A quantitative eDNA-based method to monitor fish spawning in lakes: application to European perch and whitefish
There is an urgent need to evaluate the effects of anthropogenic pressures and climatic change on fish populations dynamics. When monitored in lakes, the spawning of fish is generally assessed using traditional, mostly destructive or damaging, methods as gillnetting and collection of fertilized eggs. Over the last decade, environmental DNA (eDNA) based methods have been widely developed for the detection of aquatic species, offering a non-invasive alternative method to conventional biomonitoring tools. In particular, the emergence of new methods as the droplet digital PCR (ddPCR) offer the possibility to quantify an absolute eDNA signal in a very sensitive way and at a low cost. Here, we developed and implemented a quantitative eDNA method to monitor the spawning activity of two fish species, European perch and whitefish. ddPCR protocols were formalized based on existing and newly designed COI primers, and were applied during four spawning periods in lake Geneva. The results demonstrate the efficiency of eDNA coupled with ddPCR to identify the timing and duration of the spawning periods, as well as the peak of the spawning activity for the targeted species. In addition, the use of a control species (i.e., quantification of the eDNA signal of a fish that does not reproduce during the monitoring period) was shown to be relevant to clearly discriminate fluctuations of the eDNA signal associated to the spawning activity from the baseline eDNA signal. For future implementation, we recommend using an integrative sampling strategy (e.g., pooled samples for a give station) to smooth the local variability of the eDNA signal. These results show that we reached an operational level to use these non-invasive eDNA methods to monitor the spawning periods of these two fish species in large lakes.