Population dynamics of the freshwater crayfish Procambarus clarkii (Girard, 1852) in the River Nile, Egypt

The red swamp crayfish, Procambarus clarkii was introduced into the River Nile in the early 1980s and has become a resident in the Egyptian freshwater ecosystems, but up till now no detailed study has been done to know its stock or even its population dynamic in the River Nile. The population dynamical parameters of Procambarus clarkii were estimated during the period from 2009 to 2012; based on 3465 specimens (1455 males and 2010 females) (7.0 – 15.0 TL), collected from two locations along the River Nile. It was found that this species attains its highest growth rate during the first six months of life, after which, the increment in length is decreased with the further increase in age. The growth of this species is isometric based on the b-value of length weight relationship. From the length frequency distribution analysis, the population parameters for combined sexes were: asymptotic length L∞ = 16.45cm TL, growth coefficient K = 1.60/y, instantaneous total mortality Z = 3.65/y for male and 5.60/y for female, natural mortality M = 2.28/y for male and 1.91/y for female, fishing mortality F = 1.37/y for male and 3.69/y for female, exploitation ratio E = 0.38 /y for male and 0.66/y for female, length at first capture Lc = 9.11 and 9.25 cm TL for males and females, respectively. The relative yield per recruit (Y /R) and relative biomass per recruit (B /R) analysis for P. clarkii in the River Nile gives a maximum (Y /R) at E= 0.70 and the exploitation level which maintains the spawning stock biomass at 50% of the virgin spawning biomass E0.5 was estimated as 0.37. This indicates that there is a chance to expand the red swamp crayfish fishery by increasing the current E to that which gives the maximum Y /R but raising the exploitation ratio to that level will not be reasonable and will be associated with a very little portion of yield. So, keeping the exploitation ratio at its current value or reducing it to E0.5 value (E= 0.37) to achieve more economic return.