Comparing DNA metabarcoding with faecal analysis for diet determination of the Eurasian otter (Lutra lutra) in Vejlerne, Denmark

The Eurasian otter (Lutra lutra) is a semi-aquatic predator that lives in a wide range of aquatic habitats. Knowledge of the diet of predators is essential for the management and conservation of the species and their habitats. We assessed the diet of otters in fifty-three faecal samples (spraints) by morphological analysis of undigested prey remains and by DNA metabarcoding approaches with two different blocking primers. The analyses were conducted to compare the accuracy of the two methods. The prey items were categorised to the highest possible taxonomic resolution, and relative frequency of occurrence was applied in order to compare the morphological and DNA results. The bulk score for difference prey items in the spraints was based on volume estimates in macroscopic observations. The two methods had an overlap of identified prey species of 57.6 %, meaning a large portion of prey items found by one method was replaced by other prey items in the other method. There was a considerable dissimilarity, with a turnover-rate of ßsim=0.6 between the two methods, compared to the nestedness value of ßsne=0.0065574. The observed marked difference in ßsim and ßsne values means that the number of prey items found by both methods is low; therefore, most prey items found through DNA-metabarcoding are considered new when compared to the morphological analysis. Therefore, applying both morphological and DNA analysis is crucial in order to determine the diet of otters and, possibly, other predatory species.