EFFECT OF THE DENSITY OF CYCLOPOID COPEPOD Mesocyclops longisetus ON THE PREDATION OF Pseudoplatystoma corruscans LARVAE

Abstract

Cyclopoid copepods represent a serious risk to larviculture during the first days of the larvae's life. Aiming to evaluate the effect of the population density of copepods (Mesocyclops longisetus) on the predation of Pseudoplatystoma corruscans larvae, two experiments were carried out in 9.3 x 14.5 x 25.0 cm aquarium containing one liter of water. In the first experiment, the population density in each aquarium was of 10 larvae with 5.7 mm of total length. Five treatments were tested: A, B, C, D and E with densities of (control), 10, 20, 30 and 40 copepods per liter, respectively. Each treatment comprised three repetitions. The design of the experiment was totally randomized. The surviving larvae were counted after 1, 4, 7, 10, 13 and 25 hours from the start of the experiment. A quadratic reduction (P < 0.01) was observed in the survival rate in treatments A, B and C. Dunnett test (P < 0.05) was applied to compare the averages of the treatments with the control. After one hour of experiment no predation was verified. With four and seven hours of experiment, treatments D and E differed significantly from control. With ten, thirteen and twenty-five hours, treatments C, D and E differed from control. The survival averages, after twenty-five hours of experiment, were A = 70.00%; B = 30.00%; C = 1.67%; D = 1.00%; E = 0.33%. In the second experiment, five larvae with 14.00 mm of total length were used in each aquarium, while all other procedures were identical to those of the first experiment. In this second experiment, no predation was observed, indicating that larvae with the mentioned length or longer were free from predation, at the population density used. The results demonstrate the influence of the larvae length and the population density of copepods on the predation activity.