| Japanese eel is an economically important migratory fish with a complex life history in the western Pacific Ocean.The spawning ground is located near the west mariana ridge,and the newly hatched fish develop into Leptocephalus,with the North Equatorial Current and Kuroshio Current to the northwest China,Japan,South Korea and other waters.A few months later it drifted into coastal waters,where it became a Glass eel.It soon became a pigmented Elver and entered fresh water.Called the Yellow eel during freshwater growth,it morphs into the Silver eel a few years later,and then begins its migratory migration to complete its life cycle.El Ni?o is refers to the tropical ocean temperature anomaly and sustained warming phenomenon,southern oscillation is located in central and eastern Pacific,the temperature of tropical Marine waters,both collectively known as ENSO,caused the world within the scope of the climate change,a wide range of water temperature rise and various changes in ocean currents,a degree of impact on Marine life.Japanese eel larvae need several months to drift thousands of kilometers from spawning site to estuarine,which is affected by various Marine hydrology factors.In this study,the development and migration of Japanese eel were studied by using the methods of external morphology,otolithic dating and multivariate analysis.The main research results are as follows:1,From January to April 2019,394 larvae of Japanese eel were collected from the Yangtze estuary,with a total length of 49.58-62.64 mm,with an average length of 55.58± 2.39 mm.Body weight was 0.0785-0.1903 g,with an average of 0.1229±0.0198 g.Fullness was 0.047-0.094,with an average value of 0.071±0.013.It was successfully screened and identified that the adult age of 136 females was 139-178 d with a mean value of 155±13.1d.It was found that there was no significant correlation between the adult age and the total length(P>0.05)and the body weight(P>0.05).All 394 larvaewere in the stage of glass eel,including five phases of pigment development: VA,VB1,VB2,VIA0 and VIA0.Among them,there were 112 VA stages,accounting for 28.4%.VB1 phase 257 tail,accounting for 65.2%;VB2 stage 13 tails,accounting for 3.3%;VIA0 and VIA1 had 5 and 7 tails,accounting for 1.3% and 1.8%,respectively.It was found that there was no significant correlation between the length of the glass eel and the phase of pigment development(P>0.05),but the body weight and fullness were significantly correlated with the phase of pigment development(P<0.001).2,The 302 samples collected from 2017 to 2019 were analyzed for basic biology and otoliths.The total length in 2017,2018 and 2019 was 55.40±1.6mm,54.66±1.2mm and 55.43±2.2mm,with the mean length of 49.75-59.92 mm,48.75-59.88 mm and49.98-59.98 mm,respectively.The axial radii of the sagittal stone were 113.8-169.45 m,132.64-192.51 m and 143.87-192.09 m,respectively.The mean axial radii of the sagittal stone were 142.82 m,151.67 m and 167.21 m,respectively.The mean age was138.4±6.6d,143.8±8.1d and 155.5±13.1d,respectively.It was found that the length and age of the larvae of Japanese eel were related to the collection time.From January to April,the monthly mean length of the larvae decreased gradually,while the mean daily age increased gradually.According to NOAA data and the size of Nino index,2017 is considered as Normal year,2018 as weak La Ni?a year,and 2019 as weak El Ni?o year.Analysis showed that the El Ni?o event had no significant effect on the total length of the larvae of Japanese eel(P > 0.05),but reduced the migration rate of the larvae.According to the calculation of daily age,the number of Japanese eel larvae collected in El Ni?o years before August15 hatches accounted for 10.29%,the number of Japanese eel larvae collected in La Ni?a years only accounted for 2.48%,and the incubation time collected in Normal years was all later than August 15,indicating that the incubation time of Japanese eel larvae was advanced due to El Ni?o.3,According to the analysis of Japanese eel samples collected from the Jingjiang section of the Yangtze river from 2002 to 2017,137 tails of Japanese eel were found in519 net surveys in the past 16 years,with an average age of(1.8 ± 1.0)d,average length and weight of(31.1 ± 9.8)cm and(57.64 ± 91.94)g,respectively.89.1% of the individuals were sexually immature and 94.9% were in the yellow eel stage.The relation between total length and mass is: W=0.0002415ŚL3.483(r2=0.9537,n=137).The quantity and weight of Japanese eel in Jingjiang section only accounted for 0.08%and 0.69% respectively,and the abundance and occurrence rate of Japanese eel showed a fluctuating decline trend during the years.At the same time,it was found that the interannual fluctuation of the abundance of eel Japanese eel had nothing to do with the population supplementation of eel population in the Yangtze estuary,and its occurrence rate was affected by water temperature,water quality factors such as ammonia nitrogen and turbidity. |
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