Analyses of length-weight, length-length relationships, and condition factor of Frigate tuna, Auxis thazard, collected from coastal fishing boats: a study on Negombo fishery harbour, west coast of Sri Lanka

Auxis thazard (Frigate tuna) is one of the commercially important neritic tuna species found in Sri Lankan waters. Even though frigate tuna contributes a higher percentage than the total tuna production income of the country, less information is available related to the biometric parameters due to the lack of studies based on this migratory species. Therefore, this study mainly focuses on length-weight relationship (LWR), length-length relationship (LLR) and the condition factor of A. thazard focusing mainly on the Western coast of Sri Lanka. The study was carried out from July 2021 to December 2021 collecting 214 A. thazard fish from the fishermen. Based on the results, the LWR was W= 0.0038TL^3.3889. The pooled ‘b’ value 3.3889 indicated positive allometric growth (b > 3, t-test, p < 0.05) confirming A. thazard in the West Coast are in a suitable, healthy environment. The K value 1.51 ± 0.17 indicated relatively healthy condition in fish. The LLRs were highly correlated (r² > 0.951; P < 0.001). The FishBase reports length at first maturity as 29.5 cm. The percentage below 29.5 cm included 17.29%. Therefore, continuous monitoring of fish landings is recommended for a sustainable fishery.