Assessment of Oreochromis aureus caught by different fishing techniques at Lake Borollus, Egypt.

Evaluation of Oreochromis aureus caught from Lake Borollus was studied based on the length- frequency of samples collected by different fishing gears. Analysis of the data using (FiSAT) computer program returned the estimate of the von Bertalanffy s growth curve parameters as: L_∞ = 18.28 cm (total length); k = 0.87 year^-1 (for Lokaffa), L_∞ = 19.32 cm; k = 0.41 year^-1 (for Trammel), L_∞ = 17.19 cm; k = 0.42 year^-1 (for Basket traps) and L_∞ = 14.55 cm, K=0.39 year^-1 (for Al-qerba). Very narrow selection range was observed and its upper limit didn t exceed 11.53 cm in most of the investigated fishing gears except that of trammels which reached 12.11 cm. The total mortality coefficient Z was found to be 3.05, 1.83, 1.46, and 1.58 year^-1 corresponding the fore-mentioned fishing methods respectively. Also, their respective natural mortality coefficient M was 1.67, 1.01, 1.06, and 1.06 year^-1. The estimated exploitation rate E was 0.45, 0.45, 0.28 and 0.33 respectively. Their relative yield and relative biomass per recruit analysis showed that the stock of O. aureus is moderately exploited and more effort could be added to reach the maximum sustainable yield and at the same time larger mesh sizes of fishing nets should be used to increase the mean lengths at first capture and also to conserve the reproducible part of O. aureus population.