Interannual Variability of Yellowfin Tuna (Thunnus albacares) and Bigeye Tuna (Thunnus obesus) Catches and Its Relationship to Climate Variability in the Western Tropical Indian Ocean

Abstract:

Pelagic tuna fishery is one of the main sources of potential economic growth for small island countries in the western tropical Indian ocean (WTIO). This study analyzes annual catch amounts of the two most abundant pelagic tuna species in the WTIO, yellowfin tuna (Thunnus albacares) and bigeye tuna (Thunnus obesus), and investigates their relationship to climate variability during 1990–2014. A prominent upwelling in the northern part of the WTIO (north of 12°S) contributes to a meridional difference in the ocean environment of the WTIO, which suggests possible difference in the favorable ocean conditions for the tuna catches. Thus, regression and reconstruction analysis was conducted for ocean environmental variables with respect to the tuna catches in the northern and southern parts of the WTIO, respectively. The increase of tuna catches in the northern part of the WTIO is related to flattening of the thermocline ridge, i.e. subsurface warming, upper-ocean freshening, increase of dissolved oxygen, and weakening of upper-ocean currents. The favorable ocean environmental conditions for the increase of tuna catches in the southern part of the WTIO exhibit generally opposite characteristics compared to those of the northern part. In addition, regressed and reconstructed anomalies of ocean environmental variables with respect to El Nino-Southern Oscillation index indicate that those favorable ocean conditions in the northern part of the WTIO occur one year after the El Niño years. These results could be applied to estimate future pelagic tuna catches in the WTIO based on its relationship to the climate variability.