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KURUMA SHRIMP (Penaeus japonicus) CULTIVATION IN JAPAN

by

MOTOSAKU FUJINAGA (HUDINAGA)
175 Kunitachi, Kunitachi-shi
Tokyo, Japan

Abstract

The history of Karuma shrimp cultivation in Japan is traced from the first attempts to rear Penaeus japonicus in the laboratory in 1933 to the large-scale commercial rearing practised in 1967.

Successful rearing on a large scale was first achieved in indoor tanks, using cultures of the diatom Skeletonema costatum as food for the zoeal stages, Artemia nauplii for the mysis and post-larval stages and crushed clam meat for the juvenile stages. The scale of operations has been greatly increased and the cost of production lowered by using large outdoor tanks filled with natural seawater to which nutrient salts are added. The resulting bloom of phytoplankton makes the separate culture of diatoms unnecessary, and wild zooplankton and benthos develop to a considerable degree, so that only relatively small amounts of Artemia and clam meat need be added.

Current research is largely directed to finding suitable foods, other than potential human foods, for juvenile shrimp.

LA CULTURE DE LA CREVETTE KURUMA (Penaeus japonicus) AU JAPON

Résumé

Historique de l'élevage de la crevette Kuruma au Japon, depuis les premières tentatives (1933) d'élevage en laboratoire de Penaeus japonicus jusqu'aux grands élevages commerciaux en 1967.

L'élevage en grand a donné des résultats tout d'abord dans des viviers intérieurs, en utilisant comme aliment des cultures de diatomée Skeletonema costatum pour le stade zoé, des nauplii Artemia pour les stades mysis et post-larvaire, et de la chair de “clam” hachée pour les jeunes. L'emploi de grands viviers extérieurs remplis d'eau de mer naturelle additionnée de sels nutritifs, a permis de développer considérablement l'ampleur des élevages et d'abaisser les coûtsde production. La poussée de phytoplancton rend inutile la culture à part des diatomées; en outre, le développement considérable du zooplancton naturel et du benthos fait qu'il n'est plus nécessaire d'ajouter que des quantités relativement faibles d'Artemia et de chair de “clam”.

La recherche vise actuellement en grande partie à trouver, pour l'alimentation de jeunes crevettes, des aliments autres que ceux qui peuvent servir à la consommation humaine.

EL CULTIVO DE CAMARON KURUMA (Penaeus japonicus) EN JAPON

Extracto

Explica el cultivo del camarón Karuma en el Japón desde los primeros intentos que se hicieron para críar Penaeus japonicus en el laboratorio, en 1933, hasta la producción comercial en gran escala practicada en 1967.

Donde primero se consiguió críar con éxito, en gran escala, fue en depósitos interiores, utilizando cultivos de la diatomea Skeletonema costatum como alimento de las fases zoé; Artemia nauplii para las fases mysis y poslarvales, y carne de almeja triturada para las fases juveniles. La escala de las operaciones se ha aumentado grandemente, disminuyendo el costo de la producción mediante el empleo de grandes depósitos exteriores llenos de agua del mar natural, a la que se añaden sales nutrientes. La consiguiente floración de fitoplancton hace innecesario el cultivo por separado de diatomeas, desarrollándose en grado considerable el zooplancton y bentos silvestre, por lo que solo es necesario añadir cantidades relativamente pequeñas de Artemia y de carne de almejas.

Las investigaciones actuales se dirigen, en gran parte, a la obtención de alimentos adecuados, excluidos los que podría emplear el hombre para los camarones juveniles.

1 INTRODUCTION

The culture of Kuruma shrimp (Penaeus japonicus) in Japan began in July, 1933 when for the first time I succeeded in the artificial spawning and hatching in a culture tank. Since then 35 years have passed. During these years there occurred World War II and my work has taken many turns and twists, nevertheless, I have continued the experiment of Kuruma shrimp culture up to the present.

Now there are 11 management units of Kuruma shrimp culture in Japan. They are mainly in the Seto Inland Sea and produce 200 tons of shrimp annually. I have been favored with the assistance and cooperation of a great many people in my study on shrimp culture in Japan, but in this paper I should like to mention the names of only those persons who have given me specially valuable advice and cooperation in this study.

