138
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
definite concentrations of labeled food are prepared, the animals are washed
free of the unlabeled food and placed into two of them, one remaining as the
blank. When the incubation time ends, from each experimental vessel three
parallel subsamples of 1O-30ml are taken to estimate the radioactivity of the
food. The same volume is taken from the blank vessel, but at an intermediate
time in the incubation. All the subsamples are filtered through appropriate
membrane (or GF) filters. The latter are placed into scintillation vials and
radioassayed. The rate of filtration is then calculated as described above (see
Sect. 3.5.7.1).
3.6 Measurements of Absolute Rates of Feeding,
Food Assimilability, and Respiration
3.6.1 General Considerations
The main problem in aquatic trophology is the correct estimation of food
assimilability by aquatic animals, e.g., the percentage ratio of the assimilated
food (A) to the food ration (Q): the assimilability is calculated as I = AIQ, %
(Winberg 1964; Rigler 1971). Once the assimilability is established for a given
consumer foraging on a given kind of food, other important parameters of
nutrition, such as the amount of assimilated food (rate of assimilation A) and
the growth efficiency coefficients KJ and K2 might be estimated even without
the use of the radioisotopic technique or with its use employing versions which
do not demand uniformly labeled food organisms. In this case (if the value of
I is known), the food rations (Q) might be estimated either during the determination of the filtering rates for filterers or the grazing rate for other animals.
But (more easily) they could be estimated during short-term experiments with
the use of food labeled at the researcher's convenience, because for the estimation of food rations even labeling is not obligatory. It is necessary only to
know the Cr value of the whole body material of the food organisms. Thus: Q
= Rc x Cr!lg C Sp.-l h- 1 (see Sect. 3.5.2). Having estimated the food rations by
using these simple approaches, the assimilated food then is calculated: A = QII
lOO!lgCsp.-lh-1. The nonassimilated part of ingested food (F) is equal to the
difference: F = Q - A. With the known values of A and of the M (respiration
rate) values measured separately, the expenditure of food for growth G
(production) can be calculated as follows: G = A - M. Then secondary production coefficients KJ and K2 are calculated as the ratios KJ = Gle and K2 =
GIA.
The above considerations show quite clearly that the adequate estimation
of the assimilabiity index I is the key point in all trophological studies. Its estimation opens the way for a rather simple procedure to evaluate the balance
parameters of the expenditure of consumed food, which forms the background
for any holistic analysis and modeling in aquatic ecology. The exact estimation
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
definite concentrations of labeled food are prepared, the animals are washed
free of the unlabeled food and placed into two of them, one remaining as the
blank. When the incubation time ends, from each experimental vessel three
parallel subsamples of 1O-30ml are taken to estimate the radioactivity of the
food. The same volume is taken from the blank vessel, but at an intermediate
time in the incubation. All the subsamples are filtered through appropriate
membrane (or GF) filters. The latter are placed into scintillation vials and
radioassayed. The rate of filtration is then calculated as described above (see
Sect. 3.5.7.1).
3.6 Measurements of Absolute Rates of Feeding,
Food Assimilability, and Respiration
3.6.1 General Considerations
The main problem in aquatic trophology is the correct estimation of food
assimilability by aquatic animals, e.g., the percentage ratio of the assimilated
food (A) to the food ration (Q): the assimilability is calculated as I = AIQ, %
(Winberg 1964; Rigler 1971). Once the assimilability is established for a given
consumer foraging on a given kind of food, other important parameters of
nutrition, such as the amount of assimilated food (rate of assimilation A) and
the growth efficiency coefficients KJ and K2 might be estimated even without
the use of the radioisotopic technique or with its use employing versions which
do not demand uniformly labeled food organisms. In this case (if the value of
I is known), the food rations (Q) might be estimated either during the determination of the filtering rates for filterers or the grazing rate for other animals.
But (more easily) they could be estimated during short-term experiments with
the use of food labeled at the researcher's convenience, because for the estimation of food rations even labeling is not obligatory. It is necessary only to
know the Cr value of the whole body material of the food organisms. Thus: Q
= Rc x Cr!lg C Sp.-l h- 1 (see Sect. 3.5.2). Having estimated the food rations by
using these simple approaches, the assimilated food then is calculated: A = QII
lOO!lgCsp.-lh-1. The nonassimilated part of ingested food (F) is equal to the
difference: F = Q - A. With the known values of A and of the M (respiration
rate) values measured separately, the expenditure of food for growth G
(production) can be calculated as follows: G = A - M. Then secondary production coefficients KJ and K2 are calculated as the ratios KJ = Gle and K2 =
GIA.
The above considerations show quite clearly that the adequate estimation
of the assimilabiity index I is the key point in all trophological studies. Its estimation opens the way for a rather simple procedure to evaluate the balance
parameters of the expenditure of consumed food, which forms the background
for any holistic analysis and modeling in aquatic ecology. The exact estimation