Effects of hydrological alteration on fish population structure and habitat in river system: A case study in the mid-downstream of the Hanjiang River in China

Fish resources in the mid-downstream of the Hanjiang River have showed a serious decline in past decades due to eco-environmental degradation. This study aims to analyze the effects of hydrological alteration on fish population structure and habitat by combining the IHA-RVA method, the FLOWS method and the fish resources survey. Based on the change-points (the years 1990 and 2012) detected in the runoff time-series of the Huangzhuang hydrological station, we divided the study period into three periods: 1956–1989 (Period I), 1990–2011 (Period II), and 2012–2017 (Period III). The percent change in runoff D₀ between the different periods (I-II, II-III, I-III) are 42.2%, 69.28%, and 81.68%, respectively. The monthly average flow increased during the dry season but decreased in the wet season. The rise rate of flow showed a 46% and 35% reduction in Period (II) and Period (III), respectively. The average 1-day maximum flow reduced from 12473.6 m³/s to 4466.7 m³/s, while its occurrence time also changed from July and mid-September to mid-August. The average high pulse count in Period (III) had a 38% reduction compared to that in Period (I). The number of species of fish spawning drifting eggs decreased by 6 species, so did the carnivorous fish. In addition, the spawning grounds in the Yicheng-Zhongxiang section was seriously deteriorating. The proportion of omnivorous fish and small fish increased by 16.5% and 24.5% during the past 62 years, respectively. The trend of miniaturization and aging of fish was significant. For Coreius heterodon, the number of suitable fish habitat sections in the study area has decreased from three to zero, while Cyprinus carpio could adapt to changing environment. Our results provide important clues for a better understanding of the changes in fish population structure and habitat under hydrological alteration.