Defining the critical oocyte diameter range for optimizing spawning success in Black seabream, Acanthopagrus berda: An integrated approach

Accurate assessment of oocyte maturation is essential for improving induced breeding efficiency in sparids. This study evaluated critical oocyte diameter for induced spawning in Acanthopagrus berda using ovarian biopsy, histology, and ultrastructural analysis. Across 29 induced breeding attempts using Gonadotropin Releasing Hormone analogue, 18 resulted in successful spawning. Among the females classified based on the average oocyte diameter in biopsy as 300 – 350 µm, 350–400 µm; 400–450 µm and above 450 µm, successful spawners predominantly exhibited mean oocyte diameters of 350–450 µm. Relative spawning fecundity increased significantly with oocyte size, with females possessing 350–400 µm (231 ± 72 eggs g⁻¹) and 400–450 µm (441 ± 88 eggs g⁻¹) oocytes giving the highest values (χ² = 19.19, p < 0.001) with Dunn’s post-hoc test identifying the 400–450 µm class as significantly superior. Near-complete spawning (around 0.1 million eggs) occurred within a narrower optimal range of 360–430 µm (median 410–420 µm). Ultrastructural observations revealed progressive pore canal development from 300 µm to 450 µm, with fully formed pore canals and micropyles evident in matured (400–450 µm) oocytes. Females with oocytes less than 350 µm showed no ovulation on inducement, whereas those with 450 µm produced fully hydrated eggs, but were not ovulated. Optimal breeding outcomes occurred at 350–400 µm (85% fertilisation; 87% hatching) class, whereas 400–450 µm class showed variations in fertilisation and hatching rates. Collectively, the findings demonstrate that a critical oocyte diameter of approximately 350–400 µm is ideal for yielding enhanced fertilisation and hatching rates in A. berda for the proposed hormonal inducement even though spawning success occurs in 350–450 µm classes. • Oocyte diameter strongly predicts spawning success in Acanthopagrus berda . • Best spawning, fertilisation and hatching performance at 350–400 µm oocyte size. • Ultrastructure confirms maturation via micropore and micropyle formation. • Oocytes above 450 µm undergone hydration without successful release.