Gillnet selectivity of longtail tuna based on morphometric data in Iranina coastal waters in Persian Gulf and Oman Sea

In this study, the size selectivity of long-tail tuna (Thunnus tonggol) was examined for the gillnets of 101, 140 and 132 mm using Sechin model (1969) in Iranian coastal waters of Persian Gulf and Oman sea during the years of 2014-2015. To this aim, the transverse morphometric data for girths in three positions of the fish body such as Opercular girth (OP), girth at the beginning of first dorsal fin (D1) and girth at the beginning of first dorsal fin (D2) as well as fork length were collected by mesh size. Results from Analysis of covariance (ANCOVA) showed that girth at D1 is considered as the maximum girth of the species (SNK: P <0.05). Fish being snagged, depending on the mesh size, were constituted the considerable amount of the catches in the nets depending on the mesh size. Unlike the 114 mm nets, in mesh sizes of 101 and 132 mm the selectivity curves were not fitted to the length frequency distribution, owning to the fish being snagged by larger sizes. According to the selectivity curve, the optimal length of T. tonggol for the nets of 101, 114, and 132 mm was obtained as 41, 50 and 55 cm, respectively. Depending on the mesh size more than 80% of catched juvenile fish are Immature. As well as of target species, bycatch recorded is 26% of the total catches, which is expected to have detrimental effect on the environments. To obtain accurate results, it needs to determine later the size selectivity of long-tail tuna from the data taken from the field experiments of several mesh sizes.

Keywords:

Persian Gulf , Oman Sea , Gillnet , Thunnus tonggol , Optimal length , by-catch