Eurasian otter (Lutra lutra) has recently returned to colonize the river network of the Region Friuli Venezia Giulia in the area of Tarvisio, along the catchment “Slizza-Gailitz”. The aim of this study was to analyse the composition of the diet of otters in this newly colonized area. The traditional approach to get information on diet of predators, is based on the identification of prey remains found in their scats with the help of a microscope. This analysis, however often leads to problems as, for example, the lack of prey identification at genus/species level. To overcome these problems, an additional NGS approach has been recently widely applied, the DNA-barcoding which has proven to be valuable in several diet studies on different species of animals, although it may also lead to biases related to quantification of number of prey found in scats.
In this study, we analysed otter diet in the area of Tarvisio, by integrating these two approaches. Furthermore, we investigated the existence of a trophic selection of otters on different size classes of fish, comparing data gathered on their diet with data on the availability of fishes of different size classes along the Slizza-Gailitz catchment.
From June 2016 to February 2017, 102 spraints were collected. All spraints have been analysed with a microscope. A subsample of 50 spraints was analysed also with DNA-barcoding method, based on the hypervariable V9 region of the 18S rRNA.
Traditional analysis revealed that otters eat mainly fish, followed by amphibians, both annually and seasonally. A little presence of crayfish was also recorded from spring to autumn. Fish eaten by otters belonged to two taxa: Salmonids and European bullhead (Cottus gobio), with the first preyed more frequently. Results indicated also a selection on Salmonids of small size (75-100mm). DNA-barcoding analysis confirmed the results obtained with the traditional analysis in terms of composition of the diet of otters. Moreover, through DNA-barcoding it was possible to obtain information on the Salmonid species preyed: some of the sequences obtained, were in facts attributable to the brown trout (Salmo trutta).
Results obtained reflect trends described in other diet studies in Europe, with fish being the most preyed taxon by otters. The size of preyed Salmonids does not overlap with the size-range of angling interest, thus preventing the possibility of arising conflicts with anglers. DNA barcoding approach proved to be more informative at species-level identification than traditional approach, but the lack of reference sequences in GenBank database for many potential prey species present in the study area, prevented us to identify the other taxa found in spraints at more informative levels. In conclusion, DNA-barcoding has proved promising also for Eurasian otter diet studies but, to improve our results, further investigation is needed.