Catch composition and species diversity during dredging operations of Mediterranean coastal lagoon, Lake Manzala, Egypt

Lake Manzala, the largest Egyptian coastal lake, is one of the most valuable fish sources in Egypt, which contributes about 14% to the total annual Egyptian fisheries production. However, given that Lake Manzala suffers from major environmental and fisheries problems, an ambitious national project for the dredging, purification, and development of this lake was established in 2017. Hence, this study was conducted to assess seasonal diversity and to provide an inventory of fish species at Lake Manzala during dredging operations. Five seasonal surveys were conducted during the period from 2019 to 2021. By using different fishing gears and methods operating in the lake, a total of 7774 fish and crustaceans—belonging to 29 fish species and three crustacean species that clump into 21 families—were collected from Lake Manzala. During the study period, the family Cichlidae showed the highest occurrence of about 70%. The marine-brackish species recorded the highest aquatic environment (milieu) diversity with 34.4% of the collected species, but only 12.5% of the freshwater species. Six of the collected fish species were recorded of Indo-pacific origin, as Lissepsian migrant species. In this study, the diversity index (H) was calculated, ranging between 1.13 and 1.77. As indicated by the seasonal variation of the diversity index, the highest diversity index of Lake Manzala was evident in autumn 2019 and spring 2021 at 1.700 and 1.365, respectively. The governmental dredging and cleaning operations in the lake have a relative impact on the species composition in this area. Further studies are recommended after the completion of these operations to confirm the impact of dredging on species composition alteration.