Biological Indices as Markers of the Health of Cirrhinus mrigala under the Stress of Microplastics and Metal Nanoparticles.

Coexistence of microplastics and nanoparticles is an emerging concern for environmentalists. This study highlights the individual and combined toxicities of environmentally relevant concentrations of two particle sizes of low-density polyethylene microplastic [LDPE: 150-250 µm (M1) and < 150 µm (M2)] and Predicted No Effect Concentration [PNEC: 2.95 mg/L, 1/100 of 96 h LC₅₀ {295.319 (253.533-333.814) mg/L}] of nickel oxide nanoparticles (NiO NPs: N) on the well-being of a food fish, Cirrhinus mrigala. Length-weight relationship, Fulton Condition Factor (FCF), Relative Condition Factor (RCF), Hepatosomatic Index (HSI), Gastrosomatic index (GaSI), along with behavioural and morphological alterations, were recorded monthly during an exposure and recovery period of 60 days each. The biological indices declined significantly over the control (p < 0.001) in treatments. FCF (1.65 ± 0.05) and RCF (0.83 ± 0.03) were minimum in M2N after 60 days of exposure, but in M1N after 60 days of recovery period (1.34 ± 0.045 and 0.678 ± 0.03, respectively). Exposure and recovery values of b, HSI and GaSI were lowest in M1N. Abnormal swimming, distressed hitting against the walls, copious mucus secretion, haemorrhage, darkening of skin, clogging of intestine and feed avoidance seem to have reduced energy allocation and well-being, causing allometric growth in treatments. Minimum changes in N, cataract and exophthalmia in M2 and tumour formation in M2N clearly indicate that MPs and NPs altered behaviour and bioavailability of each other. Their coexistence in natural waters might pose a direct threat to organisms and hint at developing regulatory strategies for the discharge of microplastics and nanoparticles.