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Breathing Underwater: The Most Important Findings From Our Dissolved Oxygen Study

Updated: Jan 19

Fish Welfare Initiative recently conducted a study where we observed six fish farming ponds with known dissolved oxygen (DO) issues. 

The aim of this two-month observational study was to understand the depth of DO challenges and unearth the potential factors driving these conditions. It offers our most detailed window yet into some of the worst suffering that happens in Indian aquaculture farms.

FWI data collectors Manikanta and Ghandi collecting data for the study.

Key Findings

  • Water Quality: DO was within acceptable only ranges 23% of the time in the morning and 32% of the time in the evening. Indian Major Carp (the species of fish being farmed in this study) are able to tolerate poor DO levels, but the persistence with which poor DO was experienced likely meant that fish were significantly stressed, as they were unable to recover. This was made worse by the prevalence of other water quality issues such as ammonia (42% within acceptable levels) and pH (63% within acceptable levels).

  • Biomass: A surprisingly strong relationship was observed between fish biomass and DO levels. Higher biomass, particularly noted before “harvesting” (when fish are taken from farms to be sold to market), was often linked with lower DO levels. This relationship highlights the toll that overstocking can take, as fish consume oxygen in higher amounts than the system is able to generate.

  • Phytoplankton: Phytoplankton blooms also appeared to be a cause of DO issues, though it appeared to be less important than fish biomass. 

  • Gasping: Notably, fish gasping events – a dire sign of oxygen deprivation – tended to occur when DO levels dipped below 1 mg/L and ammonia levels increased above 0.5 mg/L (which happened 6 times during the study). This is evidence that these two stressors combine into one of the most harmful experiences for fish.


These results tell us a few things. A deeper assessment of these fish farming ponds with known DO issues shows that fish likely suffer perpetually in such conditions, both from DO itself and from further water quality problems (ammonia especially), which in concert with DO are likely very stressful for fish. More speculatively, we may compare this to severe respiratory problems in humans (such as those caused by a motor neuron disease), where “The person has great difficulty walking even short distances or climbing any stairs, feels tired when at rest, and is anxious” (The Institute For Health Metrics And Evaluation gave this condition a Disability Weight of 0.4).

These findings help to confirm our beliefs that more attention is needed on these farms, and shows that not all fish farming ponds are equal in their level of suffering caused.

Also, these results seem to show that FWI might have been wrong to focus as much on phytoplankton, and that biomass may also need to be a key part of improving DO levels. We will continue to think more about the implications this should have for the Alliance for Responsible Aquaculture and for our R&D department.

We believe these findings highlight the importance of a deep understanding of what animals are experiencing on farms. Even now after two years of operation in India, we are still learning vital information on how we may best help fish on Indian farms. 

If you are interested in seeing our comprehensive insights, you can read our full report here. Thank you to all of those who made this research project possible.

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