The growth, mortality and yield per recruitment of white-spotted conger (Conger myriaster) in the Yellow Sea and the East China Sea

The objective of this study was to provide biological reference points for the fisheries management of Conger myriaster in Yellow Sea and East China Sea. Specifically, we projected the yield per recruitment (YPR) model for C. myriaster, and obtained the biological reference points, such as F_max and F_0.1, and the suggested length of first capture (L₅₀). Through the scientific survey and the commercial fishery survey in the Yellow Sea and East China Sea in 2016 and 2017, 657 individuals of C. myriaster were collected and measured for the total length (cm) and weight (g), in which 256 samples were analyzed for their age according to the otoliths. Based on these 657 samples, the weight-length relationships were studied by a power law model. The condition factor a estimate was 4.5×10^–4, indicating the slender eel-like body shape of C. myriaster, while the estimate of parameter b was 3.3, meaning that C. myriaster tends to become relatively fatter or have greater girth as it grows longer. The von Bertalanffy growth model was used to analyze the length-age relationships based on those 256 samples with age data. The asymptotic lengthL_∞ was estimated to be 102 cm, and the estimate for t₀, the age at which weight and length were zero, was –0.19. The exponential rate K of approach to the asymptotic length was 0.21 per year, indicating the slow growth rate of this larger demersal carnivorous species. Five scientific bottom trawl surveys were conducted in Haizhou Bay in March, May, July, September and December, 2011, in which 715 samples were collected and measured. In order to estimate the instantaneous total mortality rate Z, these samples were used to project the (length-converted) linearized catch curve to length-frequency data. This catch curve indicated that the estimate of Z was 3.36 and the length of first capture (L₅₀) was 30 cm, meaning that the age of first capture is 1.47 years. The Pauly empirical formula evaluated the natural mortality M to be 0.33, according to the above growth parameters and the water temperature. Therefore, the fishing mortality F was 3.03, and the exploitation ratio was 90%. Based on all above estimates for the growth and mortality parameters, the yield per recruitment models were projected for C. myriaster. YPR increased to maximum, then decreased, when F kept increasing. Different M (0.23, 0.28, 0.33, 0.38 and 0.43) and different lengths of first capture (30 cm, 40 cm, 48 cm, 56 cm and 67 cm) were considered in the YPR model. When M increased, the estimates for F_max and F_0.1 would increase with smaller YPR, while the increase of L₅₀ would result in greater F_max, YPR_max, F_0.1, and YPR_0.1. The current YPR of C. myriaster was 27.14. When M=0.33, F_max was 0.38, one eighth of the current F, while YPR_max was 52.89, nearly double of the current YPR. And when M=0.33, F_0.1 was 0.255, 30% smaller than F_max, while YPR_0.1 was smaller than YPR_max by 5%. In conclusion, this fishery is undergoing heavy fishing pressure, with less length of first capture and extremely high fishing effort. Therefore, in order to keep this population health and the sustainable development of this fishery, we suggest to decrease the fishing effort largely and increase the length of first capture. For example, if F decreases from 3.03 to 2 and L₅₀ increases from 30 to 40 cm, YPR would increase by 89%.