2 HISTORY OF RESEARCH ON KURUMA SHRIMP

My research work on Kuruma shrimp for the past 35 years from 1933 through 1967 can be divided into 5 periods.

2.1 Period I (From 1933 to 1940)

As stated above, the first time I succeeded in the artificial spawning and hatching of Kuruma shrimp was in early July 1933. In the first attempt, however, because the food of the shrimp in the zoeal stages was unknown, the majority of the larvae died, and only a few of them survived until after their mysis stage. The survival rate of the shrimp has been improved by continued experiment year after year, and in 1940 I succeeded to grow the shrimp to marketable size. The number of shrimps of commercial size, however, was very small and it was far from artificial culture on a commercial basis. These eight years were characterized by difficulties in feeding the shrimp in the zoeal stages.

The laboratory was located in Senzoku Island, Amakusa Archipelago, Kumamoto Prefecture, from 1933 to 1934, then at Misumi, tip of the Uto Peninsula, Kumamoto Prefecture, from 1934 to 1936, and it was removed to Aio, Yamaguchi Prefecture, from 1936 to 1942.

2.2 Period II (From 1941 to 1942)

It was found in early 1941 that a diatom, Skeletonema costatum, provided suitable food for zoeal shrimp. With regard to the pure culture method of S. costatum I was taught by Dr. Yoshiyuki Matsue, at the suggestion of Dr. Ikusaku Amemiya, my respected teacher of Tokyo University. I should have taken many more years to find out S. costatum as a suitable food for zoeal shrimp if I had not been favored with the cooperation of the two professors. In this period Dr. Hiroshi Kasahara and Dr. Kasio Ota worked very hard as my assistants.

As already mentioned, almost all larvae of Kuruma shrimp died during the zoeal stages and less than one percent of them metamorphosed into the mysis stages. However, by feeding them with S. costatum in the zoeal stages, it became possible to rear more than 30 percent of them to the mysis stage. Such increased survival rate of shrimp larvae made me optimistic. It was 8 years since I succeeded in the artificial spawning and hatching of Kuruma shrimp. In 1942, again I repeated the experiment of feeding S. costatum during the zoeal stages, but unfortunately on July 16, 1942 my laboratory at Aio was totally destroyed by a high tide caused by a typhoon. Most of the experimental data and reference materials collected for many years were washed away by the high tide. However, a bright spot in the misfortune was that the draft of my third paper (Hudinaga, 1942) was in a printing office in Tokyo and escaped loss.

2.3 Period III (From 1942 to 1949)

The year of the destruction of my laboratory at Aio, 1942, was the second year of the second world war in which Japan was involved. There was a shortage of human food, and there was a general feeling that the study of luxury foods such as Kuruma shrimp should not be continued. Such public opinion made it impossible to rebuild the laboratory.

At that time I was also studying the attachment mechanism of the barnacle (Balanus), work which was subsidized by the Navy. I built a laboratory at Toyohama, tip of Chita Peninsula, Aichi Prefecture, chiefly for barnacle study, but my study on Kuruma shrimp was continued in it as before. The second world war came to an end in 1945 during my stay at Toyohama.

For several years after the second world war the Japanese people suffered very much from shortage of food and I could not devote myself to work on Kuruma shrimp. I spent my time studying literature on the life histories of marine animals. So the several years after the second world war were the most fruitless in my entire period of study of Penaeus.

2.4 Period IV (1949 – 1959)

In February 1949 I was appointed the Chief of the Research Division which was newly established in the Fisheries Agency. My main task was to rebuild and enlarge the national fisheries research organizations devastated by the war, but I could not forget the shrimp study even in the very busy office. Every Sunday I used to make field observations on shrimps on the beach of Chiba Prefecture bordering on Tokyo Bay.

It had been found before the war that S. costatum was a suitable food for zoeal shrimp. In the stages from mysis to early post-larvae, it was possible to feed the larvae on fine grains of fish meal, but the result was not satisfactory. I cherished the hope to repeat the prewar experiment and, if possible to carry out a large soale experiment. In the years after the War the supply of goods gradually increased and social conditions improved. In the summer of 1955 I established a small laboratory at Kisarazu, Chiba Prefecture, at my own expense and resumed the experiment on Saturday afternoons, Sundays and holidays.

