Improving an endangered marine species distribution using reliable and localized environmental DNA detections combined with trawl captures

The description of marine fish distributions generally relies on trawl survey observations. For rare species, sporadic catches necessitate the combination of multiannual trawl surveys to accurately describe the distribution, limiting short term monitoring. Recent studies suggest that combining traditional capture methods and environmental DNA (eDNA) detections enhance rare species' occurrence. In this study, the reliability and localization of eDNA detections (single- and multi-species) of an endangered marine species, the Atlantic wolffish Anarhichas lupus, was assessed during fine scale surveys. eDNA was detected at all six stations sampled with Niskin bottles over caves housing one or two A. lupus. Detections from samples collected with syringes by divers along a 15 m transect perpendicular to each cave were limited to the fish cave entrance. Trawl-captures and eDNA detections were then combined to test if the species distribution is improved for broad scale surveys. The station-based frequency of species occurrence was 13% with trawl captures and increased to 23% when combined with eDNA detections. Single-species detections were generally more sensitive than multi-species detections. Our results showed that a rare marine species distribution improves combining traditional methods and eDNA detections in oceanographic surveys. Strategies for integrating optimal eDNA detections in marine surveys are discussed.