Fishery Biology and Population Structure of the Blue Swimmer Crab, Portunus pelagicus, from the Red Sea, Egypt

Abstract

More than 300 MT of blue swimmer crabs (BSC), Portunus pelagicus, are annually captured from the fisheries of the Egyptian Red Sea waters. During the last 20 years, a decline has been witnessed in the production of this species. For management purposes, information about the fishery's biology and population structure is required. Hence, monthly samples of P. pelagicus were collected from the commercial catch of Hurghada landing site, Red Sea, Egypt during the period from November 2017 to October 2018. A number of 1190 specimens were measured for morphometric relationships, sex ratio, age determination using carapace width frequency, population structure, mortality rate and exploitation ratio. The study revealed that P. plagicus was targeted by trammel nets and caught by a small bottom trawler as a by-catch. Carapace width- total weight relationship showed positive allometric growth (b-value= 3.10). Higher condition factors were observed in the small individuals. Four age groups were determined and age group +I was dominant by number (60.6%). Parameters of the von Bertalanffy equation were 21.19 cm, 0.414 year-1, and -0.998 year for CW∞, K and to, respectively. The carapace width at first capture was estimated (CW50 = 8.7 mm). The mortality rates were calculated as 2.929, 1.285 and 1.644 year-1 for total, natural and fishing mortalities, respectively. The blue swimmer crab fisheries on the Red Sea is over-exploited (E= 0.65) by the trammel net. Some management measures were suggested to maintain its stock for sustainability.