As Chief of the Research Division in the Fisheries Agency, I was involved in international negotiations and conferences on Fisheries, and visited Canada, U.S.A., U.S.S.R., Italy and Switzerland. Through these trips abroad I got valuable information on shrimp culture, and particularly during the fur seal negotiations in Washington, between Japan, U.S.A., Canada and U.S.S.R., which extended from November 1955 to April, 1956. This meeting was not held on a daily basis, and sometimes it was adjourned a week or ten days. I visited various institutes while the meeting was adjourned.

One of these visits was to the Marine Laboratory of North Carolina University at Newport, where they were rearing Schizopoda using nauplii of brine shrimp as food. The idea came to me that nauplii of brine shrimp might be used as food of penaeid shrimp from mysis to early post-larval stage. I began rearing brine shrimp at the hotel, Washington D.C.. It was very hard to get sea water in Washington D.C., but the sea water was transported to my hotel every two days through the kindness of Dr. L.A. Walford.

In the summer of 1956 nauplii of brine shrimp were given to mysis larvae or early post-larvae of shrimp and this was found to produce good results, as expected. But this was a very small scale experiment. I borrowed 3000 m² of land in the Onuki coast, Chiba Prefecture, in 1957 and set up a laboratory and a culture farm of bigger size than before. With these facilities it became possible for the first time to build a pure culture tank for S. costatum of 2 m × 1 m × 1 m, a spawning and hatching tank of the same size and various culture ponds of different sizes. These facilities were used as the basis for construction of Takamatsu Culture Farm. In 1958 for the first time 10 kg of shrimps of marketable size were produced.

I resigned from the Fisheries Agency in October 1959 after 10 years and 6 months service. It was feasible neither to carry out free research study not to manage a large culture farm while I was in government service.

2.5 Period V (1959 to present)

I established a company as soon as I resigned my office in the Fisheries Agency and set up a culture farm of commercial scale in the disused salt farm, Takamatsu. In the actual management of the culture farm of commercial size, everything did not go so well as in the laboratory.

The result worthy of special mention from 1959 up to present was that a large amount of cheap fry (the first requisite for shrimp culture) was made available. Fry culture technique has continued to develop and at present it would be possible to produce more than one thousand million fry annually if there were enough demand.

However there remain many problems yet to be solved concerning the food from fry to adult stage, which is the second requisite for shrimp culture. As will be mentioned later, foods for shrimp culture are at present bivalves, such as short-necked clams, shrimps and fish. These should be used primarily for human food and not for shrimp culture. The amount of food necessary for 1 kg shrimp production is 7 to 10 kg. Accordingly, the more the cultured shrimp production increases, the more the animal protein for human consumption decreases from the view-point of Japan as a whole. Unless this contradiction is solved, there is no possibility of shrimp culture development in Japan. I have been studying to find a food which does not compete with human food and I have had some success in the laboratory, but I have not yet tried a large scale experiment. I am now planning to make a big scale experiment in the summer of 1968, and I am expecting that this problem will be solved to some extent during 1968.

3 PRESENT KURUMA SHRIMP CULTURE IN JAPAN

Until 1964, for the purpose of spawning, hatching and raising of kuruma shrimp larvae, the following standard method was used. Fill a small tank (2 m × 1 m × 1 m) with filtered sea water in an almost dark room, put the mature female shrimp into this tank, and let her spawn. Give as food mainly diatoms like pure cultured S. costatum etc., after the larvae have metamorphosed into zoeas. Give nauplius of brine shrimp after the larvae have metamorphosed to the mysis stage. Give finely crushed meat of short-necked clams for several days after the larvae have transformed into post-larvae. However, as mentioned before, late in June 1964 a method for the mass culture of larvae was developed, and the culture method of larvae became entirely changed. Thereafter this new method was further improved and now it has become easy to get 1,500,000 fry in average in a concrete tank, 10 m × 10 m × 2 m in size.

