Background: The Temengor Reservoir is the second largest reservoir in Peninsular Malaysia. Located in the northwestern state of Perak, it was selected to develop a large-scale tilapia (Oreochromis niloticus) aquaculture facility within the Aquaculture Industrial Zone (AIZ) in 2008 due to its favourable environmental conditions. No record of tilapia has ever been reported in the natural waters prior to this. However, a post-establishment study recorded tilapia sightings in the natural waters of this lake. The cultured tilapia was easily recognizable with the elongated mouth and body, and long caudal fin. It is postulated that these were escapees from the floating cages that had invaded the natural waters and would negatively impact the native fish species. To test our hypothesis, we investigated the impact of the aquaculture facility on native fish diversity through a spatial design. Methods: The study was focused on assessing the impact of tilapia culture at sites nearer to the AIZ vs more distant sites, the former with a greater likelihood of receiving escapees. Two major sites were chosen; within 5 km (near-cage) and within 5–15 km (far-cage) radii from the AIZ. Fish sampling was conducted using multiple mesh sizes of gill nets (3.7, 5.1, 6.5, 7.6, and 10.2 cm) deployed at the littoral zone of the sampling points. Species diversity, abundance, dietary habits, and habitat preference were investigated. Results: The CPUE (individual/hour) of native fish species at the far-cage site of the AIZ Reservoir was found to be significantly higher (p < 0.05) than that at the near-cage site. Principal component analysis (PCA) based on diet and habitat preferences showed that the tilapia, O. niloticus had almost overlapping diet resources and habitat with native fish species. Conclusion: We conclude that there is a correlation between the reduced catches of native species (based on CPUE) and the high presence of tilapia. Thus, appropriate actions must be implemented for strategic and effective planning in terms of native fish conservation.

With a surface area of 15, 200 ha, the Temengor Reservoir, in the northwestern state of Perak was selected among other lakes in Malaysia for the development of a large tilapia aquaculture facility in 2008 due to its favourable environmental conditions. Tilapia has never been recorded at Temengor Reservoir prior to the establishment of this facility. However, preliminary observations in a previous study detected tilapia species in the natural waters of this lake, strongly suggesting these were escapees from the floating cages which had invaded the natural waters. Following that, a study was conducted to assess the distribution of these escapees throughout the Temengor Reservoir. The cultured tilapias are easily recognizable with elongated mouth and body, and long caudal fin. The sampling was conducted using gill nets. The distribution of the escaped tilapia throughout the Temengor Reservoir was mapped based on previous data and current data on the fish checklist conducted at different sampling points of this lake, and the occurrence (presence/absence) of escaped tilapia was recorded. A comparative analysis was conducted at several number of points among three sampling areas, according to the radii from the fish cage facility; 15km (S3). The ANOVA showed significant differences (p<0.05) in Catch per Unit Effort (CPUE) weight between the three sampling areas. As tilapia is a highly successful global invasive species, the presence of tilapia at these and other locations at Temengor Reservoir should be rigorously monitored. Complete eradication of tilapia in the lake is of high urgency. We recommend intensive targeted fishing of the species in the vicinity of the cages and biological control by native predators to preserve and conserve the native fish species at Temengor Reservoir.