Abstract:

Abstract: The Oujiang River Estuary (ORE), a critical habitat for fish foraging, breeding and nursery, represents a high productive ecosystem in the northern subtropical of the west Pacific Ocean. However, due to the rapid development of surrounding cities, various seas have been occupied and marine resources have been exploited intensively, with enormous variations of the community structure of fishery resources. The common economic species, including Coilia nasus, have declined significantly, and have been replaced by younger age, lowvalue and miniaturized fish, indicating that the stability of fish community structure has been increasingly damaged. C. nasus is an important economic fish along the coast of China. It is popular among coastal residents because of its good taste and rich nutrition. It has also received widespread attention from the academic society for the decline of resources in recent years. C. nasus belongs to order Clupeiformes, family Engraulidae, and genus Coilia, and is mainly distributed in China, Japan and the Korean Peninsula. The research on C. nasus has mainly focused on the waters of the Yangtze River Estuary (YRE) in recent years, and the research contents include population, resource changes, reproductive characteristics, age identification and so on. There are few studies on Coilia nasus in the ORE. To understand the resources and its changes of Coilia nasus in the ORE, a total of twelve bottom trawling surveys were carried out in the sea from February 2017 to November 2019. The survey range of east longitude was between 121°00′to 121°54′, and north latitude was between 27°45′to 28°06′. The survey ship was “Zheyueyuxiu 00105” with a main power of 202 kW and a tonnage of 128 t. The opening area of the bottom trawling net underwater was about 6 m (width) and 1.5 m (height), the mesh of the bag net was 2.5 cm, the ship was trawling 0.75 hours per station with a speed of 34 nmile·h-1, all the surveys were carried out during the day. The sweeping area method was used to estimate the biomass distribution of C. nasus, and the biological indicators of all samples were measured. The biological parameters of C. nasus was calculated using FiSAT II, and the BevertonHolt model was constructed to evaluate the changes in resources. Results showed that a total of 1 008 C. nasus were collected in 12 bottom trawling surveys from 2017 to 2019. The average biomass in 2017, 2018 and 2019 was 16.06 kg·km-2, 4.22 kg·km-2 and 14.62 kg·km-2 respectively, while the average abundance was 0.82×103 ind·km-2, 0.13 × 103 ind·km-2 and 0.85 ind·km-2, respectively. The body length of C. nasus ranged from 15 mm to 357 mm, and the dominant body length was between 90 mm to 270 mm with a proportion of 84.36%. The body weight of C. nasus ranged from 0.08 g to 195 g, and the dominant body weight was between 0 g to 270 g with a proportion of 86.61%. From March to April, C. nasus of the ORE began to travel upstream into the freshwater area for reproductive migration. After October, when the water temperature in the ORE dropped, C. nasus descended to the sea for overwintering; in February 2019, 98 tails of C. nasus was caught in a single net at DT09 station. The body length ranged from 88 mm to 280 mm, and the body weight ranged from 3.20 g to 76.25 g, which also showed that the sea area was a wintering ground for C. nasus in winter. By analyzing the temperature and salinity of the survey stations where C. nasus appeared, it was found that C. nasus in the ORE area only appeared at the station with the surface salinity below 31‰ and surface water temperature below 25℃.Growth of C. nasus was described using Von Bertalanffy model based on the body length frequency, and the estimated parameters were  l∞=362.25 mm，k=0.440，W∞=158.38g，t0=-0.31a. The Von Bertalanffy’s growth equations of length and weight were as follows: lt=362.25［1-e-0.44(t+0.31)］; Wt=158.38［1-e-0.44(t+0.31)］3.017 8. The lengthconverted catch curve was used to calculate the total mortality coefficient Z, which was found to be 1.33, and the fitted equations were as follows: ln (N/dt) = 6.916-1.33t (R2=0.931). According to the Beverton Holt dynamic model, the growth inflection point age tλ was 2.20a, the corresponding body length was 242 mm, the weight was 47.00 g, and the capture size was 229.4 mm. According to the twodimensional analysis of relative yield / recruit (Y′/R) and exploitation rate(E), it showed the exploitation of C. nasus in ORE: E10=0.720, E50=0.401, Emax=0.803. With the current opencapture body length Lc=229.4 mm, E>0.68, Y'/R first increased and then decreased with the increase of the exploitation rate of C. nasus; when Lc=229.4 mm, E<0.68, Y′/R increased with the exploitation rate. When E increased from the current 0.68 to Emax (0.803), the corresponding Y′/R reached the maximum (Y′/R≈0.103), and when it fell to E50, the corresponding Y′/R dropped to 0.085. From the perspective of the maintenance of relative unit supplementary biomass (B′/R), the current utilization rate of C. nasus was 0.68, and about 20% of biomass resources (B′/R) were maintained; when Emax=0.803, only 11.8 % of biomass resources (B′/R) were kept, and when E50=0.401, about 50% of biomass resources (B′/R) could be kept. Natural mortality coefficient M was estimated using Pauly’s empirical formula and found to be 0.43. Accordingly, the fishing mortality coefficient (F) was equal to 0.90 (F=Z-M=0.90). The exploitation rate of ORE from 2017 to 2019 was assessed by the formula of E=F/Z, and found to be 0.68. In the present study, the results demonstrated that the exploitation rate of C. nasus was greater than the optimum exploitation rate of fish, while lower than the maximum exploitation rate. Namely, the resources of C. nasus in the ORE have been fully utilized. However, reducing the fishing pressure in an appropriate amount can better protect the resources of C. nasus in the ORE. Therefore, proposal for the establishment of a spawning ground reserve in the waters between Xizhou Island and Nanxi River Estuary to protect and restore the resources of C. nasus is made. 

Keywords: Coilia nasus; growth parameters;mortality parameters; exploitation rate; Oujiang River Estuary