| In China,C.nasus is mainly distributed in the East China Sea,Yellow Sea,Bohai Sea and the rivers leading to the sea,and is an important economic species of the Oujiang River.In recent years,due to high fishing pressure,environmental pollution and human activities,the spawning,baiting and migration channels of C.nasus have been destroyed,resulting in the decline of its resources.Thus,it is important to study the biological characteristics and the resource status of C.nasus.In this study,the growth,reproductive biology and feeding ecology of C.nasus were studied and investigated based on 1,153 samples of C.nasus collected monthly from March 2021 to February 2022 in the Oujiang River Estuary.In addition,a preliminary assessment of the resource distribution characteristics and resource utilization status of C.nasus was carried out in combination with monitoring data.The main findings are as follows:(1)Growth characteristics:The body length of C.nasus ranged from 64 to 341 mm,with a mean value of(183±65.8)mm,and the dominant body length groups were 100-140 mm(24.5%)and 220-260 mm(24.4%).Body weight ranged from 0.7 to 197.0 g,with a mean value of(33.2±33.3)g.The dominant body length group was 0-60 g,accounting for 80.1%of the total.The body length-body weight relationship equation was W=3.96×10-7L3.4151,indicating that C.nasus showed positive anisotropic growth.The effect of season on the a and b values in the body length-weight relationship equation was greater than that of sex.The growth parameters of C.nasus were estimated based on the body length frequency data,with the asymptotic body length L∞=389.0 mm,growth rate k=0.44,theoretical growth initial age t0=-0.56 a,weight growth inflection point age of 2.23 a,corresponding inflection point body length of 275.1 mm and inflection point weight of 82.41 g.(2)Reproductive characteristics:The peak reproductive period of C.nasus is June.The absolute fecundity of C.nasus females ranges from 11890 to 57843 grains,with an average of(31388±9750)grains;the relative fecundity FL ranges from 47 to 192 grains/mm,with an average of(109±31)grains/mm;the relative fecundity Fw ranges from 158 to 456 grains/g,with an average of(281±68)grains/g.The range of egg diameter was 0.40～0.98 mm,with only one peak in the distribution frequency of egg diameter,indicating that C.nasus is a one-time spawner.The first sexual maturity body length(L50)of C.nasus females was 236.8 mm by fitting the logistic equation.The absolute fecundity of C.nasus females was related to body length,body weight and ovary weight in a power function,and the correlation with ovary weight was the highest.(3)Feeding ecology:There are seven taxa and 20 species of bait organisms fed on by C.nasus.Based on weight percentage,number percentage,frequency of occurrence and relative importance index,shrimp was the most important bait group.Body length 200mm is the turning point in the feeding of C.nasus,and fish appeared in the bait taxa of C.nasus with body length>200 mm.The food composition of C.nasus has seasonal changes,mainly feeding on fish and shrimps in spring,while shrimps are dominant in other seasons.With the increase of body length,the average fullness index of C.nasus showed an overall increasing trend.The carbon and nitrogen stable isotope results showed that the body length of C.nasus was positively correlated with δ13C value and δ15N value,and the differences of δ13C value,δ15N value and C/N value of C.nasus in different seasons were not significant(P>0.05).The trophic level of C.nasus in Oujiang Estuary ranged from 3.28 to 4.39,with the mean value of(3.75±0.25),and the trophic level was positively correlated with body length as a whole.(4)Resource status:The results of the monitoring data showed that the flood season was from May to the end of June,and the operation was mainly concentrated on Jiangxinyu.According to the Length based Cohort Analysis(LCA),the number of resources(N)of C.nasus was estimated to be 19,164,424 and the resource weight was 125.52 t.In the process of resource estimation by LCA,different initial values of M and F will affect the estimation results,and the resource quantity is more sensitive to M.According to the results of the equal catch curve drawn by the B-H dynamic integrated model,the current production of C.nasus is below the optimum production area,and current fishing mortality factor is greater than the biological reference points F0.1 and Fmax.All the C.nasus in the Oujiang Estuary are in the overexploitation state.Considering the fish growth potential,fishery biomass and biological reference points,it is suggested that the catchable size of C.nasus is 220 mm or 40 g.In summary,as an important economic species in the Oujiang River estuary,the growth rate of the C.nasus population is relatively fast,but the reproductive capacity is relatively low and the resources are in an overexploited state.Therefore,this study suggests to strengthen habitat protection and reduce the impact of human activities on the reproduction of C.nasus;and to establish the catchable size to ensure the sustainable development of C.nasus resources and to satisfy the economic interests of fishermen. |
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