The biological marine system in the Mediterranean Sea off Alexandria, Egypt, was investigated to recognise its biodiversity and the relations among "ichthyofauna, invertebrates, and benthic" cover including biota and flora, as well as seabed bathymetry during 2017 using a multi-seasonal surveys by the commercial bottom trawler. Moreover, zooplanktonic community from the water column was also collected to support the picture of the biodiversity in the investigated area. The identified species were 94 fishes, 64 invertebrates, 6 benthic flora, and 304 zooplanktonic species. The ichthyofauna included 5 Chondrichthyes species (5.3% of the fish species), while Osteichthyes fishes were 89 species (94.7%) belonging to 48 families and 72 genera. The most abundant family was Sparidae (13 species). The highest abundance of fishes occurred in the summer (68 fish species 72.34%), while the lowest abundance occurred in the spring (49 species, 52.13%). Regarding the demersal and benthic biota, the most abundant phylum was Mollusca (31 species) and represented by three classes (Bivalvia, Cephalopoda, and Gastropoda). Gastropoda was the most abundant class (18 species), while the lowest Phyla was Chordata (1 species of Ascidians) and Annelida (1 species). The number of lessepsian fish species were 17 (18.1%) of the total number of species caught by the bottom trawl net. In addition, this work provided new records Aulopareia unicolor (F): Gobiidae) for the area for first time and considered the second time in Egypt. The benthic flora was represented by 6 species belonging to three phyla (Tracheophyta, Chlorophyta, and Rhodophyta). Sea grasses were represented by three species (Posidonia oceanica, Cymodocea nodosa, and Halophila stipulacea). The highest abundance of benthic species occurred in the summer (53 species with 75.7%), while the lowest one was in autumn (27 species, 38.6%). Geologically, the fishing ground constituted of hard rocks to very fine silt. The eastern part of the study area includes terrigenous Nile sediment origin, while the western side has biocalcareous sediment with shell fragments richness, coastal limestone ridges origin. The continental shelf, which runs along the study area, is portrayed by a 200-m contour line. In the water column, zooplanktonic community was represented by 304 taxa, belonging to 12 phyla, 6 phyla (Arthropoda, Tintinnida, Chordata "fish eggs and larvae", Cnidaria, Foraminifera, and Radiozoa) were dominant. Copepods were the dominant group (71.59%); its annual average abundance was 1271 ind./m3. Its most diversified season was the winter (175 No/m3.) and its average abundance was 1892.9 ind./m3. However, in spring, 118 species were recorded presenting the highest average abundance (2419.4 ind./m3). The lowest diversified season was summer (85 organisms) with density of 1150 ind./m3. The present work offers updated data regarding the marine biodiversity in Egypt, enriches the gaps in the bibliography in the Eastern Mediterranean, and gives preliminary list of species and biodiversity of bottom trawl combined with the interaction with other biosystems and features of fishing ground. These data could be used to monitor evaluate the impact of bottom trawl on the fisheries habitats and changing in ecosystems. Also, it could be used as constructive step to manage or protect such area in combination with other. It is recommended to fulfil the need for more and detailed studies in all areas by different gears to cover the gaps in marine biodiversity data.
Keywords: Biodiversity; Bottom trawl; Continental slope; Eastern Mediterranean; Egypt; Zooplankton.