Accurate fisheries acoustic surveys depend on species-specific target strength (TS) references. However, there are limited TS references for tropical reef-associated species. This study established TS–total length (TL) relationships for two commercially demersal fishes in Indonesian waters: Lutjanus gibbus (Humpback red snapper) and Cephalopholis formosa (Bluelined grouper). Using a Simrad EK-15 single-beam echosounder (200 kHz), TS measurements were conducted under laboratory conditions at two pulse durations (0.08 and 0.16ms) to assess methodological impacts. Both species of L. gibbus (TL: 17.4–42.1cm) and C. formosa (TL: 22.6–31.4cm) exhibited negative allometric growth (b < 3), indicating that body mass increases more slowly than length. L. gibbus showed a significant TS and TL relationships: TS=20.26·log(TL)-68.71 for 0.08 ms (R²=0.841) and TS=19.88·log(TL)-72.12 for 0.16 ms (R²=0.787). In contrast, C. formosa exhibited no significant TS-TL relationship: TS=24.64·log(TL)-73.51 for 0.08 ms (R²=0.370) and TS=24.65·log(TL)-77.65 for 0.16 ms (R²=0.329). Within adjusted regression, longer pulse durations reduced mean TS values by 3.94 dB (L. gibbus) and 4.13 dB (C. formosa), underscoring the need for standardized protocols in shallow-water acoustics. While robust TS models for L. gibbus enhance biomass estimation accuracy, the weak TS-TL linkage in C. formosa highlights the influence of biological variability and limited size ranges. This work advances sustainable fisheries management by providing foundational TS data for understudied species and quantifying pulse duration effects critical to survey design.