Preliminary Assessment of Age and Growth of the Red Swamp Crayfish Procambarus clarkii [Girard, 1852] in the River Nile in Egypt by Direct and Indirect Methods

The red swamp crayfish (Procambarus clarkii) is an exceptionally invasive species introduced into the Egyptian freshwater ecosystems in the 1980s for aquaculture purposes. Despite its ecological and economic significance, the age distribution of this species has not been assessed properly using an accurate method. This study is the first to validate the use of growth band counts in the mesocardiac ossicles as a direct technique for age determination in P. clarkii using 166 known-age individuals reared under various temperature regimes. The findings confirm that band counts represent age rather than number of molts. Field comparisons between northern (Giza) and southern (Aswan) Nile populations revealed maximum longevities of six and seven years, respectively, using direct methods, while indirect size–frequency analysis underestimated age at three years. Growth rates were higher at 25 °C compared to 32 °C, both in the lab and in situ. Males matured at larger sizes than females, though age at sexual maturity averaged one year in both regions. This study demonstrates the superiority of direct aging over traditional indirect aging methods, offering critical insights for managing crayfish populations and ensuring sustainable fisheries in the River Nile in Egypt.