Using a head-start conservation intervention to boost spawning numbers of the endangered Clanwilliam sandfish

Freshwater ecosystems are the most threatened on Earth, with many species facing extinction. The Clanwilliam sandfish (Labeo seeberi) is South Africa's most threatened migratory freshwater fish and is endemic to the Olifants-Doring River system in the Cape Fold Ecoregion. Non-native fish predation and river desiccation have caused a recruitment bottleneck, severely compromising juvenile survival and resulting in a declining population of aging sandfish. The Saving Sandfish Project launched an emergency head-start intervention in 2020 to reduce extinction risk. We (1) rescued juvenile sandfish from drying pools in a key spawning tributary (the Biedouw River); (2) relocated them to 6 off-stream reservoirs; and (3) released reservoir-reared sandfish back into their natal river once large enough to evade non-native fish predation. Here, we estimate survival in the reservoir environment, evaluate return rates relative to wild run size, and assess the probability of return based on conditions at release. Between 2020 and 2022, we stocked 33,391 juvenile sandfish into the 6 reservoirs. After 1 year, the estimated survival rate at one reservoir was 0.679 (range based on 95% CI: 0.385-0.973). Release and return results are presented only for the first (2020) rescue cohort. In 2021, we released 1277 sandfish from 2 reservoirs into the Biedouw River, comprising 16.6% of the 2020 rescue cohort. Mean size at release was 169 mm (SE 0.6) total length. Of those released, 994 were PIT-tagged. A total of 77 PIT-tagged sandfish were recorded during the 2022 spawning migration-a return rate of 7.7% of tagged releases in the first year of returns. Size of fish and distance from the Doring River at release were significant predictors of return probability, with larger fish released further from the Doring experiencing a higher probability of return. This program serves as a model for the conservation of freshwater fish where there is an imminent and high risk of extinction. The endangered Clanwilliam sandfish (Labeo seeberi), the most threatened migratory freshwater fish in South Africa, is at risk of extinction due to a recruitment bottleneck caused by river desiccation and predation by non-native species. We evaluate the outcome of the first cycle of a head-start program to increase recruitment and boost spawning numbers of adult sandfish in a key spawning tributary. PIT tags are used as a monitoring tool to show that 7.7% of PIT-tagged head-started sandfish returned to their natal river 1 year after release into the wild, boosting the size of the spawning migration substantially compared to the previous year.image