Incubation and oxidative stress of grass carp (Ctenopharyngodon idella) embryos exposed to different un-ionized ammonia levels

The increase in concentration of ammonia in surface water is a critical concern in aquaculture, especially in fish larvae production. The purpose of this study was to assess the effects of un-ionized ammonia (NH3) on the development and oxidative stress of grass carp (Ctenopharyngodon idella) embryos. The results showed that the rate of embryonic development to hatch was retarded with an increase in NH3-N concentration. Hatch rate of grass carp embryos in the control was greater than 90%, whereas the hatch rate in 0.879 mg L-1 of NH3-N was less than 70%. Viability of newly hatched larvae (24 h post-hatch) significantly decreased with increasing NH3-N concentrations. Viability of newly hatched larvae 24 h post-hatch in the control was 97.8%, whereas in 0.879 mg L-1 of NH3-N, the viability was only approximately 40%. This indicates that although embryos can hatch to larvae under high NH3-N conditions, the larvae were much less tolerant of NH3-N than embryos. NH3-N had a significant effect on superoxide dismutase (SOD) and malondialdehyde (MDA) levels, whereas there was no statistically significant difference in catalase and glutathione activity among different NH3-N levels. Two-way analysis of variance indicated that there was a statistically significant interaction between NH3-N and developmental stage on SOD and MDA. Such changes indicated the presence of oxidative stress when grass carp embryos were exposed in water containing pollutants and SOD plays a key role in removing the free radicals in this case.