Some biological aspects of Auxis thazard (Lacepède, 1800) in the Persian Gulf and Oman Sea (Hormozgan Province)

Abstract

Introduction: Auxis thazard is one of the pelagic fish with high economic importance and wide distribution in the waters of the Persian Gulf and Sea of Oman. Materials & methods: In this study, the reproductive and feeding habits of this fish that were collected in 5 main landing centers in Hormozgan province were investigated during a period of 12 months from January 2022 to December 2022. During this period, the biometric information of 399 frigate tuna was recorded. Results: The fork length of the females ranged between 24.1-38.5 cm (with an average of 30.1±0.14 cm) and their weight was between 0.26-0.95 kg (with an average of 0.46±0.006 kg) and in the males, ranged between 25-39 cm (with an average of 31.3±0.33 cm) and their weight was between 1.1-0.28 kg (with an average of 0.51±0.017 kg). The fork length and weight relationship for females and males frigate tuna was w=0.00005(FL)2.68 and w=0.00004(FL)2.75 respectively. The sex ratio of female: male was significantly 1:0.4 (P<0.05). Also, the sex ratio in different length classes showed that there was a predominance of females, although the frequency of males in length classes over 34 cm was significant so the sex ratios were observed equal in these classes. According to the range of ovarian maturation stages of female frigate tuna, two peaks of spawning were observed in August and January. Examining the trend of changes in GSI and HSI indexes also showed that the highest amount of these indices was in the months of August (8.15) and October (3.91), respectively. The minimum length at which the female frigate tuna reaches sexual maturity was 24 cm in the fork length, and the length at which 50% of the females reached sexual maturity was estimated at 29.48 cm in the fork length. Examining the contents of the stomach showed that the frigate tuna widely feeds on small fish, crustaceans and molluscs. The CV index showed gluttony and a high desire to feed the frigate tuna. Also, the detailed examination of the FP index showed that the Encrasicholina punctifer was assigned as the dominant prey with the highest abundance in the diet of frigate tuna at 46%. Conclusion: Frigate tuna has two peaks of spawning in August and January, and its females are more dominant in the region. This species has a carnivorous diet with a preference for Encrasicholina punctifer.