Determining gill-net selectivity for longtail tuna (Thunnus tonggol Bleeker, 1851) using artisanal fishery data in the Iranian waters of the Oman Sea

Using the catch data obtained from local fisheries fleet, length frequency, length-weight relationship and multifilament drift gillnet selectivity of longtail tuna were determined in Iranian waters of the Oman Sea (Chabahar), Iran. Multifilament drift gillnets with 100, 110, 130 and 165 mm stretched mesh size were utilized in fishing operations. For each species, fork length, total weight, head girth and maximum girth were measured. A total of 477 specimen were collected ranging from 32 to 90 cm fork length and mean length was recorded to be 55.15±0.61 cm. Head girth-fork length and maximum girth-length relationship were obtained as: Gh = 0.5379 FL + 0.4955 and Gmax = 0.5879 FL + 0.5095, respectively. All girth-length regressions were found to be linear. Using Sechin method, optimal catch size of 100, 110, 130 and 165 gillnet mesh sizes were determined as: 35, 38, 46 and 57 cm, respectively. Moreover, selection coefficient (K) was calculated 0.285 (100 mm mesh size), 0.289 (110 mm mesh size), 0.282 (130 mm mesh size) and 0.289 (165 mm mesh size) for longtail tuna gillnets. In this study, the selectivity curves were the shape of normal distribution and narrow.