Total suspended solids

Total Suspended Solids (TSS) refers to the amount of solid material above 1 µm in diameter that is suspended in the water.38 These solids can consist of various substances, including uneaten feed, fish fecal matter, algae, and other waste. The impact of TSS on fish welfare is complex and not only depends on the absolute amount of TSS, but also on the characteristic makeup of the suspended particles (such as size, composition, and abrasiveness).39 In sea cages, TSS levels are largely determined by environmental conditions, stocking density, feeding practices, and water flow, though farmers have limited ability to control TSS once elevated. Increased TSS and turbidity levels often co-occur, but measure different aspects. Turbidity primarily affects feeding behavior through visual impairment, while TSS primarily affects gill function through particle irritation and bacterial load.

Level 0 Level 1 Level 2 Level 3
≤ 10 mg/L
> 10 to ≤ 25 mg/L
> 25 to ≤ 40 mg/L
> 40 mg/L
Protocol application

Measuring TSS typically requires expensive equipment or sending samples to a laboratory. Turbidity can be used as a proxy indicator where it is not possible to measure TSS, though turbidity and TSS do not necessarily correlate.

It is recommended to measure TSS in high-risk periods, (such as storms, nearby construction, algal blooms, and where turbidity readings are high). Measuring TSS at various points is crucial for a comprehensive understanding of the system’s environment. This involves taking readings at different depths (e.g., surface, mid-column, and bottom) and at multiple horizontal locations (e.g., at the center and near the net edges).

Note the TSS source when identifiable (e.g., clay, sand, organic matter, or feed waste), as well as the spatial distribution (e.g., whether it is localized or system-wide).

Scoring notes
  • Acute exposure to elevated TSS levels (e.g., < 24 hours) has less severe welfare impacts than chronic exposure (e.g., > 7 days).
  • Validation: The thresholds were decided based on expert opinion and evidence, mostly considering typical organic biosolids (such as feed waste and fecal matter). Abrasive inorganic sediments (such as clay, silt, or sand) or algal blooms may cause welfare impacts at lower concentrations. Further, there is limited rainbow trout-specific evidence (particularly in seawater) assessing the full range of TSS levels, e.g., the 10–30 mg/L range has not been tested.
Key evidence
Source Summary of source Key result
Becke et al., 2019 Experimental study investigating the effects of suspended solids (system-generated organic particles) and unionized ammonia on 785 juvenile rainbow trout (87 g) in RAS over 13 weeks.

The control group had ~5 mg/L TSS, while the treatment had > 35 mg/L. Unionized ammonia increased at week 6 (by ~0.012 mg/L) and week 11 (by ~0.025 mg/L) in both groups.		 The higher TSS levels did not show any detrimental histological alterations in the gills. Even at levels up to nearly 70 mg/L, no direct issues were observed in the fish.

Increased TSS levels caused indirect effects, increasing turbidity and suppressing feeding behavior.

A higher bacterial load in the gills and spleen was found in the higher TSS group.
Ross et al., 1985 Experimental study investigating the impact of estuarine silt on respiratory behavior in 15 juvenile rainbow trout (66–115 g) in laboratory conditions with 60-minute exposures.

For suspended solids treatment, 5 fish were exposed to estuarine silt at three concentrations (10, 100, 1000 mg/L) for 1 hour.		 All three groups showed increases in ventilatory activity (40–65%) and coughing activity (150–270%) with exposure.

The 100mg/L group had a 690% increase in oxygen consumption, higher than the 1000 mg/L group, which had a 480% increase. Oxygen consumption data were not collected for the 10 mg/L group.

In terms of recovery, cough rates in the 1000 mg/L treatment group did not fully return to baseline levels after 3 hours, whereas they did in the lower-concentration groups. The 1000 mg/L group also showed a marked increase in swimming activity.
Noble et al., 2020 Literature review on many rainbow trout welfare indicators, including TSS. A definitive threshold value for an acceptable level has not been agreed upon. Still, an upper limit of 15 mg/L has been suggested for Atlantic salmon, and the RSPCA recommends a maximum concentration of non-suspended solids of < 25 mg/L for all life stages of rainbow trout.
Supporting evidence
  • Becke, Cornelius, Mark Schumann, Dieter Steinhagen, Paula Rojas-Tirado, Juergen Geist, and Alexander Brinker. “Effects of Unionized Ammonia and Suspended Solids on Rainbow Trout (Oncorhynchus mykiss) in Recirculating Aquaculture Systems.” Aquaculture 499 (January 15, 2019): 348–57. https://doi.org/10.1016/j.aquaculture.2018.09.048.
  • Becke, Cornelius, Dieter Steinhagen, Mark Schumann, and Alexander Brinker. “Physiological Consequences for Rainbow Trout (Oncorhynchus mykiss) of Short-Term Exposure to Increased Suspended Solid Load.” Aquacultural Engineering 78 (Part A) (August 2017): 63–74. https://doi.org/10.1016/j.aquaeng.2016.11.001.
  • Birtwell, I. K., M. Farrell, and A. Jonsson. “The Validity of Including Turbidity Criteria for Aquatic Resource Protection in Land Development Guideline (Pacific and Yukon Region).” Fisheries and Oceans Canada, 2008. https://publications.gc.ca/collections/collection_2014/mpo-dfo/Fs97-4-2852-eng.pdf.
  • Davidson, John, Christopher Good, Carla Welsh, and Steven T. Summerfelt. “Abnormal Swimming Behavior and Increased Deformities in Rainbow Trout Oncorhynchus mykiss Cultured in Low Exchange Water Recirculating Aquaculture Systems.” Aquacultural Engineering 45, no. 3 (November 2011): 109–17. https://doi.org/10.1016/j.aquaeng.2011.08.005.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Ross, L. H., Kathryn M. H. Ward, and Barbara Ross. “The Effects of Formalin, Malachite Green and Suspended Solids on the Respiratory Activity of Rainbow Trout, Salmo Gairdneri Richardson.” Aquaculture Research 16, no. 2 (April 1985): 129–38. https://doi.org/10.1111/j.1365-2109.1985.tb00302.x.
  • Smith, C. M. “Total Suspended Solids Impact on the Stress Response and Swimming Performance of Fish.” Master’s Thesis, 2025.

  1. Davis, Michael W. “Fish Stress and Mortality Can Be Predicted Using Reflex Impairment.” Fish and Fisheries 11, no. 1 (March 2010): 1–11. https://doi.org/10.1111/j.1467-2979.2009.00331.x.

  2. Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.