EFFECT OF TILAPIA Oreochromis niloticus OVER THE FISHERIES IN EL GUÁJARO RESERVOIR ATLÁNTICO - COLOMBIA

Objective. To evaluate the effect of tilapia, Oreochromis niloticus, over the fisheries in the El Guájaro reservoir, located in the Atlantico state, north of Colombia. Materials and methods. The reservoir has approximately 14.000 ha, and support the daily effort of 2.500 fisherman, coming from the eight Municipalities around. Through landing samplings carried out during 48 hours and a monthly frequency in all the ports, the abundance and composition of the captures were evaluated during 2002 and compared with the production during 1988 in the same ecosystem. Results. 38 fish species, belonging to 14 families, were identified. It was found that the production in 2002 was 431 ton/month, against 84 ton/month in 1988. During 2002, the composition of the captures was dominated by Oreochromis niloticus (53%) and Triportheus magdalenae (36%) this means a variation in the composition of the captures that during 1988 were respectively of 13% and 73%. This data however, did not affect the proportion of herbivores - carnivorous that stayed constant in 90-10 percent in the two periods of study. Conclusions. The changes in composition and abundance of the whole production suggest a displacement of a native species (Triportheus magdalenae) by an exotic (Oreochromis niloticus), which is being observed throughout the Magdalena river basin in this decade.