III/E-10
ESTIMATION OF STANDING CROPS AND RATES OF FEEDING FISH IN PONDS

by

H.S. SWINGLE
Auburn University
Agricultural Experiment Station
Auburn, Alabama, U.S.A.

Abstract

Rate of daily feeding is calculated either as a percentage of the total weight of the standing crop or on the basis of the daily increase in total weight of the standing crop. Both methods are subject to two sources of error. The first is the possible error in estimating the average weight of the population and the second is the error in estimating the number of individuals in a population at any one time, on the basis of seining samples.

Estimates of standing crops may be made from the total amount of daily feed added up to any one point from the equation:

W = IR^N, where

For estimation of the rate of change in weight “R”, it is necessary to know the pond conversion value “S” for the feed used at the rate it is being fed:

D	=	number of days feed is added during interval T,
T	=	intervals at which feeding rate is readjusted,
P	=	daily feeding rates from 0.02 to 0.05, (2 percent to 5 percent of body weight),
S	=	pond conversion value at P and T for a particular feed and for a particular species of fish.

ESTIMATION PONDERALE DU STOCK ET DE LA RATION ALIMENTAIRE DES POISSONS EN ETANG

Résumé

La quantité d'aliments fournie journellement est calculée soit en pourcentage du poids total du stock présent, soit sur la base de l'accroissement journalier du poids total de ce stock. Les deux méthodes peuvent être entachées d'erreurs au départ: erreur dans l'estimation numérique du stock à un moment donné.

Une estimation pondérale du stock peut être faite à partir du poids total de la ration alimentaire quotidienne sommée jusqu'au moment considéré à l'aide de l'équation suivante:

W = IR^N où

Pour estimer le taux d'accroissement de poids “R”, il faut connaître le taux de conversion “S” applicable à l'étang en fonction des aliments fournis et de la quantité distribuée:

ESTIMACION DE LOS RECURSOS POTENCIALES EXISTENTES Y TASAS DE ALIMENTACION DE LOS PECES EN LOS ESTANQUES

Extracto

La proporción de alimentación diaria se calcula, sea como porcentaje del peso total de los efectivos existentes de la población, sea sobre la base del incremento diario del peso total de dichos efectivos. Ambos métodos están sujetos a dos causes de error. El primero es el posible error en la estimación del peso medio de la población, y el segundo es el que procede de la estimación del número de indivíduos en una población en un momento cualquiera sobre la base de muestras obtenidas mediante redes de cerco. La estimación del efectivo en organismos puede realizarse partiendo de la cantidad total de alimento añadida en cualquier punto, de la ecuación:

W = IR^N, en la que

Para la estimación de la tasa de cambio en peso “R”, es necesario conocer el valor “S” de conversión en el estanque para el alimento utilizado en la proporción en que se suministró:

1 SOURCES OF ERRORS IN COMPUTATION OF STANDING CROPS AND RATES OF FEEDING

The successful fish culturist must supply feed to his fish at daily rates that allow efficient food utilization. In American and European fish cultures, amounts to be fed daily are calculated upon the basis of a certain percentage of the body weight of all fish in the pond (the standing crop). The percentage rate used varies with temperature and size of the fish being fed; the amount fed is readjusted weekly, bi-weekly, or monthly as the crop increases in total weight and average size.

In Israel, Tal (1956) recommended bi-weekly seining to determine the average daily growth of the common carp (Cyprinus carpio). The average daily increase during the past two-week period expressed as gain in kilos per hectare per day is then multiplied by a factor to give the amount of feed to be given the fish daily during the next two-week period. The factors recommended increase with increasing average size of the carp.

Both methods rely upon estimates of average size of fish made from seining samples and the results are multiplied by the number of fish estimated to be present per hectare, in one case to estimate the weight of the standing crop and in the other to estimate gain per hectare per day.

