三峡工程对坝下鱼类早期资源补充过程和种群动力学的影响研究——以贝氏餐为例

Hypolimnetic discharge from the Three Gorges Dam lowers water temperature downstream; sediment retention above the dam makes the nutrients trapped in the reservoir, and lower productivity downstream. Such physic-chemistry modifications for water downstream induces food resource shortage and stunting growth of fish, and consequently results in population decline. In the present project, we hypothesize that decline of fish populations below the dam is mechanized by both increased mortality during early life stage and changed population dynamic parameters. First of all, low water temperature and shortage of food resources stunt growth and ontogenetic development of fish during early life stages, which result in increased early mortality and declined year-class strength of the population. Meanwhile, delayed spawning and stunted growth for juveniles and adults may influence population dynamic parameters, e.g. older age at first maturity, reduced energy investment in reproduction, and declined fecundity, and consequently result in population decline..Influence of dam discharge on water physic-chemical properties downstream buffers with the increase in distance from the dam. In the present project, we test the above hypotheses by comparing recruitment process during early life history stages and population dynamic parameters of fish among different sections downstream the dam in the middle reach of the Yangtze River. We select Hemiculter bleekeri (Warpachowsky) as a model species. This fish is the most dominant species in the middle reach of the Yangtze River. As for its low market value, it is not targeted by commercial fisheries. Thus, recruitment process and population dynamic parameters reflect mainly the influence of the environment variations among the investigated sections of the river, which are resulted from dam discharge. In the project, abundance and dynamic of larvae of H. bleekeri will be investigated; food contents and growth will be examined for larvae and juveniles; selected survival during early life history stages depending on growth will be tested. By comparing abundance, dynamics, feeding, growth and recruitment of H. bleekeri among different sections in the middle reach of the Yangtze River, influence of dam discharge on recruitment process during early life stage will be revealed. On the other hand, population dynamic parameters, e.g., age structure and growth, age at first maturity, and fecundity, will be investigated for H. bleekeri populations in different sections in the middle reach of the Yangtze River to reveal the influence of dam discharge on population dynamic characteristics. Results from this project will reveal mechanisms underlying population decline of fishes below the dam. The results will also provide valuable information for fishery resource conservation and management.

三峡大坝下泄低温和贫营养水，影响鱼类的摄食和生长，导致坝下鱼类种群数量下降。本项目提出坝下鱼类种群数量下降机制的以下假说：1）鱼类早期个体生长发育受阻，导致早期生活史阶段存活率降低；2）繁殖季节推迟、生长减慢，使种群初次性成熟年龄推迟、繁殖力降低，影响种群动力学特征。由于大坝运行对坝下水环境的影响随距大坝距离的增加而减弱，本项目通过比较不同江段早期生活史阶段的补充过程和种群动力学特征验证以上假说。项目以长江中游最优势的鱼类贝氏餐为材料，一方面，研究仔鱼发生动态、仔稚鱼摄食与生长状况、早期生长与存活关系，揭示不同江段早期生活史阶段种群补充过程；同时，研究种群年龄结构与生长、初次性成熟年龄和繁殖力，揭示不同江段种群动力学特征。由于贝氏餐不是商业捕捞对象，其种群动态特征更能反映环境的影响。项目的开展将揭示坝下鱼类资源下降的种群生态学机制，对坝下鱼类资源保护和管理也具有重要的指导作用。

三峡大坝下泄低温和贫营养水，影响鱼类的摄食和生长，导致坝下鱼类种群数量下降。本项目提出坝下鱼类种群数量下降机制假说：1）鱼类早期个体生长发育受阻，导致早期生活史阶段存活率降低；2）繁殖季节推迟、生长减慢，使种群初次性成熟年龄推迟、繁殖力降低，影响种群动力学特征。由于大坝运行对坝下水环境的影响随距大坝距离的增加而减弱，本项目通过比较不同江段早期生活史阶段的补充过程和种群动力学特征验证以上假说。项目以贝氏䱗为材料，研究三峡坝下不同江段早期生活史阶段种群补充过程和种群动力学特征。研究结果揭示了坝下鱼类资源下降的种群生态学机制，对鱼类资源保护和管理有重要的指导作用。.分析贝氏䱗种群动力学参数表明，在离三峡大坝最近的监利江段，贝氏䱗的繁殖期比洪湖和武穴江段晚；绝对和相对繁殖力比武穴江段低，但与洪湖江段的相似；武穴、洪湖江段的卵径较监利江段大。来自三峡水库的底层低温水会降低坝下江段的水温，阻碍卵及性腺的发育，使鱼类的产卵延迟，最终使仔鱼的数量下降影响种群的补充。三峡坝下不同江段贝氏䱗仔鱼丰度的空间格局显示：丰度随着离大坝距离的增加而增加，离大坝越远距离的江段相比离大坝近的江段拥有更高丰度的仔鱼；贝氏䱗仔鱼时间生态位宽度随着离大坝距离的增加而增加。贝氏䱗仔鱼发生和分布动态与水温、流量和溶解氧密切相关，沿着坝下不同江段的环境空间梯度改变，揭示了三峡大坝运行后鄱阳湖比洞庭湖对鱼类种群成功补充具有更重要的作用。.项目同时对坝下不同江段四大家鱼和鳜仔鱼的分布、摄食、生长和存活进行了研究。四大家鱼仔鱼的丰度、摄食和生长在长江中游监利、洪湖和武穴江段呈现明显的空间差异。监利江段仔鱼的丰度最低，出现时间最晚，摄食强度最低以及生长率最慢。鳜仔鱼的丰度、摄食、生长和死亡在长江中游呈现与四大家鱼仔鱼格局相似的空间差异。三峡大坝改变的环境梯度不仅对长江中游洄游性鱼类四大家鱼仔鱼产生影响，也是决定非洄游性鱼类鳜仔鱼丰度、摄食、生长和死亡时空格局的关键因素。研究结果也支持项目的假说。