III /E-4
EVALUATION OF THE RELATIVE SUITABILITY OF VARIOUS GROUPS OF ALGAE AS FOOD OF MILKFISH IN BRACKISH-WATER PONDS

by

YUN-AN TANG¹

and

TING-LANG HWANG
Tainan Fish Culture Station of the Taiwan Fisheries Research Institute
Tainan, Taiwan

EVALUATION OF THE RELATIVE SUITABILITY OF VARIOUS GROUPS OF ALGAE AS FOOD OF MILKFISH IN BRACKISH-WATER PONDS

Abstract

An evaluation of the relative suitability of the four major groups of brackish-water pond algae as milkfish food was made by digestion experiments, observations on their suitability as food and effect on proper pond management.

The nutritive ratios of fresh Chaetomorpha, phytoflagellates, filamentous blue-greens and diatoms are 1:33.44, 1:2.47, 1:1.18 and 1:1.54 respectively. The Chaetomorpha-produced detritus has been found to have a nutritive ratio of 1:1.66.

Fresh Chaetomorpha is undesirable because the milkfish under harvestable sizes are unable to consume it due to its coarseness and fibrous nature and it interferes with the manipulation of pond fish populations. Chaetomorpha-produced detritus can serve as food for large-sized milkfish also.

A bloom of phytoflagellates is harmful to milkfish as it reduces the schooling and grazing activities by obstruction of vision, reduces light penetration and causes oxygen depletion as a result of the decomposition of dead organisms.

Filamentous blue-greens and benthic diatoms that grow on pond bottoms are the most desirable food for all age groups of milkfish. A luxuriant growth of these algae on pond bottom can crowd out undesirable algae and possibly other harmful organisms in the ponds and create better environmental conditions for the fish.

EVALUATION DES AVANTAGES RESPECTIFS DE DIVERS GROUPES D'ALGUES EN TANT QU'ALIMENT POUR LES CHANOS ELEVES EN ETANGS A EAUX SAUMATRES

Résumé

Les avantages respectifs des quatre principaux groupes d'algues croissant dans les étangs à eaux saumâtres en tant qu'aliment pour les Chanos ont été évalués à l'aide d'expériences portant sur leur digestibilité, et d'observations relatives à leurs qualités alimentaires et à leur effet sur l'aménagement rationnel des étangs.

La valeur nutritive des Chaetomorpha, des phytoflagellés, des algues bleues filamenteuses et des diatomées consommés à l'état frais est respectivement de 1:33,44, 1:2,47, 1:1,18 et 1:1,54. II a été constaté que les détritus provenant de Chaetomorpha ont une valeur nutritive de 1:1,66.

A l'état frais, les Chaetomorpha sont à déconseiller car les Chanos au-dessous de la taille exploitable ne sont pas capables de s'en nourrir en raison de leur consistance grossière et fibreuse; en outre elles gênent les manipulations de poissons en étang. Les détritus de Chaetomorpha peuvent servir également d'aliment pour les Chanos de forte taille.

Une surabondance de phytoflagellés est nuisible aux Chanos car elle gêne l'agrégation et le brout en diminuant la visibilité, elle s'oppose à la pénétration de la lumière et abaisse la teneur en oxygène de l'eau par suite de la décomposition des organismes morts.

Les algues bleues filamenteuses et les diatomées benthiques qui croissent sur le fond des étangs constituent les aliments les plus appropriés aux Chanos de tous les groupes d'âge. Lorsque ces algues se développent abondamment sur le fond d'un étang, elles peuvent conduire à l'élimination des autres algues indésirables et éventuellement des autres organismes nuisibles, créant ainsi de meilleures conditions de milieu pour les poissons.

EVALUACION DE LA CONVENIENCIA RELATIVA DE VARIOS GRUPOS DE ALGAS COMO ALIMENTO DEL SABALOTE CHANOS CHANOS EN ESTANQUES DE AGUA SALOBRE

Extracto

Se efectuó una evaluación de la conveniencia relativa de los cuatro principales grupos de algas de estanques de agua salobre como alimento para el sabalote (Chanos chanos) mediante experimentos de digestión, observaciones sobre su utilidad como alimento y sus efectos sobre una adecuada explotación racional de los estanques.

