Environmental DNA based biomonitoring for hatchery-raised fish in riverine habitats before and after recordable flood event

It is reported that the magnitudes of flood events in riverine systems have been increasing due to global climate change. Because flood events could displace fish species downstream and/or increase the mortality of fish, it is important to know how the increased floods can affect fish in river networks. In this study, we focused a hatchery-raised amphidromous fish, Ayu <i>Plecoglossus altivelis altivelis</i> in a mainstem-tributary network in Japan and examined the relative fish abundance changes before and after the recordable massive flood using the environmental DNA (eDNA) analysis. We also examined the spatiotemporal patterns of the Ayu eDNA concentrations and the relationships with tributary discharge, width, and depth to examine if the relative abundance changes could be related to tributary size. Our results indicated that Ayu tended to inhabit large, deep tributaries more than small, shallow tributaries even after the flood event. Further, the eDNA concentrations of Ayu have decreased in most study sites after the flood; however, the eDNA concentrations in certain tributaries with lower tributary size have increased during the study period. These results suggest that (1) the habitat conditions could be important for the maintenance of Ayu populations before and after flood events, (2) increase in flood magnitude along climate changes could have impacts on Ayu populations, and (3) not only large tributary, but also small tributaries could be important habitats for the target species to avoid flood disturbances.

据报道，受全球气候变化影响，河流系统中的洪水事件规模正逐渐增大。由于洪水可能将鱼类冲至下游，或提升鱼类死亡率，因此探明洪水规模加剧对河网内鱼类的影响至关重要。本研究以日本某干流-支流河网中的人工繁育两游性鱼类香鱼（Plecoglossus altivelis altivelis）为研究对象，借助环境DNA（eDNA）分析技术，探究了该鱼种在有记录的特大洪水前后的相对丰度变化。此外，本研究还分析了香鱼eDNA浓度的时空分布特征，及其与支流流量、宽度和深度的关联，以验证相对丰度变化是否与支流规模相关。研究结果显示，即便在洪水过后，香鱼仍更倾向于栖息于宽大深邃的支流，而非狭小浅窄的支流。进一步分析发现，多数研究点位的香鱼eDNA浓度在洪水后均有所下降，但部分规模较小的支流内，香鱼eDNA浓度在研究期间却出现了上升。上述结果表明：（1）栖息地条件对于洪水前后香鱼种群的维持至关重要；（2）伴随气候变化加剧的洪水规模增大，可能对香鱼种群产生影响；（3）不仅宽大支流，狭小支流也可成为目标鱼种规避洪水扰动的重要栖息生境。