Spatio-temporal dynamics and population structure of swordfish (Xiphias gladius) in Kenya's EEZ

Swordfish (Xiphias gladius Linnaeus, 1758) play a critical role in the economy and ecosystem of the western Indian Ocean; however, limited information exists on their biology and fishery dynamics within Kenya’s Exclusive Economic Zone (EEZ). This study integrates fisheries-independent and fisheries-dependent data collected from 2016 to 2025 to assess size structure, maturity, sex ratios, and the relationship between swordfish abundance and fishing activity in the Kenyan EEZ. The population is predominantly composed of immature fish, with most individuals measuring between 80 and 120 cm lower jaw fork length (LJFL) across both datasets. Approximately 93% of observer-sampled swordfish were below the maturity threshold of 163 cm LJFL, indicating that immature and sub-adult individuals dominate the catch. Maturity modeling revealed sex-specific differences: males attain 50% maturity at approximately 156 cm LJFL, while females do so at 172 cm LJFL. Weight–length analysis demonstrated positive allometric growth, and sex-ratio analysis showed male dominance in smaller size classes, with females prevailing at larger sizes, reflecting differences in growth and maturation rates.

Spatial analysis identified the western coastal waters west of 41°E, particularly during the Southeast Monsoon (June–August), as critical high-abundance areas for immature swordfish, likely associated with monsoon-driven productivity and upwelling. Substantial spatial overlap between intensive fishing activity and immature aggregations during this period raises serious concerns over the overexploitation of pre-reproductive individuals. During the Northeast Monsoon, reduced fishing effort provides a natural seasonal refuge that may support stock recovery. Collectively, these findings underscore the Kenyan EEZ’s significance as a core recruitment and aggregation area for swordfish in the western Indian Ocean. The results highlight an urgent need for targeted management interventions, including seasonal closures and minimum size limits, alongside enhanced monitoring to ensure the long-term sustainability of swordfish populations.