Los coeficientes nutritivos de Chaetomorpha fresca, fitoflagelados, algas filamentosas verde-azules y diatomeas son 1:33,44, 1:2,47, 1:1,18 y 1:1,54, respectivamente. Se ha comprobado que los detritos producidos por Chaetomorpha tienen un coeficiente nutritivo de 1:1,66.

Chaetomorpha fresca no resulta apropiada ya que el sabalote (Chanos chanos) inferior a tallas recolectables no puede consumirlo debido a la dureza y carácter fibroso de aquélla y a que dificulta la manipulación de las poblaciones de peces de los estanques. Los detritos producidos por Chaetomorpha pueden servir como alimentos también para el sabalote de tamaño grande.

Una floración de fitoflageladas es perjudicial para el sabalote ya que reduce las actividades gregarias y de alimentación de éste por obstrucción de la visión, disminuye la penetración de la luz y determina una desoxigenación como resultado de la descomposición de organismos muertos.

Las algas verde-azules filamentosas y las diatomeas bentónicas que crecen en el fondo de los estanques son el alimento más conveniente para todos los grupos de edad del sabalote. Un crecimiento exuberante de estas algas en el fondo de los estanques puede sofocar el desarrollo de algas inadecuadas y posiblemente de otros organismos dañinos en los estanques y crear mejores condiciones ambientales para los peces.

¹ Present address: FAO/TA Inland Fisheries Biologist, Philippine Fisheries Commission, Manila.

1 INTRODUCTION

The brackish-water ponds for the culture of milkfish, Chanos chanos (Forskal) in the countries of Indonesia, the Philippines and Taiwan, are distributed on the alluvial tidal flats of geologically recent deposits. The soil and the water of these flats are extremely rich because the streams running down from hinterland erode and bring down large amounts of plant nutrients, which are deposited together with the alluvium in this area. In the tropical climate, there are various groups of algae that can produce luxuriant blooms under these conditions. Some of these algae are very good for milkfish growth and advantageous for pond production. There are, however, also many kinds of algae that are undesirable for milkfish culture, either because they are unsuitable as food for milkfish, or because of some ill-effects of the algae on fish life and on the management of the ponds. Therefore, an intensive study of the relative suitability of the major groups of algae that are commonly produced in brackish-water ponds as food for milkfish, is one of the requirements for the improvement of this type of fish culture.

2 THE DOMINANT GROUPS OF ALGAE IN BRACKISH-WATER PONDS

The algae produced in brackish-water ponds can be grouped into four major groups: the green, Chlorophyceae; the diatoms, Bacillariophyceae; the blue-greens, Cyanophyceae; and the various free-living phytoflagellates. The dominant genera of the Chlorophyceae are Chaetomorpha and Enteromorpha, but the latter genus is very rare; of the Bacillariophyceae Navicula, Pleurosigma, Mastogloia, Stauroneis, Amphora, Nitzschia and various unidentified genera; of the Cyanophyceae Oscillatoria, Lyngbya, Phormidium, Spirulina, Microcoleus and possibly other genera of the order Oscillatoriales; and of the free-living phytoflagellates Chlamydomonas, Pyramimonas, Chlorosarcina, Gymnodinium, Chilomonas, and a number of unidentified flagellates.

