The statistically significant relationship obtained between known fish species and total species diversity from some IO countries has been used to predict potential species numbers for IO counties. Values predicted for some East African countries and the island nations in the western IO were then compared with the data as known from the MASDEA database for these countries. 1. Comores, 2. Djibouti, 3. Eritrea, 4.Kenya, 5.Madagascar, 6. Mauritius, 7. Mozambique, 8. Réunion, 9. Seychelles, 10. Somalia and 11. Tanzania.