Response of winter cohort abundance of Japanese common squid Todarodes pacificus to the ENSO events

The Japanese common squid Todarodes pacificus is an economically important species with one year lifespan, which is significantly influenced by climatic and environmental variability. According to the fishery data of the winter cohort of T. pacificus from 2003 to 2012, as well as environmental data and the Oceanic Nio index (ONI, which was defined by the sea surface temperature (SST) anomaly in the Nio 3.4 region), variations in the SST, chlorophyll a (Chl a) concentration, suitable spawning area (SSA) and sea surface height anomaly (SSHA) on the spawning ground of T. pacificus were examined under the El Nio and La Nia conditions. Their influences on squid abundance (defined by catch per unit effort, CPUE) were further assessed. The results showed that seasonal changes were found in SST, Chl a and SSA on the spawning ground of T. pacificus. Correlation analysis suggested that annual CPUE was significantly positively correlated with Chl a and SSA (p < 0.05), but had insignificant relationship with SST (p > 0.05). Moreover, the El Nio and La Nia events tended to dominate the changes of SSA and Chl a concentration in the key area between 25A degrees-29A degrees N and 122.5A degrees-130.5A degrees E, driving the variability of squid abundance. However, this influence varied with the intensity of each anomalous climatic event: the weak El Nio event occurred, the spawning ground was occupied by waters with enlarged SSA but with extremely low Chl a concentration, leading to low squid recruitment, the CPUE then decreased; the moderate intensity of El Nio event resulted in shrunk SSA but with high Chl a concentration on the spawning ground, the squid recruitment and CPUE increased; the moderate intensity of La Nia events yielded elevated SSA and high Chl a concentration on the spawning ground, the squid recruitment and CPUE dramatically increased. Our findings suggested that the ENSO events played crucial effects on the incubating and feeding conditions of the winter cohort of T. pacificus during the spawning season and ultimately affected its abundance.