Allometry, condition factor and growth of the swimming blue crab Portunus segnis in the Gulf of Gabes, Southeastern Tunisia (Central Mediterranean)

Though the blue swimming crab Portunus segnis (Forskal, 1775) was an early mass migrant Lessepsian to Mediterranean Sea, found since decades in various areas, it has not reached southeastern Tunisian coasts (Gulf of Gabes) until very recently. This paper studies the allometry, condition factor and growth of P. segnis along the Gulf of Gabes coasts. We collected samples in one year time (October 2015 to September 2016) using artisanal fishing gears as well as trawl net and processed 4971 crabs. The crab carapace width (CW) ranged from 45 to 168mm for males and 50 to 159mm for females. The crab carapace length (CL) was 22.02-89.20mm and 38.70-85.28mm, respectively for males and females. The carapace width-carapace length, carapace width-front, carapace width-carapace height and carapace width-weight relationships were studied in separate sexes of crab. The covariance analysis indicated a significant difference between the two sexes with respect to the carapace width-carapace length and the carapace width-weight relationships. The condition factor ranged from 0.0042 to 0.0118 with a mean of 0.0078. The condition factor decreased with increasing crab size. The Von Bertalanffy growth parameters CW infinity, K and t(0) were estimated on the basis of Length-frequency modal progression analysis methods. The mean values of L infinity (mm), K (year(-1)) and t(0) were respectively, 206.48mm, 1.34 and -0.130 for males;183.89mm, 1.42 and -0.127 for females and, 190.60mm, 1.02 and -0.177 for combined sexes.