OTOLITH CHARACTERISTICS AND REPRODUCTIVE PERIOD ESTIMATION OF SCHIZOTHORAX O’ZONNORI IN THE MIDDLE REACHES OF THE YARLUNG ZANGBO RIVER

Schizothorax o’connori, an endemic fish, distributes in the upper and lower reaches of the the Yarlung Zangbo River Grand Canyon in China, holding significant ecological and economic value. In this study, we performed observation and analysis on deposition pattern of otolith increments and the formation time of the first daily increment, utilizing artificially hatched larval and juvenile S. o’connori. Additionally, we estimated the hatching date for wild juvenile S. o’connori in the Sangri County-Jiacha County section of the middle reaches of Yarlung Zangbo River. The results showed that the first daily increment of larvae appeared on 1 DPH (8 hours post hatching) at temperatures ranging from 6.8 to 8.8℃. Subsequently, one additional increment of lapilli and sagittae formed each day, and the number of increments on lapilli and sagittae exhibited a linear relationship with age. The slopes of the equations were close to 1.0 (P>0.05), indicating a daily formation of otoliths increments. Throughout development, the shape of lapilli transformed from an almost circular shape to clam-shaped and eventually kidney-shapedi, while sagitta evolved from a wedge-shape to a long ellipse shape, eventually changing into long pointed posterior shape. The lapilli exhibited stable shapes with clear daily increments, making them suitable for daily-age identification purposes. By counting the number of daily increment and confirming daily pattern of the increment deposition, we back-calculated the hatching dates of the larvae, which ranged from 11 March to 2 May in 2022 and from 8 February to 23 April in 2022. These results provided valuable insights into the ecological characteristics, and offer a scientific basis for the conservation of natural population resources of S. o’connori.