Environmental DNA for Aquatic Monitoring: Impact of Diel Vertical Migration

There is increasing interest in using eDNA for deriving abundance indices for biodiversity monitoring and in support of fisheries management. However, eDNA concentrations are affected by animal behavior, such as diel vertical migration, which has repercussions for designing eDNA sampling strategies for deriving unbiased abundance indices. In this study, we investigated the potential impact of diel vertical migration or other diel activity variations on measured eDNA concentrations for European hake ( Merluccius merluccius ), European seabass ( Dicentrarchus labrax ) and blackspot seabream ( Pagellus bogaraveo ). For hake, in situ eDNA concentrations near the sea floor differed systematically between samples taken before sunrise and after sunset, with the average concentration in morning samples being 24% of the average evening samples. For the two other species, only a weak diel signal in eDNA concentrations was found. Modeling the dispersal and decay of eDNA particles through a Lagrangian approach revealed that eDNA concentrations might decrease to 21%–41% of their initial value during the absence of a species moving entirely up from the sea floor during the night. For M. merluccius , the coherence between observed diel variations in eDNA concentrations near the sea floor and modeling results indicates that diel vertical migration behavior needs to be accounted for when devising eDNA sampling plans. The necessity is less clear for D. labrax and P. bogaraveo .