3 THE RELATIVE DIGESTIBILITY OF VARIOUS GROUPS OF ALGAE

Determination of the amount of digestible nutrient that a particular food supplies to the fish is the simplest, but the most important method of measuring the relative values of foods. A number of digestion experiments were made with the four major groups of algae for milkfish of three different sizes, fingerlings (3 to 5 g), juveniles (15 to 20 g), and half-grown fish (60 to 100 g) which are commonly used for stocking ponds in Taiwan. For these experiments a series of concrete ponds of identical size, 6 × 4 × 0.5 m were used. Each digestion experiment was made over a five-day period. Feeds of these four groups of algae were given daily at the rate of 20 to 25 percent of the total body weight of the fish stocked in the pond. The algae of both fresh and detrital forms were given on the basis of wet weight, which includes approximately 88 to 93 percent of water. During the experiments, the faeces excreted by each group of test fish were carefully collected with a glass tube by suction. The total weight of each kind of food consumed, and the total amount of faeces excreted during the experiments were measured. The chemical compositions of different feeds and the organic nutrients in the faeces¹ from different feeding trials were determined. The digestion coefficients of protein, fat, nitrogen-free extract and fibre, the percentage of digestible protein, the percentage of total organic digestible nutrients and the nutritive ratio of each kind of food were also computed, and are given in Table I.

¹ It may be noted that the residual organic nutrients in the faeces of the fish might lose some portion into the water, and this loss might vary with the different species of fish and with different kinds of food. However, the faeces of milkfish remain in compact clots after they are excreted, so the results as indicated appear satisfactory for the purpose of this study.

Table I

Composition of four major groups of algae and their relative nutritive value as milkfish food
(The water temperature during digestion experiments ranged from 29° to 33°C and the salinity from 24 to 27 ppt)

¹ Digestive coefficient:
² Digestible protein: The percentage of protein in the food × Digestion coefficient of protein
³ Total digestible nutrients: The sum of digestible protein, fibre, nitrogen-free extract and fat × 2.25
⁴ Nutritive ratio:
⁵ Centrifuged from the pond water where Chlamydomonas and Chilomonas flagellates bloomed predominantly.
⁶ Furnished as the diatom sludge
⁷ Collected from the pond bottom where the dominant genera, Oscillatoria and Lyngbya grew

3.1 Filamentous green algae

Although fresh Chaetomorpha contains a relatively high percentage of crude protein, the proportion digested by milkfish of the above-mentioned sizes is extremely low, computed at 0.09 percent, and it has a high nutritive ratio of 1:33.44. Since it is only the digestible protein which supplies the necessary protein for increase in body weight, this kind of feed is apparently unsuitable for growing milkfish. However, Chaetomorpha can be used as feed of relatively high value for milkfish when it dies and on decomposition, its protein content becomes digestible for milkfish. The digestion experiment showed that the Chaetomorpha-produced detritus supplies 2.28 percent digestible protein, and with a relatively low nutritive ratio of 1:1.66 for milkfish.

3.2 Phytoflagellates

The phytoflagellates used for this digestion experiment were mostly Chlamydomonas and Chilomonas. These organisms were obtained by centrifuging the water where they had bloomed. This experiment showed that the phytoflagellates are the most nutritious feeds for the milkfish as they supply 3.17 percent digestible protein and 10.41 percent (including fat × 2.25) total digestible nutrients and with a nutritive ratio of 1:2.37.

3.3 Diatoms

The diatoms sludge, formed by various species of bottom-dwelling diatoms, was used for this digestion experiment. The results indicated that the diatoms are undoubtedly a nutritious food for milkfish as the diatom cells contain a high portion of digestible organic nutrients for the fish.

3.4 Filamentous blue-green algae

The filamentous blue-greens used for this experiment comprised mainly various species of the family Oscillatoriaceae. This group of algae, as shown in the results of this experiment, is valuable as food for milkfish. They furnish 1.60 percent digestible protein and have a low nutritive ratio of 1:1.18.

4 THE RELATIVE DESIRABILITY OF VARIOUS GROUPS OF ALGAE

4.1 Food and the feeding mechanism of milkfish

It is believed by some fisheries biologists that milkfish are plankton feeders. However, milkfish gill rakers are not very well adapted to filter planktonic algae from the water. Feeding experiments conducted in concrete ponds indicated that milkfish have little food in their stomachs when they were stocked in ponds containing large quantities of phytoflagellates. This was also found to be the case for fish from plankton-rich earth ponds. With the exception of the planktonic algae that die and accumulate on the pond bottom, the quantity of fresh plankton taken as food by milkfish in ponds is negligible, even by milkfish fingerlings.

