Results of PCA indicated that higher subsurface water temperatures resulted in a deeper thermocline and caused a higher cpue of YFT, but the influence of NTA on the cpue of YFT seemed to be insignificant.