Biodiversity and fisheries of the non-target catch from bottom trawl, off Alexandria, Mediterranean Sea, Egypt

•The high diversity indices were recorded in spring and summer.•A high percentage of recruits were captured before reaching their ripening stages.•22 Lessepsian species out of 89 collected ones were recorded.•The authors propose simple anterior trawl changes in cod end mesh size from 2.5 to 4.0 cm.

This article aimed to assess the species composition, seasonal distribution, length-weight relationships (LWRs); age composition and the non-indigenous species (Lessepsian species) of the non-target catch obtained by commercial bottom trawlers operated between Al Agamy and Abu Qir (west and east off Alexandria, Egypt). A total of 44 economic species and 45 non-economic species were recorded forming 51.2 % and 48.8 %, respectively of the total examined catch weight during the whole year 2014. The high diversity indices were recorded in spring and summer for economic species. The recruits of nine mostly important economic species in the region were recorded. Four species namely: Spicara maena; Pagellus acarne; Trachurus picturatus and Nemipterus japonicus had lengths between 8.5 and 10.9 cm appeared in spring while other five species namely: Pagellus erythrinus; Boops boops; Serranus hepatus; Spicara smaris and Mullus barbatus had lengths ranged between 7.00 and 11.00 cm appeared in summer. The exponent “b” of the length-weight relationship for 22 species varied between 2.1316 and 3.2415 for economic species and ranged between 2.6341 and 3.4406 for non-economic species. Age analysis showed that the life span of the majority of economic species is two years. The number of Lessepsian species recorded in this area was 22 species of which eleven were economic.