Otolith isotopic signatures in identification of natal sources and stock differences of Chinese silver carp

Background/Question/Methods

Silver carp, Hypophthalmichthys molitrix (Cuvier et Valenciennes), is one of the four major Chinese carps and an important species for the freshwater fishery and aquaculture in China. Biological surveys indicated that the spawning grounds of silver carp are located in the main branch of the Yangtze River, and sub-branches such as the Xiangjiang River and the Ganjiang River. The Poyang Lake, the largest freshwater lake in China and the main nursery area for silver carp in the middle basin of the Yangtze River watershed, is considered as an important source for juvenile recruitment. The silver carp fisheries have been declining since early 1980s, but the populationm structure of silver carp has not been fully investigated and understood. Here we report an isotopic results (d¹⁸O and d¹³C) on natal sources and stock differences of silver carp, by analyzing 135 otoliths of juvenile fish collected from the Yangtze River, the Ganjiang River, and the Poyang Lake, respectively. The main research goal was to use otoliths from the "young-of-the-year" (or age-0+) silver carp to examine the isotopic differences of juveniles between the Ganjiang River and the Yangtze River, and possible relationships with the Poyang Lake.

Results/Conclusions

For all otoliths in the age-0+ group, the d¹⁸O values ranged from -12.1 to -8.6‰ while the d¹³C values of the same data set ranged from -19.1 to -12.7‰. The isotopic data and correlation of d¹⁸O versus d¹³C showed a clear separation and significant differences (p<0.001) between otoliths from the Ganjiang River and the Yangtze River at d¹⁸O values around -10‰. The Poyang Lake otoliths were overlapped with otoliths of the Ganjiang River, but distinctly different from the Yangtze River samples. These isotopic signatures are in good agreement with the previous genetic investigation results that there are marked distinctions between one population in the Ganjiang River and the other in the Yangtze River, and the Poyang Lake population belongs to the Ganjiang River rather than to the Yangtze River. Therefore, we concluded that stable isotopic signatures of otoliths can be effectively used to study natal sources and stock differences of silver carp.