The following account of the 1964 experiment is adapted from Hudinaga and Kittaka (1967).¹

3.1 The 1964 experiment

3.1.1 Limitations of earlier method The larvae of the Kuruma prawn in the nauplius stage are sustained nutritionally by the yolk which is carried within their body and the actual consumption of food begins with the zoea stage (Hudinaga, 1935). One of the foods most suitable for the zoea stage is the cultured diatom, S. costatum (see Hudinaga, 1942). However, due to the difficulty of maintaining the cultured diatoms during certain seasons, bivalve eggs and larvae or a benthic diatom which grows in special ponds having a double bottom structure have been substituted (Hudinaga and Miyamura, 1962).

One of the characteristics of the conventional method of raising the larvae on these foods is that the culturing of food, and the collection of it, is done separately from raising the larvae, and the prepared foods are later fed to the larvae in the culture tank (Hudinaga and Kittaka, 1966).

This method is relatively costly and not well suited to production on a very large scale. Dr. Kittaka and I therefore conducted experiments in order to solve the problems of mass production and lower cost by employing a larger culture tank and applying an extremely simple method to produce a large number of young prawns in a single operation.

The preparatory experiment and study were done in September of 1963 at the Kuruma Prawn Farming Co. Ltd., Ikushima, Takamatsu, and this was followed by full scale experiments and study at the Setonaikai Fishery Development Co. Ltd., Aio, Yamaguchi, from July through November 1964. The following reports are based on the results of the 1964 experiments.

¹ The editors are grateful to the Plankton Society of Japan for permission to republish this account.

3.1.2 Method The culture tanks used for this experiment were 12 salt concentration tanks (Tanks No. 1–12) at the disused salt field located in Hanaga, Aio-cho, Yoshikigun, Yamaguchi-ken. These tanks were equipped with apparatus for supplying water and aeration for experimental use. The dimensions of these culture tanks were 10 m × 10 m × 2 m, and they were constructed on concrete slabs outdoors. The floors of the culture tanks Nos. 1–8 had gradients toward the drainages, but the culture tanks Nos. 9–12 had level floors.

Twenty aeration pipes were used for each culture tank. Tanks Nos. 1–8 were provided with sufficient aeration but tanks Nos. 9–12 received insufficient aeration due to the narrowing of the main distribution pipe.

The parent prawns used for the experiment were selected from the catch from nearby Aio Bay. They were placed in the culture tank overnight for spawning and, as a rule, they were removed on the following day.

The water supply for the breeding was obtained from Aio Bay and was filtered through 80 to 100 mesh screen net. From the time of the placement of the parent prawn in the culture tank to the early part of the post-larvae stage, a period covering 15 to 20 days, the water was not changed, but after this period, 1/5 of the total amount of water was changed each day.

In this research project no provision for using the special diatom culture tanks (as used in the past conventional method) were made, but attempts were made for the propagation of plankton-like diatoms in the culture tanks by adding nutrient salts. The nutrient salts used were potassium nitrate and potassium phosphate. The eggs of Artemia or clam meat were fed according to the stage of larval development.

The approximate number of larvae in the culture tanks, in each stage, was estimated by counting the number of larvae in 1 litre of water sampled from several locations in a culture tank.

At the time of the final collection, the young prawns were placed in buckets, several thousand in 10 or 12 1 of water in each bucket. A sub-sample of 0.2 1 was taken from each bucket, after stirring, to obtain a final estimate of numbers. This final figure was more accurate than that obtained from samples from the culture tanks.

3.1.3 Results The records of the hatching and development of the larvae appear in Table I. In the case of tanks Nos. 9–12, the production of the young prawns was very low in five of the six experiments. This was probably due to insufficient aeration and to the flat bottom of the tank becoming covered with dead plankton and waste food.

Of the 22 tests which were conducted in tanks Nos. 1–8, eight were discontinued because of poor production of larvae. Four tanks out of the 14 which were continued produced less than 600,000 young prawns. Three of these four tanks were initial test tanks, in which the experiments were preparatory. The remaining test tank contained post-larvae which were kept in the tank over 100 days. Without counting these exceptional cases, the production of the young prawns per tank was 800,000 to 1,250,000, with an average count per tank of 1,000,000. However, the duration of the rearing period was 50 days, a longer period than the average for conventional methods. If the young prawns are harvested with a rearing period of about 20 days, as in the conventional method, a further increase in production can be predicted.

