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To Feed or Not To Feed

To feed, or not to feed. That is the question.
(along with how much, how often and when)

The seemingly simple act of feeding one’s pond fish, whether they be Koi, Goldfish, Orfe or other variety, can quickly become confusing when one is bombarded with admonitions such as- “You are feeding too much!”, “You are feeding too little!”, “Too often!”, “Not often enough!”, “Wrong time of day” ad nauseum et confusium.

Perhaps if we were to take the time to examine a few basic facts surrounding the digestive processes and natural feeding behavior and food preferences of fish, the solution may reveal itself.

The focus here will be placed on Koi/Carp (Cyprinus carpio) given that this is presently the most popular of pond fish.

When examining the digestive system of a Koi/Carp, we are immediately presented with a glaring physiological difference between Koi/Carp and most other fish. Koi/Carp do not have stomachs! They instead have just one continuous gut. In a natural environment, food seems to be ingested in small particles in a relatively steady stream instead of intermittently in large units, so the storage function of a stomach probably is not missed. Digestion takes place in this intestine, which is twice the length of its body. Different enzymes are secreted by the pancreas along the intestine. The system works best if it is able to process a steady supply of food. This is why Koi/Carp are constantly rooting around for something to eat. At the back of the mouth are the pharyngeal teeth. These are aligned in three rows on each side of the mouth, with three teeth in the innermost row and one tooth each in the two outer rows. The teeth are positioned below a tough calcified pad. Food is ground between it and the pharyngeal teeth before being passed to the first part of the gut called the esophagus.

Koi/Carp cannot digest all foods efficiently. They might eat bread and sweet corn, but it is difficult for them to digest them. Fibre, especially grass and alfalfa meal, is important as it allows the intestine to grip the food and keep it moving. Worms, insect larvae and fresh vegetable matter are only partly digested and serve as roughage to keep the intestine active and healthy. Koi/Carp will also ingest silt and other indigestible detritus to bulk out their diet.

Koi/Carp is an agastric fish, meaning absorption of nutrients is highly dependent on the rate of feed passage in the gut. Variables which influence gastric evacuation time, along with feeding rate, include temperature, season, activity, body size, gut capacity, satiety, and metabolic rate. The gut transit time (length of time the food takes to pass through the gut), just based on temperature, varies greatly from 16 hours at 25ºC/77F to 60 hours at 12ºC/54F.

Koi/Carp is also an omnivorous fish that possesses excellent sight, extremely excellent hearing, highly developed taste and, according to some, a sensing mechanism akin to what bats use so effectively. They also possess an excellent memory. Once they have a negative experience with a food item, they will avoid it at all future times.

Research has shown that, even though they are continuously grazing and rooting around for tidbits and tasty morsels, Koi/Carp possess a natural feeding rhythm. The research was conducted under temperatures ranging from 4C/39F to 34C/93F using both juvenile (61g – 91g) and adult (630g – 850g) fish. There were two periods that main feeding activity occurred: 8:00 – 11:00 a.m. and 19:00 – 23:00 p.m. with adults consuming 27% and juveniles consuming 66% of their total daily intake in the two combined time periods. This would seem to indicate that while a certain time period feeding preference is evident in young Koi/Carp, this preference wanes with age.

More importantly this study showed the effects of temperature on feeding behavior. Both adult and juvenile Koi/Carp reached the maximum daily feeding rate at 28°C/82F, being 2.84% and 12.06% of body weight, respectively. At 14C/57F, the daily feeding rate of adult and juvenile reached 1.21% and 2.63% of body weight, respectively. At 6°C/43F, though both adult and juvenile would still feed, the feeding quantities were only 0.01% and 0.35% of body weight. The daily feeding rate of adult and juvenile reduced suddenly after reaching 34°C/93F and the daily feeding rate was only 0.74% and 9.45% of body mass, respectively.

Separate research was conducted to determine the effects of feeding frequency on final weight, daily weight gain and feed efficiency. Adult Koi/Carp were fed over a period of 82 days using three (3) feeding strategies – 1) hand feeding twice a day to apparent satiation, 2) continuously by means of an automatic feeder over 2 x 6 hour periods and 3) continuously feeding a fixed rate of 1.4% body weight using an automatic feeder.

Strategies 2 and 3 improved protein digestibility, whereas only feeding strategy 3 increased organic matter digestibility.

Have we arrived at an answer to the original Title question – To feed or not to feed?

Yes, but not entirely. We know that we must take certain factors into consideration.

1. Age of fish – Young fish need to consume a higher percentage of their body weight daily in order to sustain healthy growth. As the fish age, growth becomes less of a factor and the feed demand is reduced.

2. Temperature – The metabolic demand greatly diminishes as temperatures are reduced. Digestion rate is also drastically reduced as evidenced by longer gut transit time.

The chart below lists suggested daily feeding levels (% of body weight) based on temperature and body weight.
feeding chart.jpg

(This link is great for determining a close guesstimate of fish weight.

3. Frequency – It has been shown in laboratory tests that continuous feeding is most beneficial and more closely coincides with natural feeding patterns. Unless one is willing to purchase a programmable automatic feed dispenser, the continuous feeding option is an impossibility, but the greater the number of feeding periods that one can manage the more the fish will benefit.

Beyond the three aforementioned factors, the answer becomes decidedly muddled. One must also take into consideration:

Type of pond – Classic Koi or Ecosystem or some hybrid style. The more closely that a pond emulates a true ecosystem, the greater the abundance and availability of natural food sources such as phytoplankton, zooplankton and benthic organisms and consequently the lesser need for supplemental feeding.

Stocking levels/Fish load – Here the problem is not overfeeding the fish, but overfeeding the entire system. Unless an exceptional capacity for bioconversion is available, an overstocked pond cannot accommodate the additional organic matter generated by adequate feeding levels. In addition, soon after feeding, the fish experiences an increased Oxygen demand quickly followed by an increased excretion of Ammonia. Neither of these natural occurrences bode well for an overstocked pond.

Every pond is unique, so fish feeding practices must be custom tailored to each pond. There is no ‘One size fits all” approach to proper feeding, but following the above guidelines, if you are willing to expend a little extra time and effort, can get you pretty darn close to what is best for the fish and the pond.