The estimates are thus open to two main sources of error. First, the average weight of fish taken in the seine may not represent the true average weight within the population at that time. In experiments at Auburn, it was found necessary with the channel catfish (Ictalurus punctatus) to weigh a sample of not less than 100 fish from ponds ranging from 0.5 to 10 ha to obtain a relatively accurate average. Despite such care, average weights of fish taken by seining have occasionally varied as much as eight to 19 percent from the average found by draining and weighing all fish a few days later. With Tilapia mossambica or T. aurea, samples of 300 or more fish were necessary for accurate estimates. Seining samples taken at times of day when fish were not being fed often appeared to underestimate the average size. It seemed probable that smaller fish remain in the shallow water and were thus more vulnerable to seining. Seining shortly after feeding often gives abnormally high weights for larger fish due to their habit of cramming their stomachs with more than their share of feed. Corrections can be made for this by using average weights of fish of the same lengths in the calculations.

The second source of error in estimating feed required is that the fish culturist seldom knows accurately the number of fish per hectare. He usually knows the number stocked and may know the average mortality at draining. He thus may use the number stocked or the average survival in his calculations. Use of either often leads to serious errors in calculations of the feed to be given daily. With the scaled variety of common carp, at this Station survivals normally varied from 97 to 100 percent per year and here use of the number stocked caused no serious error provided the pond was protected to prevent loss of carp during heavy outflows of water. The mirror variety of common carp has given survivals from 80 to 90 percent per year, whereas tilapias had survivals from 65 to 90 percent. In these cases estimates of the fish present may be quite inaccurate.

2 USE OF AMOUNT OF FEED SUPPLIED FOR ESTIMATES OF STANDING CROPS

2.1 Calculation of pond conversion value “S”

The need for additional and better methods of estimating the standing crop of fish is quite evident. Where a standard feed is used and where conversion rates for this feed are known for a particular species and for various rates of feeding, it appeared that this information could be used to give an estimate of standing crop by adding the expected gain from the feed to the weight of fish stocked. Since the feed and wastes directly or indirectly from feeding will produce additional fish-food organisms, it is better to use the pond conversion value (S) which is calculated:

2.2 Sizes of tilapia and efficient rates of feeding

For Java tilapia, Tilapia mossambica, Nile tilapia, T. aurea, and Congo tilapia, T. melanopleura, with Auburn No. 2 pelleted feed¹ during periods when the water temperatures remained above 22°C (70°F), “S” conversions varied from 0.7 to 1.2 with an average of 1.0 when fed at the rate given below for various sizes of fish:

¹ Pelleted dry feed containing 46 percent protein, 25 percent carbohydrates and 5 percent fat (Prather, 1958).

2.3 Sizes of catfish and efficient rates of feeding

In experiments with both white catfish (Ictalurus catus) and channel catfish, “S” conversion values for the Auburn No. 2 pelleted feed, when water temperatures were above 16°C, varied from 1.5 to 2.0 with an average of 1.7 when the daily feeding rate for various sizes of fish were:

2.3.1 Variations in “S” values

Results indicated that use of a standard pond conversion value of 1.7 for both channel catfish and white catfish estimated final production satisfactorily, but underestimated the standing crops during spring and summer. This evidently was because natural fish-food organisms furnished a material part of the feed until a weight of approximately 1,000 kilos fish per hectare was reached. Examination of the data indicated that natural feeds were causing growth during summer period of approximately 2 kg/ha of fish per day. In effect, the “S” pond conversion then becomes a constantly increasing value during this period of growth. The average calculated “S” values for gradually increasing total cumulative feed, using average seining values for standing crops were as follows:

2.3.2. Estimation of the standing crop from feeding records, seining and from draining

Following are comparisons of standing crop estimates in kg/ha made from seining records, by dividing total cumulative feed by the appropriate S conversion, and that found upon draining.

While disagreements in estimates of standing crop occurred, that calculated from feed supplied appeared as accurate as that from seining records.

In other experiments in these ponds without feeding or fertilization, the standing crop has averaged 97 kg/ha at the end of 210 days. If this correction is applied, the final S conversion would be changed by approximately 0.1 - from 1.7 to 1.8. The same correction cannot be applied to the S conversion for standing crops from 200 to 700 kg and would not help in estimating these standing crops.

