Temperature, dissolved oxygen and pH are the more frequently investigated exogenous factors affecting the hatching success of fish eggs. Members of the family Claridae have a high fecundity . The insufficient supply and relatively high cost of Clarias gariepinus (Burchell 1822) fingerlings resulting from the low output of breeding attempts indicate the need to widen the scope of factors affecting the low production . L imitations arise when the choice of water is limited to groundwater. In most cases, this water source produces hard water, creating problems for eggs, larvae, and fry.
We monitored C. gariepinus embryos incubated in hard water with four different treatments of Mg:Ca hardness ratios: 0Mg:0Ca, 10Mg:0Ca, 0Mg:10Ca and 10Ca:10Mg hardness equivalent to 0 (T1), 10 (T2), 10 (T3) and 20mg/l (T4) hardness as CaCO3 . The hatching rate was determined using the volumetric method. The effect of hard water on embryonic development and survival was determined using a mobile phone-attached microscope (60X) and digital camera. The hatching rates differed significantly across the treatments (p<0.05), with T1 having the highest hatching rate (43.14%) while T4 had the least hatching rate with 0% hatching success. There was no significant correlation between any water quality parameter and the hatching success of C. gariepinus larvae from eggs incubated in hard water. All correlations were not significant (p>0.05). Embryos in T1 showed progressive development with less mortality. Embryos in T2 and T3 also developed progressively but with mortalities. There was particularly significant mortality (100%) in T4 (10Mg:10Ca). This study highlights the importance of watching out for calcium and magnesium hardness in the hatchery water supply to avoid total fry mortality, as experienced by some farmers. Levels of 10Mg or 10Ca hardness alone affect the hatchability of C. gariepinus eggs, and the full complement of 20mg/l combination of 10Mg:10Ca hardness causes total mortality of embryos. The current results should guide farmers in designing water treatment facilities if the water supply is affected by Ca2+ or Mg2+ hardness.