Determination of gill-net selectivity for Narrow-barred Spanish mackerel (Scomberomorus commerson) using morphometric parameters in the coast of Hormozgan province

Abstract

In this research, size selectivity for narrow-barred Spanish mackerel (Scomberomorus commerson) and other species such as Indo-Pacific king mackerel (Scomberomorus guttatus), Longtail tuna (Thunnus tonggol),Kawakawa (Euthynnus affinis), Talang queenfish (Scomberoides commersonnianus) and John's snapper (Lutjanus johnii) were investigated in mesh sizes of surface, mid-water and bottom drift nets in coastal waters of Bandar Abbas and in the western part of Hormozgan province (fishing ports of Gong and Bustaneh), as well as of catch composition from the fisheries. The data required for the relevant analysis was collected for the years 2012-2014. In order to determine the size selectivity of the investigated species, cumulative probability distributions of retention girth (operculum and maximum girth) at length was used. The results indicated that there is a linear relationship between length (fork length or total length) and transvers morphometric parameters in six positions of girth at fish species. Seven types of gillnets with stretched mesh sizes of 70, 76, 79, 101, 114, 132 and 165 mm and different technical characteristics were investigated for size selectivity of S. commerson, in which the optimal length were 38, 44, 49, 63, 74, 86 and 100 cm FL in the respective meshes. The capture processes of S. commerson was mostly in wedging in the all mesh sizes, whereas in the other species, depending on the morphology of fish, the retention method was different. The comparison of length frequency distribution of kingfish with mean length at maturity (Lm50%) indicated that, depending on mesh size, the frequency of the immature fish in the catch was between 4 and 100%. On the basis of optimal length at mesh size, the minimum legal mesh size of kingfish was estimated as 162 mm. Size selectivity curve was also represented for other species, which depending on fish morphology the model was inadequate in fitting the data. The results from the catch composition showed that the species diversity is different by fishing method. In mesh sizes of 70, 76 and 79 mm, the kingfish was captured in target and bycatch species depending on time by the surface drift gillnet fishery operated by the fishing boats. In this type of gillnet fishery, bycatch accounted for more than half of the total catch in which some of the species was important economically. Catch composition in gillnet fishery operated by dhows was different depending on fishing method. At the first step, it is necessary to impose a ban on gillnets with mesh sizes of 70, 76 and 79 mm in second half of the year. In the next step, for establishing the regulation on minimum legal mesh size of kingfish, it needs to do further study in seas trails aimed at determine the appropriate mesh size. Also, in the multispecies fishery, it is necessary to do a research for optimization of mesh sizes of gillnets and to implement minimum legal mesh size for the main target species.