To evaluate theinfluence of benzocaine concentration on the induction and recovery from anesthesiain different sized Nile tilapia (Oreochromis niloticus), it was developeda random block experiment design in 4 × 4 factorial consisted of fourfish lengths (6; 10; 14 and 18 cm total lenght) and four benzocaine concentrations(60; 120; 180 and 240 mg of benzocaine per liter of water). The induction andthe recovery from anesthesia were divided in three stages based on the behaviorof the fishes under anesthetic effect. The time spent in each stage was recorded.Differences in length did not affect significantly the anesthesia and recoverytime. Linear plateu regressions were generated to estimate the point which increaseson anesthetic doses does not imply in further effects. During the anesthesiainduction, the plateu occurred in concentration time of 132 mg/L, and estimatedtime of 86 seconds would be necessary to achieve the complete sedation (stage3). For permanence of the fish under total sedation, it is needed 190 g/L benzocaineand estimated time of permanence in this stage is 91.1 seconds. Benzocaine concentrationsabove 190 mg/L do not produce any additional effects that justify its use.
