Further observations on the use of length conversion equations to address length data deficiencies for swordfish, Indo-Pacific sailfish, black marlin and blue marlin in the Indian Ocean using images

Accurate length data are essential for billfish stock assessments, yet obtaining standard morphometric measurements is challenging in Sri Lanka due to common dressing practices that remove key body parts before landing. This study evaluated the use of Pectoral–Second Dorsal Length (PDL) and Pectoral–Anus Length (PAL) as proxies for Lower Jaw Fork Length (LJFL) in swordfish (Xiphias gladius), Indo-Pacific sailfish (Istiophorus platypterus), black marlin (Istiompax indica), and blue marlin (Makaira nigricans). Morphometric data were collected from artisanal and semi-industrial multi-day fishing vessels using field measurements and image-based analysis. Results showed strong linear relationships between LJFL and both PDL and PAL for swordfish, sailfish, and black marlin (R² > 0.85), confirming their reliability for length estimation. In contrast, blue marlin exhibited weaker correlations, with PDL moderately related to LJFL (R² = 0.73) and PAL poorly correlated (R² = 0.46), indicating higher variability in body proportions. These findings demonstrate that PDL and PAL provide robust conversion tools for most billfish species in the Indian Ocean, but highlight the need for additional sampling and refinement before their application to blue marlin. Species-specific validation is therefore essential when applying morphometric proxies to address data deficiencies in billfish fisheries.