Oxygen and strontium isotope markers of Lake Qinghai naked carp otoliths and their implication for fish migratory pattern

As the dominant and unique species in Lake Qinghai, the naked carp Gymnocypris przewalskii (Kessler) is characterized by upstream migration to spawn during late spring and summer in each year and back lake to live during the remaining seasons, but little is known about its migratory pattern. The migratory pattern of the naked carp is significant in understanding its living habitat and in protecting this precious fish. Otoliths are paired biogenic carbonates in the inner ear of teleost fish and grow layer by layer throughout the fish life. Owing to its metabolically inert without involvement in metabolic processes after deposition, chemical compositions of daily and annual growth increments in otoliths record the water elements of the ambient waters where the fish encountered during its life history. Therefore, microchemistry variation of otoliths has been used widely to trace migration pattern of fish and to reconstruct water chemistry and environmental conditions in the past. The distinct water chemistry of the lake and rivers within the Lake Qinghai catchment provides a rare opportunity to trace migration pattern of the Lake Qinghai naked carp and past water compositions by using otolith microchemistry. In this study, the in-situ oxygen isotope (δ¹⁸O) and ⁸⁷Sr/⁸⁶Sr ratios of the otolith increments of eight Lake Qinghai naked carps were firstly analyzed using high-resolution SHRIMP II ion microprobe and laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) at Research School of Earth Sciences, The Australian National University, respectively. The analytical intervals of δ¹⁸O and ⁸⁷Sr/⁸⁶Sr ratios along the otolith increment axes are 25 μm and 95 μm, respectively. The δ¹⁸O results show (1) that one-to-one correspondence of the δ¹⁸O values along two growth axes of the naked carp otolith demonstrates the same information at different directions of the otolith and (2) that low δ¹⁸O in the otolith incremental zones record ambient river chemistry when the fish migrate to rivers during late spring and summer. One-to-one correspondence between ⁸⁷Sr/⁸⁶Sr and δ¹⁸O compositions along the otolith increment axes is not possible, since the analytical resolution of ⁸⁷Sr/⁸⁶Sr data is much lower than that of δ¹⁸O. However, the covariance of δ¹⁸O versus ⁸⁷Sr/⁸⁶Sr compositions of the otoliths distinctly implies that the majority of the naked carps are more likely to migrate and spawn in one fix river where they were born. For example, when the otolith δ¹⁸O versus ⁸⁷Sr/⁸⁶Sr compositions of one fish point to the Buha River end-members, we infer that the fish favors to migrate to the Buha River where it was born. This implication is significant and helpful in understanding the migratory pattern of the naked carp and in protecting and managing this precious species in Lake Qinghai. This study also provides a good case for tracing fish migratory using paired stable isotopic compositions of otolith increments using high-resolution SHRIMP II ion microprobe and LA-MC-ICP-MS.