The Effect of pH and Conductivity on the Electric Behaviour of Apteronotus Albifrons (Family Apteronotidae) in a Pollution Monitoring System

Abstract

The Electric Organ Discharge (EOD) frequency and waveform of the neotropical fish, Apteronotus albifrons (Gymnotiformes) are used to monitor water quality. This study evaluates the effects of two environmental variables, pH and water conductivity on the fish's electric behaviour. Variation in pH is shown to have no effect on the EOD waveform. On the other hand, the EOD frequency decreases significantly under acidic conditions (down to pH 3.6) and increases as the pH rises (to 10.6). A closer examination shows that the most satisfactory mathematical model for describing the pH frequency variation is a three degree polynomial. For narrow pH intervals, EOD frequency and pH are shown to be linearly related with significant increases in the discharge rate taking place on the two pH intervals: 5 to 7.6 (Student t-test value ≥23.47; degree of freedom = 48; probability < 0.001) and 9.2 to 10.5 (Student t-test value ≥6.186; degree of freedom = 48; probability < 0.001). It is also possible to show that the EOD frequency varies between 6.1 and 10.8 Hz per unit acid to neutral pH (mean: 8.5 Hz; SD: 1.6) and between 2.4 and 10.2 Hz per unit alkaline pH (mean: 6.1 Hz; SD: 2.6). With reference to the influence of water conductivity, the analysis of the results with different ionic charges (in the range of 54 to 484 µS cm−1) revealed no effect neither on the EOD frequency nor its waveform. Visual observations did reveal an effect on the signal amplitude although the biomonitor was not designed to analyse this. Different hypotheses are advanced to explain the observed results in the light of which the appropriate in situ use of the electric fish to biomonitor the water quality is considered.