Analyses of biometric growth parameters, feeding ecology and fisheries aspects of Frigate Tuna (Auxis thazard), a Neritic Tuna species found off the east, west and south coasts of Sri Lanka

Abstract

Frigate tuna (Auxis thazard) is a commercially important neritic tuna species in Sri Lanka contributing to over 40% of the total neritic tuna catch of the country. Only a few studies have been conducted on biometric parameters of frigate tuna in Sri Lankan waters. Therefore, the present study was conducted to study the growth condition indices and fisheries aspects of A. thazard in selected coastal waters around Sri Lanka and the feeding ecology of A. thazard in the West coast of the country. The study was based on a primary data analysis where the data were collected from the West coast from November 2021 to February 2022 and a secondary data analysis in which the data were obtained from the Department of Fisheries and Aquatic Resources (DFAR) following the year 2017 for a comparative study off the Western, Southern and Eastern coasts of Sri Lanka. The Length–weight relationship (LWR) and Fulton’s condition factor (K) were estimated using the equations 𝑊 = 𝑎𝐿𝑏 and 𝐾=100𝑊/𝐿3 to assess the condition of the fish. The total weight ranged from 124.90 g -1405.05 g, 157.00 g -1475.00 g and 152.60 g - 890.40 g and further total lengths ranged from 23.8 cm - 44.2 cm, 21.5 cm - 46.5 cm and 25.5 cm - 44.1 cm for Western, Southern and Eastern coasts respectively. The LWRs were 𝑊= 0.0012𝑇𝐿3.6586, 𝑊 = 0.0026𝑇𝐿3.4846 and 𝑊 = 0.0147𝑇𝐿2.951, in which the pooled ‘b’ values 3.6586 and 3.4846 for the West and South coasts indicating positive allometric growth (b>3, t-test, p<0.05) and 2.951 for the East coast indicating isometric growth (b=3, t-test, p>0.05). Both male and female fish on each coast followed the same growth pattern without any difference. The K values 1.21 ± 0.42 and 1.43 ± 0.98 calculated for the West and South coasts indicated relatively healthy conditions and 0.94 ± 0.51 on the East coast indicated comparatively poor conditions. The gillnets accounted for 98% of catches on the West coast and ring nets accounted for 68% and 67% for South and East catches based on the total landings in 2017. In the primary data analysis, the estimated Relative Gut Length (RGL) confirmed the frigate tuna as a carnivore. The size classes of 30.0 - 31.50 cm and 23.50 - 24.50 cm had the highest and lowest feeding intensities based on the Gastro Somatic Index (GaSI). The most preferred prey item was shrimps. The Fishbase reports length at first maturity as 29.5 cm. The percentages below 29.5 cm include 18.83%, 39.18% and 33.99% for the West, South and East coasts, respectively. Therefore, continuous monitoring of fish landings and data collection is recommended for a sustainable fishery.