Predictive modeling of body weight from body length in koi fish (Cyprinus carpio var. koi) cultivated in Indonesia

Koi fish (Cyprinus carpio var. koi) is a highly valued ornamental species, renowned for its diverse coloration and distinctive body patterns. Although koi aquaculture in Indonesia continues to expand, scientific studies on their morphological characteristics remain limited, considering their economic and aesthetic importance. Body weight is a critical parameter for evaluating growth performance and production value. However, there is no quick or non-destructive method to measure it accurately. Breeders traditionally rely on body length as the primary indicator of growth, yet the ability to estimate body weight from length would provide a practical alternative. This study aimed to develop a predictive model of body weight based on body length in koi cultivated in Indonesia. A total of 112 koi individuals aged 4–10 months, representing six varieties (Kohaku, Showa, Sanke, Shiro, Platinum, and Matsuba), were measured. Body length ranged from 11.27 to 20.12 cm, corresponding to the typical size of koi commonly found in the market for early selection. Length was measured using a caliper from the head to the caudal peduncle, while body weight was determined with a digital scale. Linear regression analysis revealed a strong, significant relationship between body length and body weight, with a correlation coefficient (R) of 0.880 and a coefficient of determination (R2) of 0.774, indicating that 77.4% of the variation in body weight is explained by body length. The regression equation obtained was Y = -142.663 + 13.874 × Length, with a standard error of 17.090. This model is reliable for koi with body lengths of 11.27–20.12 cm, demonstrating that body length is a practical and accurate predictor of body weight. These findings provide a valuable tool for selection, feeding management, and growth evaluation in koi aquaculture.