Effects of stomach content and gonad mass on length–weight relationships and condition indices in fish species with different body shapes

Length–weight relationships (LWRs) are widely used in fisheries science to describe growth patterns and assess fish condition, yet they are based on total body weight, which includes variable non-structural components such as stomach contents and gonads. The effects of these components on LWR parameters and condition indices remain poorly understood. This study evaluated the influence of stomach fullness and gonad mass on LWRs and Fulton’s condition factor (K) in three fish species with different body shapes: Diplodus annularis, Boops boops, and Citharus linguatula. A total of 159 individuals were analyzed using three weight definitions: total body weight (TW), gutted weight (F), and gonad-adjusted weight (TWG) estimated from the literature-based gonadosomatic index (GSI) values. LWR parameters and K values were statistically compared among weight types. The allometric coefficient (b) did not differ significantly among weight definitions, indicating that internal biological components had little effect on estimated growth patterns. In contrast, body weight and Fulton’s K differed significantly (p < 0.05), with significant differences detected only in D. annularis. Sensitivity analysis identified GSI thresholds (~5% and ~13%) at which gonad mass affects weight-based comparisons, emphasizing the importance of standardized weight definitions in condition assessments.