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Toxic metal pollution influences the lives of diverse aquatic organisms and humans who consume contaminated aquatic products. However, its potential impacts on aquatic organism health and, thus, ecological health, have been neglected in many regions. This research was carried out to contribute to filling that knowledge gap. Three freshwater fish species in the Nhue–Day River basin, Vietnam, have been chosen to study the bioaccumulation of metals (Zn, Cu, Pb, and Cd) in the tissues (livers, kidneys, gills) and their effects on fish physiological health (changes in the oxidative-GST activity, and physiological biomarkers-energy reserves, respectively) from 2013 to 2017. The extensive results revealed significant spatial and temporal variations in metal concentrations in tissues of common carp (Cyprinus carpio), silver carp (Hypothalmic molitrix), and tilapia (Oreochromis niloticus), and well correlated to their concentration in the water (p < 0.05). Fish bioaccumulated metals in the following order: Zn > Cu > Pb > Cd, with more in the kidneys and livers (spring and summer) than in other tissues. Metal accumulation in O. niloticus and C. carpio was higher than in H. molitrix. Biomarker responses (except for glycogen variation) were also higher during warm seasons. Changes in metal levels in water and fish tissues caused variations in biomarkers in the respective fish tissues, particularly in the livers, as demonstrated by significant correlations of metal concentrations in water and fish tissues to biochemical and physiological responses (p < 0.05). The findings suggest that metal pollution in the river basin adversely impacts the physiological health of both wild and cultured fish. Seasonal shifts in the levels of metal accumulation and biomarkers could be connected to species-specific differences in physiology and the levels of metals in environments. This biomarker set is simple but effective in assessing the impact of metal pollution on fish health and, hence, the aquatic ecosystem. This is one of the first biomonitoring studies to assist in designing better water management strategies for the Nhue–Day River basin.

Summary The parasite fauna of Epinephelus coioides can be used as an indicator to assess different aquaculture systems. A total of 133 E. coioides were collected from fishermen (wild fish) and marine fish farms (net cages, pond farms) in the Gulf of Tonkin (Ha Long, Cat Ba and Nghia Hung), Vietnam in 2013 and 2014, and examined for the presence of protozoan and metazoan parasites. Different patterns of parasite infections were found. The monogenean Pseudorhabdosynochus spp. was predominant in the net cage system; and the protozoans Trichodina spp. and Cryptocaryon irritans were found exclusively in pond and net cage cultivation sites in Nghia Hung and Cat Ba. These three species were assessed in addition to seven further indicator species and two dominance indices, to visualize the potential of assessing different aquaculture systems in Vietnam. This suggests the use of former shrimp ponds for grouper cultivation, reducing the pressure to further extend net cage cultivation in already highly influenced mariculture regions in Vietnam.

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