Effect of stocking density and feeding frequency on nervous necrosis virus and iridovirus susceptibility in hybrid grouper (Epinephelus lanceolatus ♂×Epinephelus fuscoguttatus ♀)

Stocking density and feeding frequency are key factors affecting the cannibalism of groupers. Additionally, nervous necrosis virus (NNV) and grouper iridovirus (GIV) are two viruses associated with high mortality in the grouper breeding industry. They are key pathogens causing major economic and industrial losses. Hybrid grouper is currently the main species in grouper culture. Therefore, the purpose of this study was to explore the effects of different stocking densities and feeding frequencies on cannibalism and NNV and GIV infections of hybrid groupers. Different stocking densities (5, 10, 20, 30, 40, and 50 fish/L) and different feeding frequencies (2, 4, 6, and 8 times/day) were used to observe the effects on the survival, cannibalism, post-infection mortality, viral gene expression, antibody titer and pathological examination of hybrid groupers. The results showed that a stocking density of 5 fish/L and a feeding frequency of 2 times /day significantly increased cannibalism, whereas a stocking density of 30–40 fish/L and a feeding frequency of 4–6 times/day significantly decreased cannibalism. The highest mortality rate, viral gene expression, and antibody titer after NNV and GIV infection were observed in the group with 50 fish/L and 8 times/day, respectively, and histopathological changes were also observed. Therefore, a stocking density of 30–40 fish/L and a feeding frequency of 4–6 times/day during the early stage of fry rearing combined with the standardized operation process for fry may reduce cannibalism, increase survival, and reduce disease outbreaks. • The stocking density of 30–40 fish/L can reduce size heterogeneity and cannibalism. • The feeding frequency was 4–6 times/day which had the lowest cannibalism. • The mortality rate of NNV infection is elevated in high stocking density. • The mortality rate of GIV infection is elevated in high-frequency feeding. • Stocking density and feeding frequency are critical factors during the fry stage.