Fig. 7. Development of the electromotive force of the fish's body. The equivalent source was characterized with the air gap procedure (Cox and Coates, 1938; Caputi et al., 1989) by maintaining the fish body out of the water and measuring the drop of voltage across a known resistor. (A) Waveforms obtained using load resistors of 100 M (red) and 1 k (black). All waves except V₄ show a similar time course (note that waveforms are shown slightly shifted in time for the sake of clarity). (B) Characteristic curves of the equivalent electromotive force of the whole fish body for V₃ in a larvae (28 mm), a juvenile (68 mm) and two adults of different length (100 and 225 mm). (C) The electromotive force at the peak of V₃ plotted as a function of fish length. Two populations gathered in consecutive summers are compared. (D) As observed in A, V₄ is modified by an external load on the electric source in the larva and the juvenile but not in the adult. We characterized this phenomenon by the increment of the ratio V₄/V₃ caused by changing the load from 1 M to 1 k. The plot in D shows that this parameter is negatively correlated with fish length (r²=0.005, N=20), suggesting that maturation is a factor involved in this phenomenon.