A Preliminary Study On The Age Structure And Growth Charcteristics Of Coilia Mystus And Coilia Nasus In The Yangtze River Estuary

There are three main species of Coilia in China,They are Coilia graii,Coilia mystus,and C.nasus.Among them,C.mystus and C.nasus have higher economic value.In recent years,ecological environment and fish habitat in the Yangtze River have destroyed increasingly,leading to a sharp decrease in both resources and catches of C.mystus and C.nasus,Their catch value has been dropping year bu year,which made a certain impact on the fishermen’s life and social economy in the Yangtze River Estuary.Based on the sampling survey of C.nasus and C.mystus in the Yangtze River Estuary in May,June,July 2018 and May 2019,a preliminary study was conducted on the age structure and growth characteristics of C.mystus and C.nasus both before and after the fishing ban in the Yangtze River Basin,in order to provide basic biological information for the resource assessment and conservation of C.nasus and C.mystus in the Yangtze River Estuary.(1)The results of 194 C.mystus samples showed that their age composition was0-3 years old,and their dominant age is 0-1 years old,in which the age group 0 and age group 1 accounted for 42.27% and 32.99%,respectively.The length range was69-192 mm and the average body length was 128.89 mm.The dominant body length was 100-180 mm,accounting for 85.6% of the total samples.The weight range was1.50-30.0g and the average body weight is 9.8g.The dominant body weight was1.50-15.0g,accounting for 78.9% of the total samples.Compared with the previous studies,the trend of low age and miniaturization of C.mystus is more and more obvious in recent years.(2)According to the comparative analysis of the growth speed of body length of C.mystus from May to July in 2018,the growth speed of C.mystus in June was the fastest.The individual of C.mystus from May in 2019 was generally smaller than that in 2018,which might be related to the abnormally low temperature in spring,2019.(3)The results of 112 C.nasus samples showed that their age composition was0-3 years old,and their dominant age group was 1-2 years old,in which the first and second age group accounted for 28.57% and 56.25%,respectively.The body length range was 96-337 mm and the average body length was 224.16 mm.The dominant body length was 130-290 mm,accounting for 90.2% of the total samples.The body weight range was 2.89-153.4g,and the average body weight was 54.58 g.The dominant body weight was 2.89-100 g,accounting for 95.5% of the total samples.Compared with previous studies,both the average body length and weight of C.nasus have decreased significantly,and trend of low age and miniaturization are also becoming more and more obvious in recent years.(4)There is no significant difference between the body length growth speed of C.nasus between May and June in 2018 through the comparative analysis of body length growth speed.Compared with the sample from May in 2018,the individual of C.nasus in May 2019 was generally smaller,which may be related to the abnormally low temperature in that year,and the impact on C.nasus was more obvious than that of C.mystus.(5)The morphological study on the otoliths of C.nasus and C.mystus showed that the total length and width of the otoliths with 650μm and 500μm could be regarded as the inflexion point of otolith weight change for C.mystus,and 450μm and400μm for C.nasus in the Yangtze River Estuary.