Species diversity of gillnet catches along the Egyptian Mediterranean coast of Alexandria

The present work deals for the first time with the species diversity of gillnets operated on the fisheries coasts of Alexandria City from El-Montazah to Sidi Kirayr. A total of 6161 specimens of about 427 kg were collected during periods from Feb 2018 to Jun 2018 and from Dec 2019 to Nov 2020 using two types of gillnets: the demersal and the pelagic gillnets. All samples were identified and photographed. A total of 108 species were identified in the catch of gillnets (99 fish species; 1 molluscan species and 8 crustacean species). From that, 28 lessepsian species were recognized. The catch was dominated by Family Sparidae (25 species) and Family Scombridae (8 species). The results showed that the species and size composition of gillnets catch were depending on the type of the gillnet and its mesh size. The catch of the demersal gillnets represented 63.27% of the total production by number and 84.72% by weight of the total catch of gillnets, while the catch of pelagic gillnets represented 36.73% and 15.28% by number and weight of the total catch, respectively. Eighty nine fish species were caught by the demersal gillnets and 36 fish species were caught by pelagic gillnets. Diplodus vulgaris was the main species caught by the demersal gillnets and Boops boops and Sardinella aurita were the main species caught by the pelagic gillnets. On the other hand, according to mesh number, which is defined as numbers of meshes in half-meter length of net, the gillnets with mesh number 17–18 followed by gillnets with mesh number 20 were the main meshes for the demersal gillnets. As for the pelagic gillnets, they were restricted to mesh number 33–34. In addition, only mesh numbers 11, 14, 17–18, 20 and 25–26 (demersal gillnets) and mesh numbers 33–34 (pelagic gillnets) were operated all over the year. Only 7 fish species from the target production were represented in all seasons namely: D. vulgaris, B. boops, Diplodus levantinus, Diplodus sargus, Umbrina cirrosa, Lithognathus mormyrus, and Trachurus mediterraneus. The seasonal diversity index of fish species was H' = 1. 3652 and the large numbers of fish species recorded were in spring (61 species) followed by autumn (58 species), summer (47 species), and winter (27 species). Further biological studies are needed to identify the most dominant species caught by the gillnets operated in Alexandria.