Impacts of tilapia aquaculture on native fish diversity at an ecologically important reservoir.

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.

Seasonal benthic species composition linked to coastal defense structures (CDS) in Kuala Nerus, Terengganu, Malaysia.

Background: The natural hydrodynamic process of Kuala Nerus, Terengganu, has changed since the extension of Sultan Mahmud Airport runway in 2008. Consequently, severe coastal erosion has occurred in the area, particularly during the northeast monsoon season (NEM). Numerous types of coastal defense structures (CDS) have been constructed to protect the coastline. Despite the loss of esthetic values, the effect of CDS construction on marine organisms in the area remains unknown. Hence, this study aims to assess the ecological aspects of macrobenthic compositions at the CDS area of Kuala Nerus, Terengganu, based on the differences between the southwest (SWM) and northeast (NEM) monsoon seasons. Methods: Macrobenthos were collected from the sediment in July (SWM) and December 2021 (NEM) using the Ponar grab at 12 substations from five sampling stations. Results: The density of macrobenthos was higher in SWM (48,190.82 ind./m2) than in NEM (24,504.83 ind./m2), with phylum Mollusca recording the highest species composition (60-99.3%). The macrobenthos species had a low to moderate level of diversity (H' = 1.4-3.1) with the species were almost evenly distributed (J' = 0.2-0.8). Windward substations exhibited coarser grain sizes (38.56%-86.84%), whereas landward substations exhibited very fine grain sizes (44.26%-86.70%). The SWM season recorded a higher organic matter content (1.6%-6.33%) than the NEM season (0.4%-3.1%). However, metal concentrations in the surface sediment were within the safe range and permissible limits for both seasons, inferring that the macrobenthos composition was unaffected. Discussion: This study demonstrated that the CDS associated with the monsoon system has controlled the hydrodynamics and nearshore sedimentary processes in the Kuala Nerus coastal zone, thereby affecting the macrobenthos population, in terms of richness and density. The ecological and energetic effects of the coastal structures in different seasons have resulted in a more significant result, with the SWM exhibiting a higher macrobenthos composition than the NEM.