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Sri Lanka’s billfish fishery targets high-value pelagic species like swordfish, marlin, and sailfish, primarily as by-catch in tuna fisheries using longlines and, to a lesser extent, gillnets. Billfish catches peak seasonally, influenced by monsoon-driven migrations closer to the coast. While the catch serves local and export markets, Sri Lanka must comply with the Indian Ocean Tuna Commission (IOTC) requirements, including reporting size frequency data (Resolution 15/02). Data collection involves measuring length types such as fork length (FL) or lower jaw-fork length (LJFL). Challenges arise when billfish are sectioned at sea, complicating accurate data collection during port sampling. Therefore, an alternative method to establish length-length relationships is proposed to improve data accuracy and fulfill IOTC standards despite sectioned landings. An attempt made to drive these relationships using the measurements taken from various sources of Black marlin, Blue Marlin and sailfish. Results indicated that there are significant linear relationships of different length and girth measurements of Black marline, Blue Marline and Sailfish laded by Sri Lankan Fishermen. Therefore, it is recommended to use Length from base of the anal fin to the base of the caudal lobe and Girth measurement via beginning of 1st anal fin to generate the Upper jaw-total length and eye orbit fork length in the case of availability of the part of the these three types of fish species in order to use these length details for the management purposes.
