106
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
assimilated food (A) are to be determined during a food balance experiment,
because the digestion of a food organism by the animals proceeds selectively.
For such experiments, the food objects, planktonic algae, bacteria, or small
animals, must be grown on labeled food, e.g., they must increase their biomass
several times when assimilating the labeled carbon source C 4 COZ for algae,
14C for labeled protein hydrolysate for bacteria, dc.). To determine the
other feeding parameters above, which might be characterized by relative
values of water clearance, ingestion, or assimilation, this stipulation does not
apply. Better results will be achieved using as evenly labeled food objects as
possible in the experiments. The best criterion for all kinds of feeding parameters, based upon the determinations of comparative intensity of feeding by
animals with labeled food, appeared to be the assimilation index, CalC. This
corresponds to the percentage of the amount of carbon of labeled food which
could be incorporated into the body material of the consumer per 24h, to the
mean carbon content in the consumer's body (Sorokin 1968).
Research aimed at obtaining complete trophic characteristics of a given
hydrobiont species should begin with estimating the assimilation index, CalC.
First, it is measured for selection of probable kinds of labeled food shown by
preliminary observations of gut contents and food habits of the species under
investigation, as sources for feeding under native conditions. As a result, the
real feeding spectrum is based on the solid background of the assimilation
index (CalC). The following step of the research is to estimate the optimal concentration of the kind of food, indicated by previous determination of the food
spectrum, as a real feeding source of the given consumer. For this, the CalC
indexes are measured at a range of concentrations, from 0.05 up to 20mgl- 1
(in units of wet biomass) for a filter feeder. The optimal concentration is indicated by the point at which the curve of the dependence of CalC on food concentration levels out. The balance experiments are accomplished with the use
of an also known optimal kind of labeled food at an also known concentration. During this experiment the basic parameters of feeding are established,
expressed as absolute values of mg C Sp-l day (or h- 1 ): the food ration (C), the
assimilated food (A), and the nondigested part of food (F). All these parameters are parts of the nutrition equation: C = A + F, or C = P + M + F, if A
= P + M, where P is the expenditure of assimilated food for production
(growth), and M is its expenditure for metabolic losses (respiration). If the
latter value is known, the value of P is calculated as the difference A-M. Then
the growth efficiency coefficients KJ and K2 are calculated: KJ = PIC, and K2
= PIA. The value of M is measured separately, using either the oxygen bottle
or other respiration method. It is possible to measure it in a separate experiment. It cannot be estimated simply by measuring the radioactivity of 14COZ
respired by the animal in the balance experiment itself, because the Cr in this
case will be different from that of the ingested food.
The next step to deriving the trophic certificate for a given consumer is to
estimate the threshold concentration of food by measuring the dependence of
the absolute assimilation rate A (mg CSp-l h- 1 ) upon the concentration of the
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
assimilated food (A) are to be determined during a food balance experiment,
because the digestion of a food organism by the animals proceeds selectively.
For such experiments, the food objects, planktonic algae, bacteria, or small
animals, must be grown on labeled food, e.g., they must increase their biomass
several times when assimilating the labeled carbon source C 4 COZ for algae,
14C for labeled protein hydrolysate for bacteria, dc.). To determine the
other feeding parameters above, which might be characterized by relative
values of water clearance, ingestion, or assimilation, this stipulation does not
apply. Better results will be achieved using as evenly labeled food objects as
possible in the experiments. The best criterion for all kinds of feeding parameters, based upon the determinations of comparative intensity of feeding by
animals with labeled food, appeared to be the assimilation index, CalC. This
corresponds to the percentage of the amount of carbon of labeled food which
could be incorporated into the body material of the consumer per 24h, to the
mean carbon content in the consumer's body (Sorokin 1968).
Research aimed at obtaining complete trophic characteristics of a given
hydrobiont species should begin with estimating the assimilation index, CalC.
First, it is measured for selection of probable kinds of labeled food shown by
preliminary observations of gut contents and food habits of the species under
investigation, as sources for feeding under native conditions. As a result, the
real feeding spectrum is based on the solid background of the assimilation
index (CalC). The following step of the research is to estimate the optimal concentration of the kind of food, indicated by previous determination of the food
spectrum, as a real feeding source of the given consumer. For this, the CalC
indexes are measured at a range of concentrations, from 0.05 up to 20mgl- 1
(in units of wet biomass) for a filter feeder. The optimal concentration is indicated by the point at which the curve of the dependence of CalC on food concentration levels out. The balance experiments are accomplished with the use
of an also known optimal kind of labeled food at an also known concentration. During this experiment the basic parameters of feeding are established,
expressed as absolute values of mg C Sp-l day (or h- 1 ): the food ration (C), the
assimilated food (A), and the nondigested part of food (F). All these parameters are parts of the nutrition equation: C = A + F, or C = P + M + F, if A
= P + M, where P is the expenditure of assimilated food for production
(growth), and M is its expenditure for metabolic losses (respiration). If the
latter value is known, the value of P is calculated as the difference A-M. Then
the growth efficiency coefficients KJ and K2 are calculated: KJ = PIC, and K2
= PIA. The value of M is measured separately, using either the oxygen bottle
or other respiration method. It is possible to measure it in a separate experiment. It cannot be estimated simply by measuring the radioactivity of 14COZ
respired by the animal in the balance experiment itself, because the Cr in this
case will be different from that of the ingested food.
The next step to deriving the trophic certificate for a given consumer is to
estimate the threshold concentration of food by measuring the dependence of
the absolute assimilation rate A (mg CSp-l h- 1 ) upon the concentration of the