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.
Fish weight(g)
Temperature © |
>5
|
5 - 20
|
20 -50
|
50 -
100
|
100 -
300
|
300 -
1000
|
<17
|
6
|
5
|
4
|
3
|
2
|
1.5
|
17 - 20
|
7
|
6
|
5
|
4
|
3
|
2
|
20 - 23
|
9
|
7
|
6
|
5
|
4
|
3
|
23 - 26
|
12
|
10
|
8
|
6
|
5
|
4
|
>26
|
19
|
12
|
11
|
8
|
6
|
5
|
(This link is great for determining a close guesstimate of
fish weight. http://www.cnykoi.com/calculators/calclenw.asp)
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.
