Morphometric and Reproductive Trends Reveal Near-Equal Sex Ratio, Positive Allometric Growth, and Early Maturity of Euthynnus affinis in Southern Philippine Waters

Ichthyometric indices and gonadosomatic dynamics provide essential insights for fisheries management and policy development, emphasizing their role in sustainable resource utilization. Our study presents updated information on the sex ratio, length-weight relationship, and length at first maturity of Euthynnus affinis in Southern Philippine waters: Celebes Sea, Moro Gulf, Sarangani Bay, and Davao Gulf. A total of 1,314 Euthynnus affinis specimens ranging from 14.00 cm to 64.70 cm collected using handline, purse seine, and ring net, were analyzed from multiple fishing ports across these four study sites. We sampled the E. affinis population which exhibited a near-proportional distribution of sexes, with an overall ratio of 1:0.59. A higher prevalence of males was observed, with a calculated male-to-female ratio of 1:0.59 (p-value < 0.05, χ² = 4.36), indicating a significant deviation from an expected 1:1 ratio. The relationship between fork length (FL, cm) and weight (W, g) was expressed as W = 0.0104 FL3.1483 for males and W = 0.0137 FL3.0639 for females. The length-weight analysis revealed a positive allometric growth pattern, indicating that E. affinis becomes rotund deeper-bodied as it increases in length. The correlation coefficients (R²) were high, at 0.9760 for males and 0.9577 for females, confirming a strong relationship between length and weight. The b-values exceeding 3 further support a positive allometric growth pattern, suggesting that the species grows proportionally more in weight than in length. The length at 50% sexual maturity (Lm50) was estimated at 37.18 cm, while 95% of the population (Lm95) reached maturity at 45.36 cm. For females, Lm50 and Lm95 were 35.19 cm and 41.48 cm, respectively, whereas males reached maturity at 38.43 cm (Lm50) and 49.90 cm (Lm95). We present these findings that highlight early sexual maturation in E. affinis, underscoring the need for targeted management strategies to ensure the sustainability of its populations. Our paper reveals the near-equal sex ratio, positive allometric growth, and early maturity of Euthynnus affinis, highlighting the need for science-based fisheries management to sustain population stability. We propose establishing minimum catch size limits above 37.18 cm (Lm50) to allow individuals to reach reproductive maturity and above 45.36 cm (Lm95) to ensure that most individuals have spawned at least once before harvest. Additional measures, including protecting spawning stocks, implementing seasonal fishing closures, and continuously monitoring population trends, can help prevent overfishing and ensure long-term resource availability. Thus, localized management strategies and adaptive policies are essential to account for environmental changes and optimize economic benefits for fishers.