Banggai cardinalfish (Pterapogon kauderni) is an endangered, range-restricted apogonid endemic to the Banggai Archipelago (Banggai Islands), Indonesia, and a flagship species of the marine ornamental fish trade. Aquarium-mediated releases have established non-native populations, including in Lembeh Strait, North Sulawesi. This study compared population density, size structure, and microhabitat use of P. kauderni between introduced and native ranges using underwater visual census along 50-m x 5-m belt transects in Lembeh Strait (March 2021) and in the Banggai Islands (October 2021). Individuals were counted, assigned to three size classes (recruit, 1.8 cm, SL; juvenile, 1.8-3.5 cm, SL; adult, > 3.5 cm, SL), and recorded with associated protective microhabitats: branching coral, Diadema sea urchins, sea anemones, and seagrass. A total of 3,997 individuals from 45 colonies were recorded (42 with recorded microhabitat). Mean density was 2.00±0.62 ind m-2, with no significant difference between introduced and native ranges (Mann–Whitney U = 10.0, p = 0.69). Populations were adult-dominated overall (73.7%), while pooled descriptive proportions showed more juveniles in Lembeh and more adults in Banggai; however, station-level comparisons detected no significant regional differences. Coral and Diadema were the most frequent colony microhabitats, and 88.2% of individuals were associated with Diadema. Colonies with Diadema contained significantly more individuals (p = 0.005), and abundance increased with the number of protective microhabitats present (Spearman ρ = 0.34, p = 0.028). An exploratory colony-level comparison showed higher median abundance in colonies with Diadema rather than in colonies with none, but this result was interpreted cautiously because multiple colonies occured within the same stations. These findings provide evidence for habitat-based monitoring and management of native and introduced populations in Indonesian waters.
