Lake Mahalona is located in Towuti District, South Sulawesi Province. There are 29 species of fish found in Lake Mahalona and most of them are endemic fish. The presence of alien species deliberately or unexpected is one of threats causing the declining of the number of biodiversity in fresh waters. This study aims to determine the relative abundance of alien fish species Flowerhorn in its fishing ground area at Lake Mahalona. The research was conducted from January to June 2021 at six stations. Fish samples were obtained by using gill nets. The total Flowerhorn caught during the study were 15,677, the most caught in January was 3,346 and the least in June was 1,451. The Flowerhorn in January were the most number of fish at Kusapa (735), February and March at Muara (566 and 618), April at Tandumata (593), May and June at Badilla (434 and 375). The highest number and relative abundance of Flowerhorn fish were found at Kusapa in January (735 and 33.23%), while the lowest number and relative abundance of Flowerhorn fish were found at Lengkomali in June (129 and 4.62%). Flowerhorn as a alien fish species is distributed throughout the Lake Mahalona station. Flowerhorn fish occupy a lot of habitats with lots of aquatic plants, sandy and rocky substrates.

Lake Mahalona is one of the lakes located in the Malili Complex, South Sulawesi and is included in the Natural Tourism Park Area. Cichlasoma trimaculatum are ornamental fish from the Cichlidae family that become invasive in Lake Mahalona. A total of 29 fish species were recorded in this lake, and most of them are endemic fish. It is estimated that native and endemic fish species and populations will decline due to invasive alien species intentionally or unintentionally in Lake Mahalona. The study to determine the size distribution and length-weight relationship of C. trimaculatum was conducted in Mahalona Lake. The study was conducted at 11 stations in October 2020. Experimental gill nets used to catch Bilih. The total length of C. trimaculatum ranged from 10.0 ± 1.57 cm (Kusapa and Badilla stations) to 18.0 ± 1.57 cm (Benu station), and the weight ranged from 19.0 ± 19.2 g (Kusapa station) to 140.0 ± 19.2 g (Muara station). The correlation coefficient (r) value for this fish ranges from 0.8685 - 0.9691. The growth pattern of C. trimaculatum was a negative allometric characteristic. An exception in Muara dan Tandu Mata stations was positive allometric.