Dear Johansen,
Your problem is quite similar to the one I encountered with tilapia
Oreochromis niloticus, an omnivorous-detritivorous fish. I believe it's a
most general problem in fish feeding behavior studies. The way I solved it
was inspired using enclosed bibliography. I considered stomach content
being composed of two components. A particulate one that can be easily
identified by optical microscopy, and an unparticulate component, which
remains indetermined.
If you need qualitative composition of the indeterminate componant, you can
use immunoassays as described by Feller. I didn't use these techniques,
because I couldn't (I was working in Africa, without equipment), but it
should be valuable.
Bowen adapted classical nutrition technics. I considered two indicators of
food quality : energy content, and amino acids. In short, you can convert
your optical measurements into energy or AA, using biovolume/energy and
biovolume/AA ratios. You also can measure this parameters on the stomac
content in its whole. By substracting the values for the stomach content in
its whole to values for the particulate components, you get as estimate of
the composition of the unparticulate component. Then, you can compare this
value to the natural feeds in the environment and to the fish growth.
With all these results you are able to take into account this undetermined
component of the food content, whose feeding importance may be very high.
Your's cordialy
Lionel Dabbadie
Ahlgren M.O., 1990 (a). Nutritional significance of
facultative detritivory to the juvenile white sucker
(Catostomus commersoni). Can. J. Fish. Aquat. Sci. 47 :
41-48.
Ahlgren M.O., 1990 (b). Diet selection and the contribution
of detritus to the diet of the juvenile white sucker
(Catostomus commersoni). Can. J. Fish. Aquat. Sci. 47 :
49-54.
Ahlgren M.O., Bowen S.H., 1991. Growth and survival of
tadpoles (Bufo americanus) fed amorphous detritus derived
from natural waters. Hydrobiologia 218 : 49-51.
Ahlgren M.O., Bowen S.H., 1992. Comparison of quantitative
light microscopy techniques used in diet studies of
detritus-consuming omnivores. Hydrobiologia 239 : 79-83.
Bowen S.H., Lutz E.V., Ahlgren M.O., 1995. Dietary protein
and energy as determinants of food quality : trophic
strategies compared. Ecology 76 (3) : 899-907.
Dabbadie L., 1996. Etude de la viabilite d'une pisciculture
rurale a faible niveau d'intrant dans le Centre-Ouest de la
Côte d'Ivoire : approche du reseau trophique. These doct.,
Univ. Pierre et Marie Curie, Paris 6, France : 208 p.
Feller R.J., 1984. Serological tracers of meiofaunal food
webs. In : Heip C. (ed). Developments in hydrobiology, vol.
26. Biology of meiofauna. Actes coll., Ghent, Belgique,
16-20 août 1983. Dr W. Junk (Publ.), Dordrecht, Netherlands
: 119-126.
Feller R.J., 1992. Potential applications of immunoassays
in studies of flatfish recruitment. Netherlands J. sea
res. 29 (1/3) : 1-5.
Feller R.J., Ferguson R.B., 1988. Quantifying stomach
contents using immunoassays a critique. In : Yentsch,
C.M., Mague F.C. et Horan P.K. (eds). Lecture notes on
coastal and estuarine studies, 25. Immunochemical
approaches to coastal, estuarine and oceanographic
questions. Actes coll., Portland, USA, 5-7 octobre 1986.
Springer-Verlag (Publ.), New York, Etats Unis (399 p) :
295-303.
Getachew T., Bowen S.H., Abebe E., Tadesse Z., soumis
pour publication. Seasonal effects on diet quality for
phytoplanktivorous Oreochromis niloticus in Lake Tana,
Ethiopia. J. Environ. Fish. Biol. : 18 p.
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