68 / 2023-12-28 18:47:36

Fish Habitats in the Yangtze River Headwaters: Implications and Insights

Yangtze River Headwaters,overwintering ground,spawning ground,water temperature,Salinity,geothermal spring

The fish diversity and distinct ecological and hydrological processes, such as bottom-freezing in winter and severe diurnal fluctuations of water temperature, is unique in the Yangtze River headwaters. New assessments of fish diversity and the ecological and hydrological requirements of critical fish habitats, including overwintering and spawning grounds, remain unexplored. Through continuous in-situ observations from 2019 to 2023, combined with ultrasonic telemetry and geochronology techniques, the overwintering ground, spawning ground, and hydrological requirements were revealed. Results showed that a critical 2.5-km-long river segment in the southern headwaters was discovered as an important overwintering habitat for fish. An estimated 93,000 fish overwintered here, with 32,000 adult Schizopygopsis microcephalus inhabiting the deeper core area. The adult fish showed high fidelity, leaving in mid-to-late May and returning between late October and early November. However, Ptychobarbus kaznakovi, similar in size to Schizopygopsis microcephalus, did not overwinter here. This overwintering habitat is part of a unique thermal spring river ecosystem in the Yangtze headwaters, forming a stable, complex ecosystem with fish, waterbirds, an "underwater forest," and amphibians discovered for the first time. The constant 21°C water temperature and 0.34 m³/s flow from the thermal spring were essential, providing base flow and heat for the ecosystem. The water temperature in the core area and at the spring river mouth had a significant linear relationship (Y_core area = 0.8X_spring - 7.0, R²=0.52). The core areas for overwintering were formed by concave bank erosion. The adult Schizopygopsis microcephalus shifted its diet from algophagous in the summer to carnivorous in the winter. Preliminary geological dating studies suggest that this overwintering ground has been around for over 29,000 years. Schizopygopsis microcephalus matured in early June and spawned before the flood season. The spawning ground was only 2.5 km from the overwintering habitat and consisted of evenly spaced, patchy spawning nests. The nests were funnel-shaped, and fertilized eggs were strictly buried under the gravel. The maximum diameter of the gravel was 4-5 cm, about 1/10 the total length of the female fish, with a median grain size of 2.2 cm. The hatching of fertilized eggs of Schizopygopsis microcephalus required a diurnal process of lower and higher temperature stimuli, with a dual threshold range of (12, 28)°C. The salinity tolerance threshold for fertilized eggs was 7psu. These results will provide a critical foundation for conservation and research into the response of geothermal spring–supported ecosystems to climate change.