<![CDATA[Kappaphycus alvarezii is one of the leading commodities in the aquaculture sector due to its potential as a source of carrageenan. This study aimed to analyze the carrageenan content of Kappaphycus alvarezii and to determine the optimum harvest age for obtaining the highest carrageenan yield. The study employed an experimental method using a Completely Randomized Design (CRD) consisting of six harvest-age treatments: P1 (20 days), P2 (25 days), P3 (30 days), P4 (35 days), P5 (40 days), and P6 (45 days), with three replications for each treatment during a 45-day cultivation period. The observed parameters included absolute weight, specific growth rate (SGR), carrageenan yield, tissue histology, and water quality. The results showed that all treatments exhibited increased growth. The highest absolute weight was recorded in the 40-day harvest treatment (P5), reaching 1012 ± 138.32 g, while the highest specific growth rate (SGR) was also observed in P5 at 6.93 ± 0.12% day⁻¹. The highest carrageenan yield of the basal thallus of Kappaphycus alvarezii was obtained in the 20-day harvest treatment (P1), reaching 38.23 ± 8.22%. Histological observations revealed progressive tissue development in the basal thallus with increasing harvest age, characterized by the continued development of the medullary tissue. Throughout the study, water quality parameters remained within the optimal range for the growth of Kappaphycus alvarezii. Based on the results, it can be concluded that different harvest ages had a significant effect (p < 0.05) on the carrageenan yield of the basal thallus of Kappaphycus alvarezii. The average carrageenan yield ranged from 24.25 ± 5.81% to 38.23 ± 8.22%, with the highest yield obtained at the 20-day harvest age. ]]>
