Think like a fish: Autumn and winter - the transition and dormant seasons

As summer fades into autumn, and temperatures continue to drop in winter, aquaculture systems face a unique set of challenges. Cooling waters slow down biological activity, alter fish metabolism, and affect stratification in ponds, which can lead to dangerous turnover events and cold-related health risks.

AUTUMN: The transition season

Autumn marks the decline in water temperature, often triggering changes in behaviour and physiology. As surface water cools, it can disrupt the stratification in deeper ponds, potentially causing a sudden mixing of oxygen-depleted bottom layers with the surface (turnover event). This sudden change can stress animals, lower oxygen levels, and increase the risk of cold-related health problems.

Key autumn challenges:

• Reduced feed intake and digestion efficiency due to cooling water

• Risk of turnover events releasing anoxic water or hydrogen sulfide

• Increased ammonia and nitrite from organic matter decomposition

• Oxygen depletion as summer algal blooms decay

Vet-recommended actions:

• Gradually reduce /cease feeding as temperatures drop (based on species)

• Monitor temperature and dissolved oxygen throughout the water column

• Remove excess organic material (e.g., sludge, leaves) to prevent anaerobic decomposition

• Prepare emergency aeration protocols for turnover-related oxygen crashes

• Increase water quality monitoring of the vertical water column

WINTER: The dormant season

In colder regions or climates, winter significantly slows biological and metabolic activity. Aquatic animals may enter a semi-dormant state, making them more susceptible to disease latency and stress from poor water quality.

Key winter challenges:

• Oxygen depletion under ice or dense surface vegetation

• Suppressed immune systems and risk of latent infections (e.g., Koi Herpes Virus, mycobacteria)

• Low appetite leading to waste accumulation from uneaten feed

• Reduced biological activity slows natural filtration

Vet-recommended actions:

• Limit or cease feeding depending on species and temperature thresholds

• Maintain minimal but consistent aeration to prevent oxygen depletion

• Avoid unnecessary handling or movement of stock

• Plan disease screening, vaccination, or treatments before winter fully sets in

Signs of trouble to watch:

• Unusual lethargy beyond normal dormancy

• Surface gasping or clustering near remaining oxygen sources

• Sudden mortalities after turnover events

• Poor water clarity or foul odours indicating decomposition

Autumn and winter demand conservative management and close observation. A well-prepared system ensures aquatic animals survive the cold and emerge healthier in spring.

Digital checklists available!
Ready to take your seasonal tracking paper-free? Use our autumn and winter checklists which are easy, interactive online forms to complete your weekly audit on any device, and get automatic records emailed to you (and your vet) for instant review. No printing, no filing—just faster, smarter farm management.

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Next in the series: Part 4: Year-round aquatic health – creating a seasonal care calendar for success