Predation on native fish eggs by invasive round goby revealed by species-specific gut content DNA analyses

Conservation of riverine fish often aims to improve access to spawning grounds and restore longitudinal connectivity by removing migration barriers, and involves substantial investments. However, these investments also enable non-native predators to invade upstream into spawning areas and potentially adversely affect the recruitment of threatened freshwater fish through egg or fry predation. Detecting egg predation is often challenging. Visual inspections of fish gut contents may underestimate predation of soft materials such as eggs and fry, which limits the discovery of predators preying upon these life-stages. DNA-based detection assays may offer a more sensitive tool to assess predation of soft materials. A conservation issue was confirmed by developing and applying a species-specific DNA-based detection assay: invasive round goby (Neogobius melanostomus) prey on the eggs or fry of the threatened common nase (Chondrostoma nasus) in Switzerland. DNA-based detection assays were also developed for five other valuable native fish species, including endangered salmonid and cyprinid river spawners. The applicability of the assays was confirmed in a series of laboratory and field feeding experiments involving eggs and fish tissue. In addition, this work provides a guiding framework for conservation managers regarding the use and applicability of different DNA-based detection approaches for gut content analysis. The results of this study could inform local conservation measures - such as temporary reductions in the density of round goby at spawning sites prior to spawning - and demonstrate how targeted application of species-specific molecular markers may advance freshwater fish management.