Temperature moderates eDNA-biomass relationships in northern pike

Support for eDNA as a quantitative monitoring tool is growing worldwide. Despite advances there are still uncertainties regarding the representability of the eDNA signal over varying spatiotemporal scales, influence of abiotic forcing and phenological changes affecting behavior of the study organism, particularly in open environments. To assess the spatiotemporal variability and predictive power of quantitative eDNA analysis, we applied species-specific real-time quantitative PCR on water filtrates during two visits to 22 coastal bays in the Baltic Sea. Within bays, we collected water along four transects across each bay and compared the pooled eDNA concentration to temporally matched catches from standardized angling targeting the northern pike ( Esox lucius ) a species for which reliable monitoring data is lacking. We found the variability in eDNA concentrations between transects to be moderate (21%) but still considerably lower than across bays and visits (52%), suggesting small scale spatial differences are of less importance during spring when pike spawn. Standardized angling catches, bay area, and water temperature together explained 48% of the variance in eDNA concentrations. DNA concentrations decreased with increasing bay area, likely indicating a dilution effect. Notably, the relationship between eDNA and standardized catches was positive but varied with temperature and the eDNA-abundance relationship was only significant at higher temperatures, which also coincided with a higher proportion of spawning/spent fish. We conclude that temperature is a key moderating factor driving changes in pike behaviour and spring DNA-dynamics. We recommend that future surveys focus on larger spatiotemporal scales during times when the influence of changing temperatures is minimized. ### Competing Interest Statement The authors have declared no competing interest.