This study investigated the relationships among parental body weight, egg size and number, and offspring performance at the larval, juvenile and grow‐out stages of bighead catfish (Clarias macrocephalus) in Mekong domesticated (Can Tho; CT) and wild (Ca Mau; CM) strains. One‐year‐old broodstock from each strain were separated into two size groups, with those in the large (L) group having, on average, approximately double the body weight of those in the small (S) group. Both strains exhibited a strong linear increase in absolute fecundity (slope = 58.8, p < 0.01 for CT and slope = 52.3, p < 0.01 for CM) and a slight increase in egg diameter associated with increasing female body weight (slope = 0.0002, p = 0.03 for CT and slope = 0.0003, p = 0.06 for CM). However, there were no linear correlations between female body weight and yolk‐sac volume or larval body length at hatching (p > 0.05). Despite similar fecundities, the CM wild strain had larger eggs and more favourable larval properties than the CT domesticated strain (p < 0.01). However, these advantages did not result in better growth and survival of CM offspring at any stage. The CT strain showed improved growth in offspring with parents in the L group compared to those with parents in the S group across the three stages (35.7%, 50.0% and 43.9% respectively), while there was no significant increase in growth in the CM wild strain (p > 0.05). Parental body weight and strain had no effects on survival and feed conversion ratios (FCRs), with the exception that juveniles of the CM S group had the lowest SR (38.8 ± 6.0%) and FCR (1.09 ± 0.08) due to disease infection. The CT L group grew fastest and consumed the highest amount of food with similar feed efficiency to the other groups in juvenile and grow‐out stages (p < 0.05). These results indicated that the pertinence of ‘the bigger the better’ paradigm varied between growth stages in the two bighead catfish strains.

Information on the reproductive biology of fish species is essential for fisheries management, conservation and culture potential assessment. Therefore, this study aimed to understand the sex‐based morphological differences and reproductive characteristics of Cephalocassis borneensis, an ariid species with a rapid decline of wild populations. Fish samples were collected monthly from local fishermen (n = 1092) in the Vietnamese Mekong River over a year. Morphological analyses revealed that sexual dimorphism was observed in mature, but not immature, fish. The length at first maturity (Lm) was smaller for males (11.5 cm) than for females (12.5 cm). Mature males had larger head length and pre‐pectoral distance than mature females, increasing the space for oral incubation of fertilized eggs and larvae in males. Conversely, females were larger in three head parameters (head angle, head width and head depth) and three abdomen parameters (body deep, ventral fin length and distance between pectoral and ventral fin) involved in ovary development. Paternal mouth‐brooding behaviour is an important reproductive strategy in C. borneensis to increase offspring survival. Monthly variations in gonado‐somatic index and condition factor (K) and the presence of maturation stages indicated that C. borneensis spawns year‐round, mainly in the rainy season from June to October. This species' fecundity was relatively low, from 10 to 31 eggs per female of quite large sizes (about 7.30 ± 0.68 mm in diameter). Besides, non‐functional oocytes (hyaline eggs) of smaller size (<3 mm) were found in the females' ovaries. Low fecundity with large eggs and paternal care indicated that the species is an equilibrium strategist. These characteristics are critical in developing ariid species conservation plans, such as setting the time and mesh‐size for fishing, and domestication programmes in artificial conditions.