The immune gap in fish is not a buzzword but a gap in the calendar. In November the water in your tub cools faster than the immune system of your medaka can keep up, while bacteria like Aeromonas simply keep dividing. If you know that window, you will not stand in November in front of a tub of fish that suddenly fall ill.
Medaka has a reputation that works against this. The Japanese rice fish (Oryzias latipes) was described in 1846 by Temminck and Schlegel and survives outdoors in the Netherlands, even in an unheated tub. That hardiness is right, but it applies to cold that comes slowly. The tricky week is the one in which the temperature drops to six degrees for three nights and the fish gets no time to adjust.
What the immune gap in fish actually is
Between roughly 10 and 15 °C two things happen at once. Immune defence in a fish runs on enzymes, and those slow down as the water gets colder. Aeromonas and related bacteria are hardly affected: at 12 °C they still divide, just more slowly than at 22 °C. The bacterium loses a little, the fish loses more.
That ratio is the immune gap in fish. It is not a disease and not a pest, but a difference in speed that lasts a few weeks, roughly from early November until the first real frost. In a 200 litre tub it plays out more sharply than in a 3000 litre pond, because a small body of water cools much faster at night and warms again during the day.
Why Aeromonas waits until the water is cold
You do not bring Aeromonas hydrophila in with a new fish; it is already in the water and on the slime layer of your fish, often for months without anything happening. It is opportunistic and attacks when the immune defence briefly drops. That makes diagnosis hard, because the cause lies weeks before the first fin rot.
- white edges or frayed tissue on the tail and dorsal fins;
- red or open patches on the flanks, sometimes with a white edge;
- a swollen belly while the fish has not been eating;
- rubbing along the wall, or hanging motionless in a corner;
- gills that look pale and move faster than normal.
One symptom says little, three at once say a lot. In that case test ammonia and nitrite first before you pour anything into the water. A nitrite spike after a period of heavy feeding gives the same signals as an emerging infection, and a bactericide solves nothing then.
How cold water suppresses medaka immunity
In a cold-blooded fish the body temperature follows the water, and with it the speed of everything the immune system does. White blood cells move more slowly to a wound, antibodies are produced more slowly, and the mucus layer that protects the skin thins out. The fish is not defenceless, it is simply slower than the bacterium.
We know this about this fish not from the tub but from the lab. Medaka is a model organism in genetics, and since its genome was published in 2007, its immune system has been one of the best described of any fish. The cold sensitivity of that system is therefore not pond talk.
What makes it harder is the daily rhythm. A clear November night cools a shallow tub to 8 °C, and a sunny afternoon brings it back to 15 °C. Every swing costs energy that goes to temperature regulation and not to immune defence. A few days the fish can sustain that; after that it can't.
Stress belongs to the same story. Cortisol, the hormone released under stress, suppresses immune defence directly. Netting, moving, a tub that is refreshed all at once, a cat that sits on the rim for hours: small events with a large effect in a month when the margin is already small. More about the species and its origin is at Wikipedia and in the species profile on FishBase.
Feeding and stress: the two levers you control yourself
The digestion of a medaka slows down along with the water. If you feed as much in November as in July, part goes through undigested and comes out as waste. That waste is food for the bacteria you least want to feed, and it consumes oxygen from the water. Overfeeding is one of the most common causes of such a peak this month.
Stopping is not necessary, tapering off is the art. As long as your fish come up to the surface actively, they get food; as soon as they slow down and linger on the bottom, the portion goes down. Below 10 °C a small amount once a week is enough, and in frost feeding can stop completely. A healthy medaka does not starve in winter, a polluted tub does.
The second lever is calm. Every action you could take in September without thinking costs more in November. A fish you move to a tub four degrees warmer stays on alert for two days. Do that kind of thing once, properly, in the morning, and not on a day when you are going to change the water anyway.
Your tub through November: what you do weekly
A fixed round of a quarter of an hour a week does more against the immune gap in fish than intervening once a dead fish is lying there. That round runs from source to consequence: first the water in which the bacteria sit, then the food, then the fish themselves.
- Remove leaves, dead plants and food leftovers before they sink to the bottom.
- Test ammonia and nitrite, and note the value with the date beside it.
- Change 20 to 30% of the water, with water of roughly the same temperature.
- Measure the temperature in the morning and in the afternoon; more than 4 °C difference in one day is a signal.
- Look at each fish separately for fin edges, flanks and behaviour, not the tub as a whole.
- Provide shelter if the tub is open and shallow, so cold nights hit less hard.
- Write down what you changed. Without notes every autumn is guesswork again.
| Approach | Effect on bacterial pressure | Cost to the fish | Sensible in November |
|---|---|---|---|
| Keep feeding as normal | Feeds Aeromonas | Nothing in the short term, a lot in the long term | No |
| Halve the portions | Less waste, less division | Nothing | Yes |
| Full water change | Lower for a moment, then back | Temperature shock and stress | No |
| Change a third | Stays low | Limited | Yes |
| Adding salt | Slightly slows division | Burdens plants and fry | Only on advice |
In the middle of the immune gap one intervention is extra risky: emptying a tub in one go because one fish is ill. That removes the bacterial film that kept the water stable, and puts the rest of the stock into a fresh system with no defence in the filter. If in doubt about a treatment, a water sample to a vet is more useful than a treatment on a guess.
What helps a healthy medaka through winter
The immune gap in fish cannot be closed, because you cannot let the water cool without the immune defence moving with it. What you can do is keep the gap narrow: little waste in the water, no temperature shocks, and fish that are not extra stressed in the weeks when they are least able to cope. None of those three costs money.
The gain lies in the order. Whoever starts tapering off in the first week of November keeps fish in November that swim clearly and have good colour. Whoever waits until the first fin rot is visible is treating a tub instead of a fish. By then the bacterium is everywhere, and the question is no longer whether anything will be lost, but how much.
Further reading
- Algae in the medaka pond: what 16 years of research teaches
- Removing sludge before winter: this is your last chance
- Caring for medaka through the seasons
FAQ on the immune gap in fish in autumn
Is the immune gap the same as an Aeromonas infection?
No. A gap is the period in which the immune defence lags behind the bacterium, and the infection is what happens when the bacterium uses that room. You can have a gap without sick fish, but not an infection without a gap.
At which water temperature is the risk greatest?
Around 12 °C, in the band of 10 to 15 °C. The bacterium is still active there and the immune defence of your medaka is already working at half speed. Once the water drops below 8 °C the division rate falls further and the window narrows.
Should I stop feeding completely in November?
No, tapering off is enough. As long as the fish come up actively, they may have a small portion. Stopping feeding only becomes relevant with persistent frost or when the fish no longer react to food.
Does salt or a bacterial treatment help in autumn?
Salt slows the division of Aeromonas slightly, but burdens water plants and fry, and it does not close the gap. A product in the water does little as long as the source stays put: leftover food, dead leaves and a filter that has not been cleaned. Deal with that source first.
Why do fry and old fish get sick sooner?
Fry have not yet built up a full immune defence and are still growing, which costs extra energy. Old fish have less reserve. In November both groups have less margin than a healthy fish a year old.
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