Growth, physiological responses, and histopathological changes of Oreochromis niloticus fingerlings exposed to aqueous gold nanoparticles (AuNs) at high stocking density.

Despite the ubiquitous usage of nanoparticles (Ns), there are still significant gaps of knowledge regarding the impact of gold nanoparticles (AuNs) on one of the economically important fish species as Nile tilapia (Oreochromis niloticus). Therefore, this study was conducted to investigate the impact of gold nanoparticles (AuNs) on the growth performance, serum biochemistry, antioxidant enzymes levels, digestive enzyme activity, and histopathological changes of different studied tissues such as gills and liver in Nile tilapia (Oreochromis niloticus) that exposed to various concentrations of AuNs (0, 5, 10, 15 and 20 μgL-1) respectively as aqueous exposure, particularly in the context of heavy metal exposure. Fish were randomly distributed in 15 glass aquariums (70 L) at a stocking rate of 20 fish/aquarium (treatment = 3 aquariums: 60 fish). Fish were within an initial average weight of 4.02 g ± 0.27 and fed a commercial diet containing 30 % crude protein (CP) at a feeding rate of 4 % of biomass. Results showed improvements in growth with AuNs exposure, but the survival rate decreased with increasing concentrations of AuNs exposure. The 20 μg L-1 AuNs group exhibited the highest specific growth rate (SGR: 3.45 %/day), representing a 17.7 %/day increase compared to the control (2.93 %/day), alongside a 44.7 % reduction in survival rate (52.55 % vs. 95 % in controls). Intermediate concentrations (e.g., 5 or 10 μg L-1 AuNs) improved SGR (3.16 and 3.23 %/day) with less severe survival impacts (92.5 and 75 %, respectively). Also, increasing doses of AuNs led to negative effects on physiological status and histological study of the liver and gills. Fish that were exposed to ≤5 μg L-1 exhibited the highest survival rate and enhanced resistance to heavy-metal challenge. It can be recommended that aqueous exposure to gold nanoparticles requires further studies, and it appears that doses less than 5 μm may have positive effects on growth and immune functions.