in a form which they can pick up easily: if it is more or less soluble or in the form of
free swimming bacteria then it may be of little use to the fish. It has to be precipitated in
a detrital aggregate or has to be taken up by microorganisms which themselves grow in
clumps and form larger aggregates for the fish to ingest. Whether this is in fact detritus
per se, i.e., non-living material, or whether it is aggregations of living microorganisms is
another matter. Detrital foods in ponds must be of the right consistency to be available to
fish as well as having the right protein and carbohydrate makeup.
BOWEN: In this regard, it is interesting that a study of manured pond culture of tilapia
published recently by Schroeder in Israel looked at microbial production and estimated
it. He found that even when he considered that source of nutrition for the tilapia, he
could not account for all the production of fish. It remains a great mystery how those
fish grew so well, because even if they were cropping 100% of the microbial production,
he could not account for all the growth of the fish. He suspects that there must have been
a non-living source of nutrients, i.e., organic matter derived from the manure.MORIARTY: There are a lot of problems in relating microbial production to fish growth.LOVSHIN: We have talked about various forms of detritus, and I am interested in
knowing whether detrital matter has to stay in the water and be worked on by bacteria
for some time before it is usable or whether the tilapias can use it fresh, like fresh cut
grass?BOWEN: There are two main categories of organic compounds to consider here with
respect to macrophyte material such as palm leaves, grass clippings etc.: structural com-
ponents that are insoluble in water, and non-structural components which are largely
soluble. It is the soluble compounds which are valuable for the nutrition of fish. Cellulose
and lignin are not used directly by fish, but soluble proteins, soluble carbohydrates, and
lipids, which will go in solution soon after waste plant material is put into the pond will
all have a real impact on the fish's nutrition.LOVSHIN: What type of detrital matter are you talking about here?BOWEN: I distinguish between morphous detritus and amorphous detritus. Amorphous
detritus is the material which has been formed from dissolved organic matter. Its formation
is not by fragmentation of leaves, for example, but it is via the dissolved organic com-
ponents. There are many ways in which you get from dissolved organic to particulate
amorphous organic matter. Microbial action is one. Interface reactions either at the
air-water interface or the water-mineral interface are others. Simple chemical polymeriza-
tion in solution is another. UV light also has a physical effect on polymerization. All
these factors play a role, but the importance of each of them is not understood. The
material which I have found in my studies which plays an important role in the nutrition
of S. mossambicus is the amorphous organic matterthe amorphous detritus.LOVSHIN: I think it would be a very important and very interesting line of study to see
what material we could throw in the fish pond, or what combinations of materials,
to get from these processes the maximum amount of detrital matter which could then be
utilized by the fish.MORIARTY: That is a subject that really does need to be examined in much more detail
because detritus is used as such a broad term that in different situations you can have
different processes happening.BOWEN: Schroeder's work shows (or strongly suggests) that an approach to improve
detrital feeding would be a very productive one. There is a huge body of literature now
on decomposition processes in aquatic environments, which gives some very good ideas of
where to begin searching for the manipulation which would simulate detritus formation
and give a product that has a high nutritional value.