Fisheries biological characteristics of Sthenoteuthis oualaniensis in the spring season in the El Nino year of 2016 in the Zhongsha Islands waters of South China Sea

Based on the 572 samples of Sthenoteuthis oualaniensis collected by light falling-net fishery during March to May in the spring season in the El Nino year of 2016 in the Zhongsha Islands waters of the South China Sea, the fisheries biological characteristics and the influence of El Nino were studied. The result indicated that the mantle length (ML) range was from 59.5 mm to 207.5 mm with a high concentration in 90 mm to 150 mm accounting for about 66.24 percent for females and from 56.5 mm to 236.5 mm with a high concentration in 60 mm to 120 mm accounting for about 78.81 percent for males, respectively. The body weight (BW) ranged from 7 g to 318 g with a dominative weight from 20 g to 140 g accounting for about 70.18 percent for females and ranged from 7 g to 381 g with a dominative weight from 0 g to 80 g accounting for about 92.14 percent for males, respectively. The ration of the somatic body weight (SBW) to BW ranged from 46.59 to 86.67 percent with an average of 70.19 percent and from 42.86 to 91.11percent with an average of 73.12 percent for females and males, respectively. There were significant differences between the BW and SBW growth, and the relationships between the ML to BW and ML to SBW were best described by the power function for females and exponential function for males by the Akaike’s information criterion (AIC), respectively. The ratio of females to males was 41.43:58.57, and the samples were most composed of immatue individuals accounting for about 61.7 and 87.79 percent for females and males. The feeding stage was mainly in 0, 1 and 2 grade accounting for 89.42 percent and all of the samples and the food was mainly composed of squids, pelagic fishes, mollusc and crustacean. The result indicated that the average sea surface temperature of the sample waters of the El Nino year of 2016 was 0.213 degrees centigrade lower than the common year of 2015, and the fisheries biological feature of S. oualaniensis was affected by El Nino with a decrease in the body size probably.