长江安庆江段刀鲚耳石微化学及洄游生态学意义

Otolith microchemistry of the estuarine tapertail anchovy Coilia nasus from the Anqing section of the Yangtze River and its significance for migration ecology

刀鲚(Coilia nasus)属辐鳍鱼纲、鲱形目、鳀科,是我国长江下游一种传统的名贵经济鱼类。长江安徽江段为刀鲚的主要分布区之一,但近年来由于人类活动干扰、栖息地衰退及过捕等原因,资源量急剧下降。近期发布的《国家重点保护野生动物名录》水生野生动物调整方案已拟将刀鲚列入国家二级重点野生动物保护名录。为客观了解长江安庆江段刀鲚的生境"履历"及探讨其洄游史,利用X射线电子探针微区分析技术对2008年7月捕获于长江安庆江段的10尾刀鲚(均为传统意义上的长颌鲚)进行了耳石微化学研究。结果表明:这些个体均具典型的溯河洄游习性。从耳石核心到边缘定量线分析结果表明,耳石的沉积过程中元素Sr/Ca比值波动显著,不仅有对应淡水生境的低值(<3)阶段,还具有对应河口半咸水、海水生境的高值(>3)阶段。这种现象亦得到了16色Sr元素面分布分析结果的印证。根据耳石淡水系数、Sr/Ca比值高低波动特征及Sr元素面分布图谱的差异性可以将这些刀鲚分为3种淡-海水生境转换生活史类型。这种现象明显地反映出长江安庆江段刀鲚的群体具有个体组成和洄游模式的多样性。

Owing to its nutritional value and delicacy, the estuarine tapertail anchovy Coilia nasus （Actinopterygii, Clupeiformes, Engraulidae） is a highly valuable commercial fish from the Yangtze River of China. Unfortunately, wild resources of C. nasus have declined drastically because of anthropogenic activities, habitat degradation, and overfishing. Recently, C. nasus has been proposed to be listed as a Class II protected species on China＇s List of Wildlife under Special State Protection. It is noteworthy that many mainstream sections in the lower reaches of the Yangtze River are the main distribution areas of C. nasus, including the Anqing section in Anhui Province. Because of empirical limitations with the current method of catch-survey analysis, it remains difficult to effectively understand the spatial and temporal dynamics of the diadromous migration and habitat utilization history of C. nasus in this river section. Alternatively, elemental microchemical patterns in otoliths have proven effective natural ＂fingerprints＂ that can provide a powerful approach to estimate precisely the diadromous migratory history of C. nasus. In the present study, a JXA-8100 electron probe X-ray microanalyzer （EPMA） was applied to investigate the otolith microchemistry characteristics of 10 C. nasus anchovies collected from the Anqing section of the Yangtze River in July 2008 for investigation of habitat ＂fingerprints＂ and migratoryhistory. All the C. nasus individuals were traditional long jaw estuarine tapertail anchovies, with supramaxillae longer than the head. Microchemical results confirmed that all of the C. nasus fish were typical anadromous individuals, i.e., the anchovies were born and hatched in freshwater habitats, but later migrated into estuarine brackish or high salinity seawaters as juveniles to grow, and then ascended to freshwater habitats again to reproduce. The results of EPMA line transect analysis generally showed that the otolith Sr/Ca ratio fluctuated significantly between low （〈 3, reflecting the freshwater habitat） and high （ 〉 3, reflecting the estuarine brackish water or seawater habitat） value phases. The evidence was supported by the 16 color Sr content profiles （red [ highest ] through yellow and green to blue [ lowest ] ） obtained by EPMA mapping analysis. Furthermore, three types of migratory life histories could be separated among C. nasus individuals of this study based on differences in the otolith freshwater coefficient, Sr/Ca ratio, and Sr content map profile. Namely, these anchovies have the ability to adopt short-term, moderate-term, or long-term ontogenetic strategies in freshwater habitats, before they shift their habitats to estuarine brackish or high salinity sea habitats. These phenomena clearly reflect the flexible habitat use patterns, diversity of population composition, and fluctuation migration strategies of C. nasus anchovies in the Anqing section of the Yangtze River. In addition, although from previous studies it was believed that juvenile C. nasus spent the winter months in sea areas, the present study has provided new evidence to demonstrate that juvenile C. nasus may overwinter in freshwater habitats. All the aforementioned results provide an important theoretical basis for a more accurate understanding of the migration patterns, population compositions, and habitat backgrounds for the life history of C. nasus in the Anqing section of the Yangtze River.