Spatio-temporal distribution of yellowfin tuna Thunnus albacares and bigeye tuna Thunnus obesus in the Tropical Pacific Ocean in relation to large-scale temperature fluctuation during ENSO episodes

Abstract

Historical catch and effort data collected from tuna longliners during 1962 and 1997 are used to investigate how the spatio-temporal distribution of the two most economically important species of tunas in the Tropical Pacific Ocean (TPO) (i.e. yellowfin Thunnus albacares and bigeye Thunnus obesus) varies with the ocean temperature fluctuations that occur during El Niño/Southern Oscillation (ENSO) events. Relatively high hook rates of both species were mostly associated with regions where sea surface temperature (SST) increased during El Niño or La Niña years. Conversely, relatively low hook rates were associated with regions where SST fell during La Niña years, especially in the eastern TPO. The hook rates of these regions varied concurrently or within 3 months after the onset of an ENSO, as determined by the Southern Oscillation Index (SOI). We speculate that the displacement of the preferable water column and the change in availability of fishing gear during ENSO are the two possible reasons causing the change of hook rates. During El Niño periods, the part of the water column preferred by bigeye tuna was expended in western regions of the eastern TPO and compressed in the east. This appeared to lead to a displacement of bigeye from east to west. During La Niña episodes, yellowfin tuna appeared to undergo a meridian displacement and expend their preferred range northwards.