Feeding your fish in winter

A fish is an ectotherm, its internal temperature follows that of the water to within half a degree. When the pond drops from 20 to 8 °C, its entire metabolism follows the same trajectory: heart rate, swimming speed, production of digestive enzymes, and oxygen consumption.

In practical terms, a fish that digested a meal in six to eight hours at 22 °C may take several days to process the same quantity at 8 °C. This is not a matter of appetite but of chemistry, enzymes work progressively more slowly as temperature falls.

This thermal dependence explains why no universal feeding schedule works. A mild coastal winter and a cold continental winter inland call for neither the same dates nor the same rations, and altitude shifts the schedule by several weeks further. The thermometer decides, never the calendar.

Air temperature drops far more quickly than water temperature. A 5,000-litre pond has considerable thermal inertia and typically runs two to four days behind the frosts forecast in the weather report.

Take your reading where the fish actually live, near the bottom, not in the first few centimetres of the surface, which cool down first. In winter, a difference of 2 to 3 °C between the surface and the deepest point is common in a pond one metre deep.

One reading per day, always at the same time and in the same spot, is more than sufficient. It is the trend over several days that should drive your decisions, not a single reading taken on one particular morning. A remote probe means you never have to lean over a frozen coping stone.

Below around 10 °C, the intestinal flora of cyprinids slows down and the production of proteolytic enzymes drops sharply. A fish may still swallow a pellet by reflex, but it can no longer process it.

This undigested food ferments in the digestive tract. The risk is far from theoretical, it manifests as intestinal inflammation, bloating, and in severe cases the death of the fish a few days later, often wrongly attributed to the cold itself.

The second effect concerns the water. Rejected pellets break down on the bottom, feed heterotrophic bacteria, and release ammonia into a pond whose biofilter is only running at a fraction of its capacity. The details of this mechanism are covered in our guide on the nitrogen cycle.

Wheat Germ is a plant-based food whose starch and lipids are more easily absorbed in cold water than the animal proteins found in growth pellets. This is why European manufacturers offer it in a so-called winter or all-seasons formula.

Expect a protein content in the region of 25 to 32 per cent, compared with 38 to 45 per cent in a summer food, bearing in mind that these figures vary considerably from one brand to another. The Wheat Germ floating pellet remains visible at the surface, making it easy to remove anything left uneaten after five minutes.

The switch doesn't happen overnight. Mix both foods for around a week from 16 to 18 °C onwards, this avoids a sudden refusal and gives the intestinal flora time to adapt.

In summer, the commonly cited figure is between 1.5 and 2 per cent of body mass per day, with some sources going up to 3 per cent in peak heat. The number seems high until you remember what it applies to, 10 kilograms of fish is already around ten adult koi. For a domestic pond stocked with two or three kilograms, the daily ration drops to 30 or 40 grams, a small handful.

Around 14 °C, this ration falls to approximately 0.5 per cent of body mass, and not every day. Around 10 °C, it drops further still, to one or two tenths of a per cent, a negligible quantity on a pond scale.

The practical rule remains the most reliable: feed only what is consumed within five minutes and remove any surplus with a net. If the fish do not come to the surface within that time, they are not hungry, and the next meal should simply be skipped.

In winter, the useful feeding window is short. Water temperature reaches its peak in the early to mid-afternoon, between 13:00 and 15:00 approximately, a few hours after the height of sunlight.

This is when fish are most capable of digesting food. Feeding early in the morning, when the water is at its coldest, is asking them for an effort they simply cannot make. An evening feed is even worse, as the overnight drop in temperature catches them mid-digestion.

Choose a mild, sunny day, check the temperature before reaching for the feed bucket, then offer a single feeding. Over a typical November, this often amounts to just three or four meals in total, which comes as quite a surprise to beginners used to the summer routine.

Colour-enhancing pellets, rich in spirulina and carotenoids, are designed for warm water and an active metabolism. Their high protein content makes them unsuitable below 15 °C.

The same applies to growth foods intended for young fish, to high-fat sticks, and to protein supplements such as dried larvae. Bread, lettuce and kitchen scraps never have a place in a pond, in winter least of all.

One final point concerns storage. A bag opened in June and then forgotten in a damp garage will see its vitamins degrade and its fats go rancid. Buy pack sizes suited to your winter consumption, which will be far lower than you might expect.

Sturgeon do not hibernate. Their metabolism remains active in cold water and they continue to search for food at the bottom of the pond, even under a layer of ice. Withholding food for three months, as one does with koi, weakens them durably and causes avoidable losses every winter.

They require a specific sinking food, rich in protein and fat, which cyprinids cannot intercept at the surface. Two feedings per week is a commonly recommended frequency during cold periods, to be adjusted according to the size of the fish and what you observe.

Finally, keep an area of the pond free of ice, without it, you will be unable to feed them at the very moment they need it most.

A continuous layer of ice cuts off gas exchange between the water and the atmosphere. Decomposition gases build up beneath the surface while oxygen slowly depletes, a situation made all the more dangerous when the bottom is laden with silt and dead leaves.

Maintain a permanent opening using a frost-protection bell or an air diffuser positioned at mid-depth, never at the lowest point. A low-wattage pond heater does the same job by keeping one area frost-free, without attempting to warm the entire volume. The aim is not to raise the temperature but to keep a passage open, and above all, not to disturb the warmest layer of water where the fish have taken refuge.

Never break the ice by striking it, as the shock wave can be fatal to the fish. If the opening has closed over, simply place a pan of hot water on the ice until it melts through.

In an indoor aquaponic system kept between 22 and 26 °C, the season has no influence on feeding. The fish eat at their usual rate, the biofilter remains efficient and the crops continue to take up the nitrates produced.

The real consideration then becomes lighting, since natural day length falls below ten hours, along with heating costs. An immersion heater controlled by a thermostat consumes less energy when the room is kept warm and the tank is covered at night.

Do bear in mind, however, unheated conservatories and greenhouses, which are often in between. Water temperatures there frequently drop to between 8 and 14 °C, in which case the full winter protocol applies in exactly the same way as for an outdoor pond.

Spring proves trickier than autumn. Fish emerge from winter lean, their reserves depleted and their immune systems at their lowest, at precisely the moment when pathogenic bacteria are recovering faster than they are.

Start feeding again with wheatgerm, in very small amounts every two or three days, as soon as water temperature holds steadily above 8 to 10 °C. Only return to a protein-rich feed once 15 to 16 °C is consistently confirmed, increasing the ration gradually over two to three weeks.

This is also the period when a visual health check is essential: white spots, wounds, marked weight loss. A fish weakened by winter is far easier to treat when spotted in March than in May.

The protocol is the same everywhere, but its duration varies dramatically. Around the Mediterranean and in southern Europe, water rarely drops below 10 °C and a complete rest period may last only a few weeks, or may not be necessary at all.

In temperate oceanic zones and across Western Europe, the rest period generally runs from mid-November to the end of March, with local variations of several weeks depending on altitude and pond aspect.

Further north, in Scandinavia, Poland, or the Baltic states, the fasting period can exceed five months, and depth becomes a survival criterion. A minimum of 1.20 metres is commonly targeted there, compared with 80 centimetres to 1 metre in a temperate climate.