Study of the gill net selectivity of Longtail tuna, Thunnus tonggol (Bleeker, 1851) using body-length relationship in the Oman Sea (Sistan and Baluchistan coastal waters)

Longtail tuna (Thunnus tonggol) is an important commercial species. This species is often caught in the Oman Sea by gill net. This study was conducted to study the selectivity of longtail tuna gill net based on the length-body girth relationship from January to July 2019 monthly in Bryce, Ramin, and Konarak sampling areas. For this purpose, the caught fish were collected and the data of Longtail tuna was measured from the eye, pre-operculum, operculum, and maximum body height at each stage of sampling. Then, the net selection curve and the optimal length of the longtail tuna were calculated by the gill net selectivity model based on the net mesh size. From the results, the maximum fish length was estimated at 40, 50, 56, 65, 68, and 80 cm for the net mesh sizes of 10.1, 11.4, 13.2, 14.6, 15.2 and 17.8 cm, respectively. According to Lm50 of longtail tuna at 73.3 cm fork length, the fish caught with mesh size of 17.8 cm has a fork length greater than the average sexual maturity length of the fish. Therefore, taking into account the catch of the target species, T. tonggol, the results showed that mesh size of 10.1, 11.4, 13.2, 14.6, 15.2 cm are not suitable for catching T. tonggol in the Oman Sea. The use of 17.8 cm mesh size with the purpose of catching adult T. tonggol would be suggested for sustainable and responsible fishing for this species.