Variation of fishing ground gravity of tuna free-swimming school caught by purse seiner in Western and Central Pacific Ocean and its relationship with Southern Oscillation Index

Based on the data of fishing positon, fishing date and catch of free-swimming school from the logbooks of Chinese tuna purse seine fishing fleet operating on the Western and Central Pacific Ocean during 2013 and 2017, we analyzed the relationship between monthly and yearly variation of fishing grounds gravity of free-swimming school and the South Oscillation Index (SOI) obtained from the US Climate Analysis Center. The results show that more catch was found in the fishing areas with sea surface temperature (SST) above 29.5 ℃. The fishing ground gravity was mainly distributed between 160°E and 175°W. The gravity center had an eastward tendency from 2013 to 2015 without evident monthly variation. The "warm pool" and the gravity center of the Western and Central Pacific Ocean moved towards the west when SOI was positive, while the gravity center moved eastwards when SOI was negative. Correlation analysis shows a significant negative correlation between SOI and fishing ground gravity (the correlation coefficient was −0.27, P<0.05), indicating that there was a close relationship between the change of fishing ground gravity center and climate change. The study helps understand the variation law of fishing ground of tuna purse seine fishery in the Western and Central Pacific Ocean.