| Coilia nasus is one of the important economic fish species in the the Yangtze River.It has two ecological types: anadromous and resident.In the history,the yield of anadromous Coilia nasus was very rich,and the fishing season was long from February to March each year.In recent years,Coilia nasus resources have declined seriously,the fishing season time has decreased,the migration distance has been shortened,and the different ecological types of Coilia nasus populations have shown a certain trend of miniaturization and low age.It is imperative to study the conservation biology of C.nasus.The habitat provides spawning and nursery grounds for C.nasus,so that C.nasus habitat protection is regarded as one of the most effective ways.Therefore,the protection of C.nasus requires a comprehensive study of its early resources.At the same time,feeding is an important factor affecting the growth,development,survival and death of early fish resources.The study on feeding status and difference in the early life cycle of C.nasus is helpful to explore the survival and supplementary mechanism of C.nasus population and protect it more effectively.Based on this,the study focused on the larval and juvenile C.nasus in the the Yangtze River in Hukou,Anqing and Jingjiang section from May to August 2016.The temporal and spatial patterns of larval and Juvenile C.nasus in the Yangtze River were analyzed.The monthly food intake of larvae and juvenile C.nasus collected from different sections of the Yangtze River was studied,and the food composition of the larvae and juvenile was analyzed and compared.Fish ecology was used to analyze the community characteristics of larval and juvenile C.nasus and the importance of larval and juvenile C.nasus in larval and juvenile community.The purpose of this study was to analyze the early resource status of C.nasus in the Yangtze River and to evaluate the difference of larval feeding habits of C.nasus in different sections the Yangtze River,so as to provide scientific basis for the conservation and management of C.nasus.The main results are as follows:1.Results on the temporal and spatial distribution of larvae and juvenile of C.nasus in the Yangtze River showed that the average density of larvae and juvenile C.nasus in the Jingjiang section was the highest during the survey period.There was a short peak period between late June and early July in larval and juvenile C.nasus in the surveyed sections,which may be related to the spawning type of C.nasus.The outbreaks of larval and juvenile C.nasus in the Hukou section were earlier than those in the Anqing and Jingjiang,and it was inferred that this phenomenon was caused by resident C.nasus spawning in the Hukou section.The difference in the densities of C.nasus in the three sampling sites along the river sections may be due to the differences in coastal topography and water dynamics.The water flux in each section was at a low level when the larval and juvenile C.nasus appeared the peak,the water temperature was 25.87 °c,25.27 °C and 21.67 °C,respectively,which all meet the spawning requirements for C.nasus.Results on spatial distribution and densities cluster analysis of larval and juvenile C.nasusin different months show that there is a certain similarity between the Anqing and Jingjiang section,while the Hukou section is separated.The larvae and juvenile C.nasus density was less in the Hukou section,but the density was higher than that of in the Anqing section in August,which indicated that the spawning population was more in the Hukou section in August.2.Results on the larval and juvenile C.nasus feeding composition in the Yangtze River showed that the feeding intensity was highest and the empty stomach rate was lowest in the Hukou section.The each section food composition in the Yangtze River is Copepoda and Cladocera,but the proportion is different.The feeding intensity highest in July in Hukou and Jingjiang section,but in Anqing section the peak was appeared in August.Hukou and Anqing sections had the highest empty stomach rates in May,while Jingjiang section had the highest rate in July.In this study,the food composition classifications were analyzed to genus level by 13 groups.The main groups were Sinocalanus,Cyclopinae and Schmackeria.The food composition of larvae and juvenile C.nasus was consistent with that of Zooplankton in the Yangtze River.The larval and juvenile C.nasus feeding groups of 5~10 mm and 10~15 mm in the Jingjiang section were more than those in the other two sections.It was inferred that the species of zooplankton in the water environment of the Jingjiang section were higher than those of the other two sections.Results of PCoA analysis showed that the feeding composition of C.nasus in different body length groups was different,which reflected the process of feeding transition during the development of C.nasus.The food composition is more similar at Hukou and Jingjiang section.The middle-high body length group(10~15 mm and 15~20 mm)food composition in Anqing section is similar to the low body length group(5~10 mm)in Hukou section.It is inferred that the zooplankton biomass of C.nasus growing area in the Anqing section is lower than that in the Hukou section.The reason for this phenomenon may be that the zooplankton species and density in water connected by lakes and the sea are higher than those in other main stream section in the Yangtze River.The feeding difference of larvae and juvenile C.nasus at different development stages were analyzed.25 species of food groups or species were identified,including 13 species of copepod,which include fragments,9 species of Cladocera,1 species of fry and 1 species of shrimp.With the development and the body length increase of larvae and juvenile C.nasus,the feeding rate,total feeding amount and feeding intensity are increasing.The feeding rate,total feeding amount and feeding intensity increased with the growth body length,and the food composition changed significantly.The feeding frequency of larvae and juvenile C.nasus on Sinocalanusdorrii increased during its development,showing a certain preference.3.Results on the community characteristics of larval and juvenile fishes in Hukou reach showed that there were 39 larval and juvenile fishes belonging to 8 orders,12 families,37 species,1 subfamily and 1 family.Hemiculter Bleekeri is the first dominant species in this section.The following dominant species are Gobiidae,Parabramis Pekinensis and Pseudolaubuca Sinensis,respectively.In some extent,it reflects the status of adult fish resources in the Yangtze River.The six periods were divided into two groups: late May to early July and late July to early August.Hemiculter Bleekeri,Culteralburnus and C.nasus reached the peak in late June,and bream,the four major Chinese carps and Mandarin fish reached the peak in early July.The density of larval and juvenile fish in the north-south coastal sampling sites was higher than that in the center of the section.By using Redundancy Analysis(RDA),the relationship between the density of larvae and juveniles and environmental factors was analyzed.The correlation analysis between water level and density of larvae and juveniles showed that there was a significant correlation between them(P<0.01).The population of larval and juvenile C.nasus accounted for 1.56% of the total larval and juvenile population.It is a common species in the larval and juvenile communities of Hukou section.The peak of larval and juvenile density occurred in late June before its density peak.It may be a strategy to avoid competition with other fish species for food,to ensure that their offspring have a good feeding environment.RDA analysis showed that there was a positive correlation between larval and juvenile C.nasus and water temperature,transparency and flow rate. |
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