OBJECTIVES

Despite annual spawning, wild sandfish populations continue to decline due to recruitment failure: juvenile survival is virtually nonexistent due to predation by non-native fish (bass and bluegill) and excessive water abstraction. With no young fish replenishing the aging population, the species is rapidly approaching extinction. This project aimed to scale up the rewilding efforts initiated in 2020, in which juvenile fish are 1) rescued from the Biedouw River before they succumb to alien fish and declining flows, 2) relocated to off-stream farm reservoirs (‘sandfish sanctuaries’), and 3) released back into the wild once they reach a ‘bass-proof’ size. The strategy is designed to bypass the mass mortality of the juvenile stage, providing thousands of new recruits to build up the wild population.. Each cohort of “head-started" fish strengthens species resilience and helps to secure species viability. The project’s objectives are to significantly increase wild sandfish numbers and assess the effectiveness of rewilding efforts across the catchment. Additionally, our partnership with the local primary school aims to engage students in hands-on conservation, fostering environmental stewardship and inspiring the next generation of conservation leaders.

OUTCOMES AND IMPACT

Validating the Head-Starting Strategy | The project successfully scaled up the rewilding intervention, achieving the largest single-year release to date. The team released 3,052 head-started sandfish from sanctuary dams back into the Biedouw River, PIT-tagging 1,603 of those individuals to track their long-term survival. Most importantly, a catchment-wide survey provided the first empirical proof of the strategy's success: the team recaptured four adult PIT-tagged sandfish in the mainstem Doring River that had been released as far back as 2021 and 2022. This confirmed that head-started fish can survive to adulthood, successfully disperse out of the sanctuary tributaries, and integrate into the broader wild population.

Monitoring Ecosystem Dynamics | While the survey proved that rewilding adds valuable adults to the system, it also confirmed that natural recruitment remains low, underscoring the ongoing necessity of the head-starting program. Additionally, the field team documented a novel threat: the invasion of the African sharptooth catfish across the Doring River system. Observations of catfish preying on invasive bluegill suggest that the ecosystem's predator dynamics are shifting, providing early data that will inform future habitat management.

Discovering Climate Refuges | The field season was severely impacted by below-average winter rainfall, which suppressed the typical spawning run in the Biedouw River. However, this drought led to an interesting hypothesis: that adult sandfish may bypass the Biedouw in favour of the neighbouring, higher-flowing Tra-Tra River during low flow years. Responding to a stranding event in the Tra-Tra allowed the team to map potential over-summering refuge pools for juveniles. Understanding this alternative migration route opens a vital new geographic frontier for protecting the species during extreme climate events, though it remains clear that more research is needed into this neighbouring system!

Fostering Community Stewardship | Learners from the Wupperthal Primary School gained hands-on experience by assisting the field team in rescuing stranded adult sandfish and participating in monitoring surveys in the upper catchment. This foundational partnership is actively building local capacity and inspiring the next generation to take ownership of their local river systems.