Effects of stocking density and grading on behaviour, cannibalism and performance of African catfish (Clarias gariepinus) fry

African catfish hatcheries in Europe require economical fry availability throughout the year. Purpose of this study was to identify the best suitable stocking density and appropriate grading regime for African catfish fry of around 0.1 g in 3 experiments (E1, E2 and E3). In E1, 3 stocking densities (10, 20 and 30 fry L-1) without grading were compared for behaviour, mortality, cannibals and growth. In E2, additional daily manual grading removed shooters or potential cannibals. In E3, 3 different handling procedures (manual grading on days 2, 4, 6 and 8: 2dH, manual grading on day 5: 5dH, self-grading on day 5: 5dS) under highest stocking density (30 fish L-1) were applied. Each experiment lasted 10 days and the fish had a mean initial weight of 0.08, 0.06 and 0.06 g fish-1, respectively. In E1 and E2 stock activity increased with increasing stocking density but aggressive behaviour was lowest at the higher stocking densities. The stocking density had no significant effect on mortality rates or growth performance from 10 to 30 fish L-1 between groups. However, regular removal of cannibals reduced the total mean mortality rate from 25 % in E1 to less than 5 % in E2 and E3. The mean feed conversion ratio (FCR) improved from 0.68 in E1 to <= 0.55 in E2 and E3. The tested handling procedures revealed no significant differences in fry behaviour and performance, while final stocking densities increased with decreasing handling stress. This suggests that African catfish fry of about 0.1 g are kept optimally at high stocking densities of 30 fish L-1 or 30,000 fry m- 3 inside regular breeding containments, with effective self-grading every 5 days in order to reduce total mortality, maximize production and minimize the amount of labor.