Diatoms are found in both planktonic and benthic forms in brackish-water ponds. Very common planktonic genera include Synedra and Melosira. These were found whenever the pond water was examined, but they never grow so abundantly as to make the pond water turbid, as did the phytoflagellates. The benthic diatoms, such as the Navicula, Pleurosigma, Mastogloia, Stauroneis, etc., together with the various species of filamentous blue-green algae, form the algal pasture on the substratum. Since milkfish of various growth stages suck up foodstuffs from the pond bottom, these pasture algae furnish the most suitable forms of milkfish food.

Milkfish of various sizes often feed on the algal scum floating on the water surface, but show no special preference for food from the upper layers of the water.

4.2 Palatability

The palatability of food is of great importance in feeding fish for high production. Unless the food is palatable, milkfish will not eat enough to increase their body weight efficiently. It also seems probable that well-liked foods are even digested somewhat better than those which are equally nutritious, but less palatable. Schuster (1960) states that the milkfish either in the sea or in ponds resort to facultative feeding. However, there is evidence that the palatability of food to milkfish in ponds varies greatly with the different kinds and groups, and different growth stages and conditions of the algae, and also with the different growth stages of the milkfish itself.

Experiments conducted in concrete ponds show that milkfish usually have empty stomachs and lose weight when they are exclusively fed with fresh Chaetomorpha. Milkfish below harvestable size are unable to eat this coarse, fibrous alga. Chaetomorpha-produced detritus can serve as food for relatively large milkfish, but it seems to be less palatable than those algae which are fresh, tender and soft.

Observations made in both concrete and large excavated ponds indicate that the fresh filamentous blue-greens and the diatoms are the most palatable groups of algae produced in the brackish-water ponds. Probably due to their softness and tenderness, these algae are preferred by milkfish of all growth stages, including larval fish.

Feeding experiments with milkfish of various sizes show that the detritus produced from the diatoms is more palatable than that formed by any other group of algae. However, milkfish also like the detritus produced from filamentous blue-greens and from phytoflagellates, with no particular preference for either one. But even large milkfish dislike Chaetomorpha-produced detritus.

5 THE RELATIVE DESIRABILITY OF VARIOUS GROUPS OF ALGAE FOR POND MANAGEMENT

Dense growth of Chaetomorpha as a submerged weed interferes with the movements of the milkfish and limits the holding capacity for fish stocks in ponds. Moreover, these obnoxious algae interfere with the harvest of fish from ponds with gill nets or draining of the pond water. Periodical thinning of the fish population, by selective gill netting of the larger fish, is one of the most important ways of manipulating the population for increased production. So, despite its nutritive values as feed, Chaetomorpha is not desirable in milkfish ponds.

Milkfish growth ceases when a bloom of free-living phytoflagellates occurs in the pond. These algae may obstruct the grazing activities of the fish by blinding them. Furthermore, such a bloom obstructs the penetration of sunlight and the dense shading hinders the growth of the desirable filamentous blue-greens and benthic diatoms on the pond bottom. It has also been found that the death of the phytoflagellates following a heavy bloom often causes a high mortality of the fish due to depletion of the dissolved oxygen in the pond water. Thus the phytoflagellates as a group are harmful for the management of milkfish ponds, and they give the fish farmers a great deal of trouble.

Besides supplying the most palatable and nutritious food, the filamentous blue-greens and the benthic diatoms forming the pasture algae facilitate pond management and furnish the milkfish with better environment. A luxuriant growth of the pasture algae can crowd out the undesirable and harmful algae by minimizing the amounts of plant nutrients available to them. Snails that compete with milkfish for food can also be crowded out from ponds by a dense covering of the stratum with algal pasture. Milkfish appear lively, schooling round the pond, and actively grazing on the fresh, tender and soft pasture algae in clear pond water. This favourable environment, however, can develop only when these pasture algae thrive luxuriantly on the floor of the pond. Therefore, good growth of algal pasture is essential for the accelerated growth of the milkfish.

6 REFERENCE

Schuster, W.H., 1960 Synopsis of biological data on milkfish, Chanos chanos (Forskal) 1775. FAO Fish.Biol.Synops., (4) : pag.var.