Fish kills - causes and what to do
The sight of dead fish on the surface is a signal that something serious is happening with the water. There can be several causes, and quick, accurate action can limit the losses - and help establish what happened.
The most common causes
- Oxygen deficiency (an oxygen crash) - the most common cause. It appears in heatwaves, windless nights, after a bloom collapses, and under the ice in winter. See oxygen monitoring and oxygen crashes.
- Poisoning - a sewage discharge, run-off from fields (pesticides, fertilisers), toxic substances; often sudden and affecting various species/sizes.
- Cyanobacterial toxins - during an intensive bloom.
- Sudden changes in conditions - temperature, pH, a sudden inflow of different water.
- Diseases and parasites - usually slower, selective.
What to do first
- Safety - do not enter the water if you suspect poisoning; protect animals and people.
- Oxygenation (in farm ponds) - if it is an oxygen crash, aeration/letting in fresh water can save part of the stock.
- Document - note the time, place, weather, symptoms, take photos; if you can, measure oxygen, temperature, pH and take a water sample (see sampling).
- Report - if you suspect pollution/poisoning, notify the appropriate services (including the environmental inspectorate, the water manager, and in case of danger - the emergency services). A prompt report increases the chance of identifying the culprit.
How to establish the cause
What says the most is measurements taken at once: very low oxygen points to an oxygen crash; an unusual pH/conductivity or a smell - to a discharge. Context helps (weather, events upstream) - see interpreting results.
How to prevent it
- Monitor oxygen in periods of risk (summer, winter under the ice),
- limit the inflow of nutrients (fewer blooms),
- in ponds - provide aeration and do not overload the stock.
In practice
The best protection is early warning. LimnoLog with an oxygen sensor and a threshold alert can notify you before the situation becomes critical - and a full set of measurements serves as documentation of the event.
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