Turbidity

Clear water allows fish to see their surroundings, detect feed, and respond to social and environmental cues. Turbidity is a water quality parameter that relates to water clarity, indicating how materials in the water scatter light. Turbidity is most accurately measured by nephelometry in Nephelometric Turbidity Units (NTU). High turbidity can reduce fish’s ability to see, affecting behaviors such as feeding.

While turbidity and Total Suspended Solids are often discussed together, they do not necessarily always correlate with each other.37

It is difficult to give recommendations that accurately reflect welfare in all cases, as turbidity can increase from suspending different materials, each of which may have varying impacts on welfare. The given thresholds should be applied with caution.

Turbidity (NTU)

Level 0 Level 1 Level 2 Level 3
≤ 4 NTU
> 4 to ≤ 20 NTU
> 20 to ≤ 40 NTU
> 40 NTU

Secchi disc depth

Level 0 Level 1 Level 2 Level 3
> 6 m
4–6 m
≥ 2 to < 4 m
< 2 m

Overall: Overall score is determined by the most severe subindicator

Protocol application

Turbidity can be measured with a nephelometer. Handheld nephelometers can provide spot checks, while submersible sensors can provide continuous monitoring. Secchi discs are often used in sea cage systems, being lowered into water until no longer visible, measuring visibility depth in meters. Scoring under both measures has been provided.

It is recommended to measure turbidity once a day, with increased monitoring during high-risk periods (such as storms, nearby construction, algal blooms). Turbidity should be measured at consistent times, avoiding post-feeding periods, as turbidity may temporarily increase due to leftover feed. Measuring turbidity 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 the bottom), upstream and downstream of cages, and at the place of highest fish density.

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

Scoring notes
  • Nephelometers should be calibrated using a consistent methodology, with the same measurement angle and depth.
  • Inorganic sediments are generally more harmful than organic particles, causing gill irritation and reducing feeding efficiency. Algal turbidity may indicate water quality issues, but it is less physically damaging. Localized, temporary waste particles are less concerning than other sources of environmental turbidity. It is critical to identify the source of turbidity for an appropriate management response.
  • Storm- or construction-related turbidity is usually temporary, but can be severe and require immediate attention.
  • Acute exposure to elevated turbidity levels (e.g., < 24 hours) has less severe welfare impacts than chronic exposure (e.g., > 7 days).
    • Short-term elevations (e.g., < 24 hours) are generally minimally harmful to welfare if not recurring, especially at levels of 10–20 NTU.
    • Sustained exposure (e.g., 20–40 NTU for 24–48 hours at the higher end, or multiple days to weeks at the lower end) increases the risk of impaired feeding, behavior changes, and gill stress. Sustained exposure at this level requires management action.
    • Chronic exposure over multiple weeks to months can result in severe impairment, especially at levels > 25–30 NTU.
    • Where there is chronic exposure, consider temporary fish relocation if possible.
  • Validation: There is limited rainbow trout-specific data and the majority of data comes from freshwater sediment studies, which may not translate to turbidity in sea cages. The Secchi disc depth thresholds have been developed based only on one paper (Montes et al. (2018)), which was conducted on Atlantic salmon in Chilean fjords, where the focus of the paper was the impact of algal blooms. The thresholds represent expert judgment and precautionary principles rather than validated, evidence-based welfare boundaries. Thresholds should be applied with caution. The thresholds also do not account for particle composition, which is more important than absolute NTU values.
Key evidence
Source Summary of source Key result
Birtwell et al., 2008 Report and literature review investigating the appropriateness of using turbidity as an indicator to protect aquatic organisms (including salmonids) during land development in the Pacific and Yukon Region (freshwater). Canadian and British Columbia Guidelines consider clear water to be water with < 8 NTU, allowing an increase of 8 NTU over the long term and 2 NTU over the short term (< 24 hours). < 3 NTU is assessed to have no risk, and < 8 NTU is assessed to have a low risk of significant impairment for clear water fish if maintained for a duration of at least 3 weeks.

Continuous exposure to 9–20 NTU was considered to carry a moderate risk of significant impairment if maintained for a duration of at least 5 days. Another reference considered slight impairment on behavioral effects to manifest after exposure to 10 NTU for 1 day, 15 NTU for 8 hours, or 23 NTU for 3 hours.

An exposure to 21 NTU or higher for over 3 hours was considered to pose a significant threat to significant impairment. Exposure to 25 NTU (for dry conditions) and 100 NTU (for wet conditions) was considered a harmful level.

Cited studies included results demonstrating that turbidity of 25 NTU resulted in reduced growth rate for juvenile coho salmon exposed for about 2 weeks. Additionally, experimental results showing a 62% reduction in brook trout growth at 40 NTU were extrapolated to estimate a 50% reduction in growth at 25 NTU. Rainbow trout exposed to 23 NTU turbidity exhibited altered behavior, including more frequent movement between cover and feeding areas, longer exposure to predators, and reduced foraging efficiency. Growth was also negatively impacted, likely due to impaired prey capture and increased metabolic demands during feeding.
Bash et al., 2001 Literature review on the impacts of turbidity and suspended solids on salmonids, with a focus on salmonids in freshwater environments. Turbidity levels can have a range of effects on salmonids. This includes gill trauma, osmoregulation, behavioral avoidance, and territoriality. However, salmonids are able to cope with some level of turbidity.

One reviewed paper suggested that salmonid populations not normally exposed to high levels of natural turbidity could be negatively affected by relatively low levels of turbidity (18–70 NTU).

