Using eDNA and SCUBA surveys for detection and monitoring of a threatened marine cryptic fish

Quantification of species’ spatial distributions and population trends is crucial for successful conservation efforts. Obtaining sufficient data, however, often difficult in the marine environment, especially rare fish invertebrate species that are small, cryptic very to detect. This study sought understand effort required search undiscovered populations cryptic, shallow vegetated coastal habitats track numbers, using Critically Endangered red handfish (Thymichthys politus) as a representative species. A sampling strategy was designed combination environmental DNA (eDNA) structured underwater scuba surveys life-like 3D-printed ‘handfish replicas’ estimate detectability ultimately determine whether current monitoring adequately covering remaining habitat occupied by Tested over scales hundreds metres kilometres, eDNA assays performed relatively poorly situ, detecting presence only ~13% samples collected from centre known, yet low-density, site. In contrast, searches independently placed replicas divers indicated mean detection probabilities at finer (~100 m) ranged 57 97%. Near certain (95% probability) an adult achievable with one three (300 m2 belt transects), depending on complexity. While other will vary searches, these findings give insight into general inhabiting environments. Such knowledge not helps refine efforts known threatened species, but may also assist identifying inconspicuous whose declines otherwise go unnoticed.