THE MINIMUM EFFECTIVE SPERMATOZOA EGG RATIO FOR ARTIFICIAL-INSEMINATION AND THE EFFECTS OF MERCURY ON SPERM MOTILITY AND FERTILIZATION ABILITY IN CLARIAS-GARIEPINUS/

The optimal ratio of spermatozoa : egg (15 000 : 1) for artificial ins emination of African catfish Clarias gariepinus gave fertilization and hatching rates of 80 and 67%, respectively. Below a sperm:ova of 3000 : l fertilization success decreased significantly. Excessive sperm (> 15 000 : l) partly inhibited fertilization success. Sperm motility was decreased significantly by 0.001 mg l(-1) Hg2+ as HgCl2, but its effe ct on fertilization was dependent on the sperm : ova ratio, since exce ss sperm masked the effect of the pollutant. The most sensitive sperm: ova ratio for monitoring pollutant effects on fertilization success wa s 1500 : l, which corresponds to half the minimal amount that yields a high fertilization rate in artificial insemination. There was a good correlation between fertilization and hatching rates (r = 0.83; P < 0. 05). Although both fertilization and hatching rates provide equally go od indicators of fertilization success, the more rapid fertilization r ate test is recommended since it requires only 12 h. (C) 1998 The Fish eries Society or the British Isles.