Nonlinear relationship between Silver Carp density and their eDNA concentration in a large river

Although environmental DNA (eDNA) is increasingly being used to survey for the presence of rare and/or invasive fishes in aquatic systems, the utility of this technique has been limited by a poor understanding of whether and how eDNA concentrations relate to fish density, especially in rivers. We conducted a field study to systematically test whether the eDNA released by a model invasive fish, Silver Carp (Hypophthalmichthys molitrix), was related to the density of this species in a large river. We quantified fish density throughout the 460 km long Illinois River using hydroacoustic surveys at 23 sites while concurrently collecting 192 surface water samples for eDNA analysis. We found that Silver Carp numerical density and biomass density were positively and non-linearly related to eDNA concentration and detection rate. Both eDNA concentration (copy number) and detection rate increased rapidly as Silver Carp density increased but plateaued at moderate densities. These relationships could prove useful for estimating Silver Carp relative abundance in newly invaded locations where population numbers are low to moderate. Future studies should explore the causes of this nonlinear relationship as it would ultimately benefit aquatic species monitoring and management programs.尽管环境DNA（environmental DNA, eDNA）正日益被用于水生生态系统中稀有或入侵鱼类的存在调查，但我们对eDNA浓度与鱼类密度之间是否存在关联、以及如何关联的认知不足，这极大限制了该技术的应用潜力，这一局限在河流环境中尤为显著。为此，我们开展了一项野外研究，以系统性检验作为研究模型的入侵鱼类银鲤（Silver Carp, Hypophthalmichthys molitrix）所释放的eDNA，是否与大型河流中该物种的种群密度存在关联。我们在全长460千米的伊利诺伊河（Illinois River）设置23个采样点，通过水声学调查（hydroacoustic surveys）量化了整条河流的鱼类密度，同时同步采集了192份地表水样本用于eDNA分析。研究结果显示，银鲤的数量密度与生物量密度，均与eDNA浓度及检出率呈显著正相关的非线性关系。随着银鲤密度升高，eDNA浓度（拷贝数，copy number）与检出率均快速上升，但在中等密度区间趋于平稳。上述关联关系可用于估算种群数量处于中低水平的新入侵区域内，银鲤的相对丰度。未来的研究应进一步阐明该非线性关联的形成机制，这将最终助力水生生物监测与管理工作的开展。