Turbidity Standards in Aquaculture Water

Turbidity is a critical optical parameter in aquaculture, reflecting the concentration of suspended particles that obstruct light penetration. It directly influences feeding behaviour, respiratory function, primary productivity, and dissolved oxygen dynamics. 

Consequently, establishing appropriate turbidity standards is essential for maintaining healthy culture conditions and ensuring compliance with effluent regulations.

Two Measures of Water Clarity

In aquaculture practice, turbidity is characterised by two complementary metrics. Secchi disc transparency, measured in centimetres, indicates the depth at which  disc disappears from view——higher values denote clearer water. Nephelometric turbidity units (NTU) , measured by a turbidimeter, quantify light scattering——higher NTU values indicate greater turbidity. Both approaches are used to assess suspended solids and plankton density in culture ponds.

Optimal Ranges for Culture Ponds

For productive pond aquaculture, water clarity must balance sufficient light for phytoplankton photosynthesis against excessive turbidity that stresses fish. The traditional quality criterion for pond water is summarised as “fertile, active, tender, and refreshing”. Under this framework, transparency between 20 and 40 cm is generally considered suitable. Values below 20 cm indicate over‑fertilisation and potential deterioration, while values above 40 cm suggest insufficient natural food organisms.

In NTU terms, recommendations vary by source and system. Some guidelines suggest 20–30 NTU is appropriate for good fish culture, while others cite 2.7–4.2 NTU as favourable. For recirculating aquaculture systems, stricter control below 5 NTU is often required due to the sensitivity of filtration equipment.

Species‑Specific Requirements

Turbidity tolerances differ markedly among cultured species. For filter‑feeding fish, transparency of 20–25 cm supports adequate plankton concentrations. For carnivorous or pellet‑fed fish, 30–35 cm is preferred. In shrimp farming, transparency is typically managed at 20–40 cm during early stages and 30–40 cm in later phases. These ranges ensure sufficient light for benthic algae without stressing the animals.

Effluent Discharge Standards

Turbidity is closely related to total suspended solids (TSS), a key regulated parameter in aquaculture effluent. The Chinese Fishery Water Quality Standard (GB 11607-89) mandates that anthropogenic increases in suspended solids shall not exceed 10 mg/L, and settled solids must not harm fish, shrimp, or shellfish. For discharge from culture operations, the recommended industry standard sets TSS limits at 50 mg/L for primary grade and 100 mg/L for secondary grade. Some provincial regulations impose tighter limits, such as 45 mg/L for primary discharge.