Fish Are Dying or Feeding Less: A Safe First-Response Checklist for Aquaculture Farms

Short answer: when fish die or stop feeding, assume two possible causes at once — an environmental or system failure (oxygen, temperature, water quality, equipment) and an infectious disease — until a qualified professional has ruled one out. Record what you observe, check the readings your normal operating plan already collects, pause non-essential movement of fish and equipment, protect unaffected units, and call an aquatic animal-health professional or diagnostic laboratory. Where national rules require it, notify the competent authority.

Educational guide, not a diagnosis or treatment plan. Reviewed 13 October 2026; technical details must be confirmed for your species, system and country by a qualified aquatic animal-health professional. No single sign identifies a specific pathogen, and this guide gives no medicine or chemical doses.

Why consider both environment and disease?

The same visible change — fish gathering at the surface, reduced appetite, lethargy or a rise in mortality — can follow a dissolved-oxygen drop, a temperature swing, a pump or filter failure, or an infection. FAO's aquatic health and biosecurity programme frames health management as preventing, detecting and responding to both kinds of risk (FAO), and NOAA notes that fish health depends on the environment, husbandry and pathogens together (NOAA Fisheries).

What should be checked in the first hour?

• System alarms and equipment: aeration or oxygen supply, pumps, filters, power, heaters or chillers.
• The water-quality readings your operating plan already measures (for example dissolved oxygen, temperature, pH, ammonia, nitrite or salinity) — record the time and the unit tested.
• Which tanks, ponds or cages are affected, and which are not.
• Recent events: stocking, transfers, grading, handling, feed changes, new water source or weather.

What should be recorded?

Write facts, not conclusions. A dated, signed record helps the professional and any authority understand the event and avoids losing details later. Use the one-page incident log below.

One-page incident log

How do you protect unaffected units?

• Pause non-essential movement of fish, water, nets and equipment between units where appropriate.
• Work from unaffected to affected units, with dedicated or cleaned equipment, boots and clothing.
• Remove mortalities promptly and store them as your professional or laboratory instructs, in case samples are needed.
• Limit visitors and vehicles until the cause is understood.

These are general biosecurity principles described in FAO's smart biosecurity work (FAO); the exact measures depend on your species, system and local rules.

Which professional or authority should be called?

Reporting duties differ by country. In the United States, for example, suspected reportable diseases go through a veterinarian to USDA APHIS and state officials (USDA APHIS). For other countries, check your national veterinary service and the listed diseases in the WOAH Aquatic Code (WOAH).

What should a farm avoid doing?

• Do not assume one sign identifies a specific disease.
• Do not start medicines, antimicrobials or chemical treatments without a professional diagnosis and local approval.
• Do not move or sell fish from affected units before taking advice.
• Do not discard the incident record once the event ends — it is part of your farm's health history.

How can planning reduce the next event?

Many events trace back to design margins: oxygen supply, backup power, water exchange and separation between units. Check those margins with the oxygen demand calculator, the water consumption calculator and the biosecurity zoning guide. For a project of USD 250,000 or more, you can describe the upgrade in a structured project request; our team reviews it and compares suitable options. Planning tools do not predict or guarantee health outcomes.

FAQ

Can I tell which disease it is from the symptoms?

No. Similar signs can come from water-quality problems or from different infections. Confirmation needs a qualified professional and, usually, laboratory testing.

Should I treat the fish straight away?

Not without a professional diagnosis. Unnecessary treatment can harm fish, staff and the environment and may break local rules on medicines.

Do I have to report the event?

It depends on the suspected disease and your country's rules. Ask your professional and check with the competent authority.

Sources: FAO aquatic health management and biosecurity; FAO smart aquaculture biosecurity; NOAA Fisheries — aquaculture fish health; FAO aquaculture biosecurity pathway. Last reviewed 13 October 2026.