Invasive Freshwater Fishes and the Decline of Native Biodiversity: A Review of Tilapia and Pangasius Impacts on Pakistan's Indus River System

Abstract

Few examples could be more dramatic than the global invasion of tilapia (Oreochromis spp.) and pangasius (Pangasianodon hypophthalmus) to illustrate the leading cause of biodiversity loss: invasive freshwater fishes. Both species were intentionally introduced to enhance aquaculture and food security, and have become feral populations that are competing with, displacing and otherwise interfering with native fish populations. This review aims to highlight the pathways, biological attributes and ecological processes involved in the success of these two invasive species and in particular in South Asia, including the Indus River system in Pakistan. Across studies, there are converging patterns: tilapia and pangasius grow rapidly, have broad environmental tolerances, are highly fecund, and are generalist feeders, meaning they outcompete native fish for food and spawning space, destroy habitat structure, affect food-web dynamics, and in some cases spread disease. The native species richness of the rivers examined at the habitat level has declined from approximately 72% of the individuals in low-disturbance upstream rivers to approximately 22% in heavily modified downstream reaches, reflecting an increase in invasive dominance in the latter. The species richness, Shannon diversity and evenness are significantly lower at sites invaded than at similar non-invaded sites. Such pressures reduce the complexity of fish communities and encourage biotic homogenization and have the potential to drive freshwater systems into alternative stable states that are less favourable but more difficult to recover. Meanwhile, the characteristics that make these species a nuisance to native biodiversity also make them beneficial to aquaculture-based livelihoods, and thus there are continuing conflicts between aquaculture and conservation of native biodiversity. Weak biosecurity governance, poor long-term monitoring, and inadequate public awareness are the key factors that cause invasions to be unknown until they are firmly established, and it suggests that better regulation, restoration of habitats, and long-term ecological monitoring are priorities to manage this trade-off.