Assessing impacts of introduced aquaculture species on native fish communities: Nile tilapia and major carps in SE Asian freshwaters

Herbivorous or omnivorous tilapia and carp species form the backbone of tropical inland aquaculture and fisheries enhancement and have been introduced widely outside their natural ranges. Perceptions of their impact on native fish faunas vary widely, but there have been few rigorous assessments. To quantify the impact of tilapia and carp stocking on native fish communities in freshwater wetlands of the Mekong region, we conducted observational and experimental impact-control studies replicated at the wetland level, at a total of 46 sites in Lao PDR. The studies were designed as paired comparisons of wetlands where the non-native species (Nile tilapia Oreochromis niloticus, mrigal Cirrhinus cirrhosus, rohu Labeo rohita and bighead carp Hypophthalmichthys nobilis) were stocked in substantial numbers with similar wetlands where the species were absent. Stocking of these non-native species was associated with significant increases in total fish biomass, by 180% in the observational study and by 49% in the experiment. Native fish biomass was not affected by stocking of the non-native species. No significant impacts on native fish species richness, diversity indices, species composition or feeding guild composition were detected, except for moderately negative effects on Simpson diversity and equitability in the observational study. In the experiment, no effect had a point estimate exceeding −14%, or a 95% confidence limit exceeding −35% of the non-impacted value. Use of these non-native tilapia and carp species in fisheries enhancement in mainland SE Asia supported substantial increases in harvestable biomass while having only mild impacts on native fish communities. Escapes of the same species from pond and cage aquaculture facilities are likely to result in lower biomass in the wild and have lower impacts than enhancement stocking. We encourage building on our results through a systematic and stakeholder-participatory evaluation of the significance and acceptability of benefits and risks associated with the use of these non-native species. This evaluation should be extended to the relative benefits and risks of replacing non-native species with native ones in aquaculture and enhancement, considering that risks associated with using the current set of non-native species are increasingly well known and appear mild-to-moderate, whilst ecological and genetic risks of releasing domesticated types of native species are poorly known.