Influence of El Nino/La Nina on the abundance index of skipjack in the Western and Central Pacific Ocean

Abstract:

Skipjack tuna, Katsuwonus pelamis, is an important economic species in the central-west Pacific Ocean, and this species is vulnerable to the change of marine environment. In this paper, according to the abundance index(unit net yield, CPUE) of skipjack tuna collected from the tuna large purse seine fisheries in the central-west Pacific Ocean from 1995 to 2010, combined with the El Nino/La Nina events(expressed by NINO 3.4 index), the influence of El Nino/La Nina on the abundance index of skipjack was studied with the time series analysis. The results showed that the yearly and seasonal abundance index of skipjack varied greatly in 16 years, in which the maximum monthly CPUE appeared in February 1995 with 30.37 t/net, and the minimum occurred in October 1997 with 5.35 t/net. The effect of Nino 3.4 index on monthly CPUE lagged 0-2 months in time, but the synchronization(lag 0 month) had the highest correlation. The Nino 3.4 index and monthly CPUE had significant negative correlation. According to the level of 16 years abundance index of skipjack and Nino 3.4 index, we chose four months(August to February of the next year) of CPUE values as studying object, and divided 16 years into two periods:1) First period was before 2000, this time period had a strong El Nino(SSTA>1.0℃), strong La Nina(SSTA<1.0℃) and normal years(0.5℃ < SSTA < 0.5℃). When the strong El Nino year occurred(1997), the abundance index of skipjack was at a lower level with monthly CPUE of 7.05 t/net. When a strong La Nina(1998) occurs, the abundance index was high with monthly CPUE of 19.61 t/net. 2) The second period was after 2000, which contains weak El Nino(0.5℃ < SSTA < 1.0℃), weak La Nina(-1.0℃ < SSTA < -0.5℃) and normal years(-0.5℃ < SSTA < 0.5℃). In the years of weak El Nino, the abundance index of skipjack(such as, the average CPUEs in 2002 and 2006 are 15.27 t/net and 16.81 t/net respectively) fluctuated around the mean CPUE value(15.73 t/net), but still lower than that in the La Nina year's(19.90 t/net). Both the CPUEs during weak El Nino and weak La Nina were higher than that in the normal year(14.16 t/net). In conclusion, the temporal and spatial abundance index of skipjack varied with both the changes of marine environment and the primary productivity caused by the El Nino/La Nina phenomenon.