Shifts in the size structure of an exotic prey may prevent native top predator recovery

Here, we demonstrate a correlation between the low abundance of the native predator, trahira (Hoplias malabaricus), and size structure of the introduced Nile tilapia, as well as its impact on the recovery of the native predator population. We sampled Nile tilapia and trahira populations from seven water bodies in Brazil over time. Our findings show that Nile tilapia reached larger sizes in areas with high predator biomass, while dense, stunted populations were found in areas with low predator biomass. Specifically, lakes with low predator biomass were dominated by small adult tilapia and had a higher biomass of juveniles compared to lakes with higher predator biomass, indicating a release of juvenile tilapias from predation. While the native predator seems to play a role in shaping the size structure of tilapia, in systems with dense, small tilapia populations, the native predator struggles to recover after a decline, likely due to juvenile competition for zooplankton. Overfishing of the native predators also might operate on the reduction of the trahira population leading to the change in the tilapia size structure. Reducing the fishing of trahira in environments with tilapia, culling juveniles, and small adults of tilapias may alleviate intraspecific competition, thereby promoting growth in tilapia and alleviating the interspecific competition between juveniles and facilitating the recovery of native predator populations. We recommend incorporating size-structured interactions into management practices for exotic species to mitigate and prevent adverse impacts on predators and other native species.