Fig. 4. Otolith growth rate during the 300 p.p.m. thiourea (TU) treatment from 15 to 27 d.p.h., followed by recovery in fresh water from 28–41 d.p.h. Filled circles indicate the control group without treatment; open circles indicate the experimental group (N=12 for each group). Data are shown as means ± s.e.m. for clarity. Arrows indicate the dates of treatment with tetracycline that produced the fluorescent rings as shown in Fig. 3.