2.4 Equation for estimation of standing crop from feed supplied

From these data it would appear that relatively reliable estimates of the standing crop or yield may be made from the equation:

(1)

where	I	=	initial weight of fish stocked
	F	=	weight of feed added
	S	=	pond conversion value for cumulative amount of feed added and for the species of fish stocked.

For example channel catfish, where I = 50 kg; F = 2210 kg Auburn No. 2 pelleted feed; and S = 1.7, with the range 1.5 – 2.0.

Calculations of standing crops from the feed should be useful in checking the accuracy of estimates from seining. It is evident that both are subject to certain inaccuracies, but from different sources.

2.5 Equations relating to expected weight of fish

From the foregoing results, it also appeared possible to devise equations relating feed supplied to the standing crop of fish to be expected after a certain period of feeding, as follows:

(2)

when D = number of days on which feed is added during interval T;

Example: Where T = 4 weeks, D = 24 when feeding is 6 days weekly, then, for the channel catfish where P = 0.03 and S = 1.6,

The general forms of the equations are:

(3) or log W = log I plus N log R

Number of “T” intervals of feeding required to reach a certain weight are:

(4)

Initial weight required to reach a specified total weight is:

(5) log I = log W - N log R.

The above equations are useful in planning methods of management and in understanding the factors affecting production.

2.5.1 Limits of feeding within which the equations apply.

Range of use is limited by the maximum safe rates of feeding/hectare/day, which, when using Auburn No. 2 pelleted feed with the following species are:

In addition, feeding must be at water temperatures above 16°C.

Since the above are limitations in the usefulness of equations (3), (4), and (5) it is evident that for channel catfish fed at the three percent rate, we can calculate as follows the maximum W for which the equation holds.

2.5.2 Examples of calculations

Where 40 kg of channel catfish are stocked and fed Auburn No. 2 pellets six days weekly, with feeding readjustments to three percent of weight of standing crop weekly for a period of 30 weeks, then

I = 40, D = 6, T = 1 week, N = 30 T., P = 0.03, and S = 1.7;

then, log W = log 40 + 30 log 1.106, Equation (3);

log W = 2.91486

W = 821 kg

If W exceeds 1,300 kg/ha, it is necessary first to calculate the weeks of feeding required to reach 1,300 kg using equation (4),

If we are attempting to produce 2,000 kg of catfish, the remainder of the time we cannot feed more than 39 kg of feed per day. If we feed six days per week, the increase in weight of catfish per week is:

Then the added weeks needed to increase from 1,300 kg to

The total time required is then 35 + 5.1 = 40.1 weeks. If we wish to reduce this time, we must start with a higher weight of fish: that is, either more fish of the same average size, or the same numbers of fish of larger size.

2.5.3 Calculation of feed required for maintenance

If we use a much larger initial weight of fish, another factor must be considered. The feeding rate with Auburn No. 2 pellets required to maintain weight of fish without growth is approximately 0.5 percent of their weight per day six days per week. If we start with 50 kg of fish in one case and 200 kg in another and continue feeding (six days weekly) for a 30-week period, the feed required for maintenance of this original weight for the former is:

50 × 0.005 × 30 × 6 = 45 kg,

and for the latter:

200 × 0.005 × 30 × 6 = 180 kg.

The daily difference in maintenance requirements is first met and only the remainder of the feed is available for growth.

Where small fish (less than 15 cm) are grown to large size, usually no correction is needed in the equations given. However, a correction is necessary when high weights or large fish are fed for a short period of time. Equation (1) then becomes:

I	=	initial weight of fish stocked,
F	=	total weight of feed supplied, and
M	=	I × 0.5% × number of days fish were fed.

In equation (2) the correction can be made by subtracting 0.5 percent from the percentage of daily feeding, P.

3 REFERENCES

Tal, S., 1956 A criticism of the methods of calculation of carp feeding. Bamidgeh, 8(3):44–51

Prather, E. E., 1958 Further experiments on feeds for fathead minnows. Proc.Conf.stheast.Ass. Game Commrs., 12:176–8