Study on age structure and growth characteristics of 

Coilia mystus spawning population in the Yangtze Estuary

Abstract: The main purpose of this study is to explore the relationship between the age and growth of Coilia mystus breeding population. C. mystus is one of most important economic fish in the Yangtze Estuary, whose resources have collapsed in recent years. Its main distribution area is in the Western Pacific Ocean, including the waters of China, Korean Peninsula and Japan. C. mystus is a semianadromous migratory fish, which usually lives in the coastal areas and migrates to estuarine area during reproduction. There are three ecological groups of C. mystus in the southeastern coast of China, namely the Yangtze River type, the Minjiang type and the Pearl River type. This study focused on the Yangtze River type, whose mature breeding colonies migrated from the coastal shallow sea clusters to the brackish and fresh water areas of the Yangtze Estuary near the Chongming Island to spawn and form fish floods. In the 1980s, the average annual output of C. mystus accounted for about 48.6% of the total output of fish and shrimp aquatic products in the Yangtze River Estuary. However, due to water pollution, overfishing and other factors in recent years,  C. mystus resources have declined seriously and fishing season has failed to form with a trend of miniaturization and younger age, indicating that the Yangtze River population has collapsed. Our literature search showed that there had no relevant study on the age structure and growth characteristics of the breeding population for C. mystus in the Yangtze River Estuary since 2016. It is urgent to carry out relevant research about the resource assessment and conservation work for this species in the Yangtze River Estuary. Therefore, this paper used a spawning population of C. mystus collected from the Yangtze River Estuary in May, June and July 2018 and May 2019, and 194 samples were collected from the catches to study their age structure and growth characteristics. It is expected to provide basic data for the evaluation of the effect of comprehensive ban on fishing in the Yangtze River on the protection and management of Coilia mystus in the Yangtze Estuary. The results showed that the age structure was from 1 to 3 years old, among which age1 and age2 individuals were the dominant group, accounting for 82.48% of the total number of samples. This study and most other studies based on otolith age identification methods showed that there were 35yearold individuals in the breeding population of the Yangtze River Estuary, among which 12yearold group was the dominant group, accounting for almost 80% of all individuals. Body length ranged from 69 mm to 192 mm and body weight ranged from 1.5 g to 30.3 g. The average body length was (128.87±25.26) mm and the average body mass was (9.90±6.61) g. The relationship between body length and body weight was W=2×10-6L3.17. The asymptotic body length L∞ and asymptotic body mass W∞ in the growth equation of body length and body weight were 212.33 mm and 48.22 g, respectively. The growth coefficient k was 0.43 and the theoretical growth starting age t0 was -0.88. Male C. mystus tended to have uniform growth rate (b =2.87), while female C. mystus tended to have allallic growth rate (b =3.31). The age of inflection of body weight growth curve was 1.81 years old, and the length and body weight at the age of inflection were 145.44 mm and 14.52 g, respectively. Compared with the results of previous studies, both body length of C. mystus at inflection age and asymptotic body length in the Yangtze Estuary increased in recent years, indicating that the trend of miniaturization might have slowed down. However, whether the results of this study indicated that the trend of individual miniaturization in the Yangtze River Estuary in recent years might be effectively curbed remains to be verified by further data monitoring, and the reasons for this need to be further explored. The surface water temperature in the Yangtze Estuary during the flood season is one of the environmental factors that affect the reproductive characteristics of Coilia mystus in the Yangtze Estuary. After the water temperature rises, the fullness of the reproductive population and the weight of gonads of Coilia mystus decrease. The reason may be that the increase of temperature in the water environment leads to the enhanced metabolism of fish, the slowdown of nutrient accumulation in the body, and finally affects the maturation of their population. The surface water temperature in the waters of the Yangtze River Estuary from April to June showed an overall increasing trend from 1982 to 2014, and there was a modal shift around 1997, which meant that after 1997 the average surface water temperature in the waters of the Yangtze River Estuary from April to June was higher than that of previous years. The average rise was about 2℃, and it was significantly negatively correlated with the total catch of Coilia mystus and the catch of a single ship during the whole flood season. Further research is needed on the response of changes in biological characteristics (such as individual miniaturization) to environmental factors such as water temperature. The response law of the age structure and growth characteristics of C. mystus reproductive population needs to be further studied, under the dual effects of environmental changes and fishing ban measures in the Yangtze Estuary, the results of this study can be used as background information on the biological characteristics of C. mystus in the Yangtze Estuary before the implementation of comprehensive fishing ban, and can be of reference value for further evaluation of the effect of the fishing ban in the Yangtze River.

Keywords: Coilia mystus; age; growth; Yangtze Estuary