Evaluation of the Stocking Efficiency of a Rheophilic Cyprinid in a Large River System using Spatio-temporal PIT Tag Detections and Survival Analysis

The common nase (Chondrostoma nasus) was a formerly abundant European riverine fish that has suffered a steep decline due to anthropogenic influences. To address this decline, stakeholders initiated a stocking program for this species in the River Inn, Germany. To evaluate stocking success, 1,650 hatchery-raised age 1+ and 2+ juveniles were tagged with passive integrated transponders (PIT-Tags), released into three habitat types (backwaters, fish passes and mainstream), and monitored over two years using 38 fixed antenna arrays distributed over 150 km. Detection rates decreased substantially over time, but still reached 7.2% after two years. A continuous-time survival model revealed that survival rates were lowest immediately after stocking but stayed relatively constant after an acclimatisation phase of 240 days. Nase used all habitat types and accumulated in areas with greatest habitat diversity including naturelike fishways. Our results highlight that the selection of the stocking site had a measurable effect on survival. They also suggest that a combined optimization of stocking and restoring of diverse riverine habitats is likely to increase the efficiency of nase conservation programs.