Spawning periodicity, fecundity and egg size in laboratory-held stocks of a substrate-spawning tilapiine, Tilapia zillii (Gervais)

Inter-spawning-interval (ISI) in serially-spawning Tilapia zillii averaged 26 days in environmentally-controlled aquaria, and exhibited a complex and variable relationship to fish size. Mean number of days elapsed spawn-1 (based upon both spawning and non-spawning fish amongst the 101 females studied) was considered the best estimate of spawning periodicity and averaged 61 days. The shortest spawning cycle was just 7 days. Fecundity increased with increasing maternal size and ranged from 461 to 11,640 eggs clutch-1 (mean = 3606 ± 280). Egg size varied only within a relatively narrow window. Variations in egg size did not, generally, relate to maternal size; mean egg diameter was 1.5 mm. Regression analysis revealed strong and significant relationships between fish size (total length and body weight) and fecundity, relative fecundity and total egg volume. Relationships between fish size and various indices of egg size (mean egg diameter, mean egg dry weight and mean egg volume) were far weaker.