Some biological aspects and molecular variations in frigate tuna, Auxis thazard of the coastal waters around Sri Lanka

Abstract

Auxis thazard (frigate tuna), is a commercially important fish species in Sri Lanka. Although Sri Lanka is fortunate to have a large resource of tuna, little information is available on the biology, genetics and stock structure of A. thazard. Hence, the present study was conducted to study the biology, genetics and stock structure of A. thazard in coastal waters around Sri Lanka. The fish samples were collected from October 2015 to September 2017 from day-boats operating in the Western, North Western, Eastern and Southern Provinces of Sri Lanka. For this period, the length-weight relationship and Fulton’s condition factor (K) calculated for A. thazard using standard length measured in centimetres and weight measured in grams were shown as W = 0.1091L 3.3385 and 1.93, respectively, indicating a relatively healthy growth pattern and a comparatively unpolluted habitat. Studies on the stomach contents revealed that A. thazard are non-selective carnivors, feeding on diverse animal prey items such as small fish species, shrimps and cephalopods available in the surrounding waters. The fecundity of female A. thazard was shown to be 48,056 to 267,000 eggs. The calculated GSI values showed that the peak spawning period for male A. thazard extends from May to August and for the females from May to July. The phylogenetic analysis of the mitochondrial D-loop region sequences of 75 selected samples representing all the geographical regions studied showed that the fish of different regions are clustering together. Hence, for fisheries management strategies, A. thazard found in the coastal waters of the Western, North Western, Eastern and Southern Provinces of Sri Lanka could be considered as a single stock.