I got into a discussion about koi and algae consumption on one of the other boards and found I had limited success in changing minds regarding koi’s ability to digest algae. It was mentioned that koi feces shows no, or little digestion of algae that passed thru the koi. And it was suggested that learned minds felt algae was not part of a carp’s diet. So how can this be?

Well, a good part of these misunderstandings came from snippets of information and anecdotal conclusions. And to a great degree, I blame the aquaculture industry with it’s myopic focus on growth in the shortest period of time, for confusion regarding what a koi IS.

If you were to study the contents of carp stomachs from around the world, you would think it would become clear just what it is that wild carp eat! But it doesn’t- especially IF you focus only on WHAT a local carp population is feeding on as a primary source of protein. In clam and cockle infested waters , for instance, carp feed heavily on— you guessed it- clams and cockles! In areas were zebra mussels are a problem- carp are right there eating the B-jesus out of those mussels! And you can find lots of locales where carp feed on crustaceans as a main protein. But if you step back and look at the forest thru the trees, you will quickly realize that koi and cousin carp are :
1) True omnivores
2) true scavengers.
3) non specific in their diet items or ‘ Generalists’ as opposed to specific in their sources of protein or prey item.

This, in fact, is the second secret of carp’s amazing ability to adapt. If temperature and ‘less than ideal’ water condition are one and two, then certainly being a Generalist and a Scavenger is their third greatest adaptation gift.

So here is the punch line- koi get a large amount of their diet from decaying vegetable/ plant and animal matter. And often this is reduced to the general category of detritus type food. Meaning decayed algae, decayed plant and dead bacteria, fungi and dead or borrowing animal plankton. When a koi is ‘working the mud’ it is deriving small but continual flow of micro detritus in various stages of mineralization or decay. And studies have shown that in some cases ( setting and season), 60% of the diet is from this detritus mix.
In the case of algae, we see a primitive ‘plant’ that grows and dies constantly. And it’s leaking content is then recovered by the rest of the biomass within a system. But this is not a light switch. The gradual decomposition attracts the carp to the scent of valuable nutrition ( via the barbels) . And grazing produces a smorgish board of microbes, broken down algae walls and associated fungi.

This is the real secret of mud pond effect on koi I think. It was made very clear at the last quality koi farm harvest when we saw all the ‘mouth holes’ visible in the drained mud ponds. It really made a great visual of just what those koi are doing in those murky waters all season.

It might interest some to know that one reason common carp were released in open water in America in the early 1900s was to control plant growth. They do this two ways- 1) by uprooting the plants . And 2)direct ingestion of plant parts- and not always just once! Because a portion will pass through the fish and become part of the detritus layer, the plants can be, and are, eaten a second time or more!! This transfer of nutrition then reduces the amounts available for things like duck weed and eventually make water and it’s food source , unavailable for water fowl. This is then the down side of having carp manage your food chain.

And finally, recent studies show carp , as true opportunist omnivores and generalist scavengers, as having a gut that houses SPECIAL and SPECIFIC bacteria for processing plant material and managing detritus in plant form. In addition, one powerful gut enzyme is specific for breaking down cellulose, the hard waxy protective material in cell walls of most plants.

In the model of algae , picture this- decaying and live sunken algae being broken down by bacteria as a normal mineralization process- the fish ingesting that material as general detritus, its bodies bacteria breaking it down further with raw enzymes assisting and finally, if that is not enough– a second ‘ ride’ through the system! Carp are up there with rats, house sparrows and cockroaches as one of the worlds best and most adaptable survivors! Long live the Carp!

-JR