Several papers reviewed suggested that even very low levels of turbidity (e.g., 6–7 NTU) had some negative impacts on welfare, e.g., reduced feeding and avoidance behaviors. In one study, coho salmon juveniles were exposed to ~60 NTU, at which point disruption of social interactions, including aggressive behaviors, was observed. Once turbidity dropped to ~20 NTU, these interactions were re-established, marking a behavioral threshold for visibility and social stability.

A number of studies indicated that aggressive behaviors decreased at higher turbidities, although fish preferred to avoid high-turbidity environments.
Sigler et al., 1984 Experimental study investigating the effects of turbidity on the preference and growth of juvenile rainbow trout and coho salmon in laboratory conditions over 14 to 21 days.

There was a treatment group that had a turbidity level of nearly 0 NTU. The exposure group was exposed to turbidities between 25 NTU and 86 NTU.		 Generally, fish in clear waters had significantly higher growth rates than fish in turbid waters. The authors noted that large amounts of suspended particles could reduce fish’s ability to see food.

Consistently fewer fish chose to stay in the turbid channels compared to the clear water channels.
Montes et al., 2018 Statistical analysis of an extensive database spanning from 1989–2016 in southern Chile, investigating harmful algal bloom thresholds for farmed Atlantic salmon. The primary focus was to quantify critical thresholds of A. catenella and P. cf. verruculosa.

Covariates included Secchi depth readings and fish behavior (where the data was recorded as ‘normal’ or ‘abnormal’).		 Secchi depth was a fixed factor in the analysis with four levels: 1–2m, 2–4m, 4–6m, and > 6m. The deeper the Secchi reading, the higher probability of observing ‘normal’ behavior.

At Secchi readings with a depth of 1–2m, there was at least a 50% chance of observing abnormal behavior, regardless of the abundance of algal cells. The authors noted that this Secchi depth should be treated as a critical threshold.

Fish had lower rates of abnormal behavior at greater Secchi depths, even where the algal cell abundance was the same. For example, at around 568 algal cells/mL, where the Secchi reading was 1–2m there was approximately an 80% probability of abnormal behavior, while where the Secchi reading was > 6m, there was only approximately a 30% probability of abnormal behavior.
Supporting evidence
  • Barrett, Jeffrey C., Gary D. Grossman, and Jordan S. Rosenfeld. “Turbidity-Induced Changes in Reactive Distance of Rainbow Trout.” Transactions of the American Fisheries Society 121, no. 4 (July 1992): 437–43. https://doi.org/10.1577/1548-8659(1992)121%3C0437:ticird%3E2.3.co;2.
  • Bash, Jeff, Cara Berman, and Susan Bolton. “Effects of Turbidity and Suspended Solids on Salmonids.” Washington State Department of Transportation, November 2001. https://www.wsdot.wa.gov/research/reports/fullreports/526.1.pdf.
  • 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.
  • 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.
  • Eaton, L., and K. A. Sloman. “Subordinate Brown Trout Exaggerate Social Signalling in Turbid Conditions.” Animal Behaviour 81, no. 3 (March 2011): 603–8. https://doi.org/10.1016/j.anbehav.2010.12.005.
  • Gregory, Jim S., and J. S. Griffith. “Winter Concealment by Subyearling Rainbow Trout: Space Size Selection and Reduced Concealment under Surface Ice and in Turbid Water Conditions.” Canadian Journal of Zoology 74, no. 3 (March 1996): 451–55. https://doi.org/10.1139/z96-052.
  • Gregory, Robert S. “Effect of Turbidity on the Predator Avoidance Behaviour of Juvenile Chinook Salmon (Oncorhynchus Tshawytscha).” Canadian Journal of Fisheries and Aquatic Sciences 50, no. 2 (February 1, 1993): 241–46. https://doi.org/10.1139/f93-027.
  • Montes, Rodrigo M., Ximena Rojas, Paulina Artacho, Alfredo Tello, and Renato A. Quiñones. “Quantifying Harmful Algal Bloom Thresholds for Farmed Salmon in Southern Chile.” Harmful Algae 77 (July 2018): 55–65. https://doi.org/10.1016/j.hal.2018.05.004.
  • Rowe, David K., Tracie L. Dean, Erica Williams, and Joshua P. Smith. “Effects of Turbidity on the Ability of Juvenile Rainbow Trout, Oncorhynchus mykiss, to Feed on Limnetic and Benthic Prey in Laboratory Tanks.” New Zealand Journal of Marine and Freshwater Research 37, no. 1 (March 2003): 45–52. https://doi.org/10.1080/00288330.2003.9517145.
  • Sigler, John W., T. C. Bjornn, and Fred H. Everest. “Effects of Chronic Turbidity on Density and Growth of Steelheads and Coho Salmon.” Transactions of the American Fisheries Society 113, no. 2 (March 1, 1984): 142–50. https://doi.org/10.1577/1548-8659(1984)113%3C142:eoctod%3E2.0.co;2.
  • Ward, David, Ken Sheehan, and Ben Vaage. “Turbidity as a Management Tool to Constrain Rainbow Trout Downstream of the Paria River.” U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, March 22, 2018. https://www.usbr.gov/uc/progact/amp/twg/2018-01-25-twg-meeting/AR14.pdf.

  1. Sorenson, D.L., M.M. McCarthy, E.J. Middlebrooks, and D.B. Porcella. 1977. Suspended and dissolved solids effects on freshwater biota: a review. United States Environmental Protection Agency, Report 600/3-77-042, Environmental Research Laboratory, Corvallis, Oregon, USA.