Perpustakaan Digital ITB

We evaluated the isoline distribution of thermocline characteristics (the upper and lower boundary temperatures and depths) in the bigeye tuna (Thunnus obesus) fishing grounds in the tropical Atlantic Ocean. We plotted the thermocline characteristics contour on a spatial overlay map using data collected on a monthly basis from Argo buoys and monthly CPUE (catch per unit effort) from bigeye tuna long-lines from the International Commission for the Conservation of Atlantic Tunas (ICCAT). In addition, frequency analysis and the empirical cumulative distribution function (ECDF) were used to calculate the optimum ranges of thermocline characteristics of the central fishing grounds. Our analysis suggested that there were significant seasonal variations in the upper boundary temperature and depth of thermocline in the central fishing grounds, which significantly influenced the temporal and spatial distribution of the bigeye tuna population. The overlay maps suggest that the central fishing ground of the bigeye tuna is located in the enclosed curve region between the higher value (70 m) and the lower value (40 m) of the upper boundary depth, from December to March of each year. After that, the fishing grounds spread to the north and south on both sides. CPUE is highest in areas where the upper boundary depths of the thermocline are between 40 and 60m to the north of the equator. This extends to between 60 and 100m to the south of the equator. The CPUE is highest in areas where the upper boundary temperature of the thermocline is between 24 and 26.9 °C in the low-latitude Atlantic Ocean, but decreases to 18 °C offshore of Namibia during July to October. The overlay maps showed that the lower boundary temperature and depth of thermocline have little seasonal variation. The lower boundary depth of the thermocline isoline depth showed that the higher depth area (>250 m) was shaped like the letter ‘W’. Similarly, the lower boundary temperature of thermocline isoline depth suggests that the temperature is higher besides the equator and lower around the equator area. The lower boundary water depth in the central fishing ground of the bigeye tuna ranged between 150 m and 250 m in the low-latitude Atlantic Ocean over the year, whereas the CPUE was low when the depth exceeded 250 m. In addition, the lower boundary water depth in the central fishing ground of the bigeye tuna decreased to between 140 and 160 m offshore of Namibia from April to October. The lower boundary temperature in the central fishing ground was about 12 °C in the lowlatitude Atlantic Ocean and 14 °C offshore of Namibia. Conversely, if the temperature is >15 °C, the CPUE tends to be low. The numerical computation results suggest that the optimum ranges for the upper boundary thermocline temperature and depth were between 23 and 26.9 °C, and 30 and 70 m, respectively. The optimum ranges were 12–14.9 °C and 150–230 m for the lower boundary thermocline temperature and depth, respectively. We documented the distribution interval range of thermocline characteristics for the central fishing ground of the bigeye tuna in the Atlantic Ocean. Our results provide a reference for improving the efficiency of the bigeye tuna long-line fleet and aids tuna resource management in the Atlantic Ocean.