Effect of impoundment on the growth and fishery of Bagrus docmac in lake Nasser

Abstract

AbstractChanges of faunistic composition in impounded reaches of regulated rivers have been related to changes in environment from lotic to lentic. Rarely has this assumption been tested, however, Bagrus docmac is a long‐lived freshwater catfish and was once a commercially valuable species in Lake Nasser. Its production declined abruptly following the early years of impoundment of the lake. This decline is significantly correlated to the increase of Tilapia catch. A modified model for the surplus production is introduced and estimated the potential yield as 195·7 metric tonnes. Relative growth rate of weight over length interval showed that the fish ranging between 20 to 40 cm in standard length have had the highest rate of weight gain which is attributed to gear selective mortality. Fish (18‐83 cm in standard length) vertebrae (N = 105) are used for growth analysis, after their annuli are validated. Age‐groups ranged between I to VI with an average age of 3·14. Age‐groups II suffered high selective mortality which caused a fishing‐up effect that took about four years to appear in the catch. Thus, the problem of Bagrus may be safely attributed to the effect of fishing gears which are mainly used to catch the relatively smaller Tilapia.