TABLE I

Records of breeding experiments in an outdoor concrete tank, 10 m × 10 m × 2 m, using unfiltered sea water

* Pn = n days after metamorphosis to post-larva

3.1.3.1 Relation between number of parent prawns and number of young produced A female prawn will release nearly all her eggs within two days. The number of parent prawns which produced the larvae is shown in Table I. The relationship between the number of prawns and the number of larvae produced is shown in Fig. 1. This relationship is not linear. That is to say, it is necessary to have more than 30 mature female prawns to produce 1,000 × 10³ young prawns, and yet increasing the number of mature females to more than 100 has little effect on the number of young prawns produced. In this experiment about sixty percent of the females actually spawned.

3.1.3.2 Effect of environmental conditions on hatching and development of larvae The larvae of the Kuruma prawn are sensitive to changes in environmental conditions in their nauplius and zoea stages and are especially susceptible in the zoea stage. Table II shows the water temperatures and weather conditions during early stages of development of the larvae.

Water temperatures, from the later part of July to the middle of August, averaged about 29°C at 6 a.m., and the depth of the culture tank was sufficient to keep the maximum daytime water temperature under 31°C. There were no observations of critical effects on larvae caused by increases in water temperature.

Fig. 1. Relation between the number of mature female prawns and the number of young prawns produced per tank.

The good results of spawnings in the latter part of August may have been due to the post-larvae developing at a comparatively lower temperature, resulting in a higher survival rate. Where the spawning took place at the end of September the larvae did not reach the mysis stage due to the lower temperature during the zoea stage.

The zoea stages were thought to be sensitive to decreases in the specific gravity of the water and to direct sunlight, therefore, the rearing of the larvae in the zoea stages has usually been conducted in indoor culture tanks. Among the outdoor experiments there were five which had one or more rainy days through the nauplius and zoea stages, and eight had fine weather through all the stages. In these cases, only one experiment was discontinued due to the poor survival of larvae. Therefore, the rearing of the larvae in outdoor culture tanks was not affected by the one day of rainfall nor by the successive days of strong sunlight.

TABLE II

Some environmental conditions during breeding experiments

* Owing to the good breeding condition, zoea passed into mysis within 3 days

** B: Number of bright days
C: Number of cloudy days
R: Number of rainy days

3.1.3.3 Fertilization with nutrient salts The inorganic nutrient salts dissolved in the water of Aio Bay are at about the level N: 100–200 mg/ton and P: 15 mg/ton, and consequently, in certain conditions the propagation of diatoms could be attained with this seawater as it is. However, in order to expedite the propagation and to maintain the desired propagation level of diatoms for a longer period, supplementation of the nutrient salts is necessary.

The amounts of fertilizer used, as shown in Table III, differed, depending upon the days and the tanks, since the experiments were conducted under trial conditions. In the final analysis fertilization with 200 g of potassium nitrate and 20 g of potassium phosphate, dibasic, per day per tank, from the day after hatching, became the standard quantity. It provided 180 mg of N and 20 mg of P per day for every ton of water.

Although most of the diatoms produced by adding fertilizers will be consumed by the larvae in the zoea stages, a considerable amount of diatoms will also be consumed by zooplankton which enters the culture tank through the filtering screens. Furthermore this zooplankton can be used as food for larvae after the mysis stages. From the above point of view, the addition of nutrient salts has a great significance in culturing young prawns.

TABLE III

Amount of fertilizer added and foods given during breeding experiments

* Canadian product. Hatching rate: about 40 percent

A week after the larvae have reached the post-larva stage, part of the population of diatoms which have been propagated in the tank will be replaced by pigmented protozoa, green algae, bacteria, etc., but the regular addition of fertilizers brings better results in culturing the larvae.

3.1.3.4 Feeding of Artemia Artemia are considered the best food for the mysis larva and the pelagic post-larva (Hudinaga and Miyamaura, 1962; Hudinaga and Kittaka, 1966). However, the use of Artemia was confined to the pelagic period of the post-larva in this experiment.

The amount of Artemia fed into each individual culture tank is shown in Table III and the relationship between this and the number of larvae produced is shown in Fig. 2. Examination of the culture tanks which produced larvae of more than 800 × 10³ showed that the highest results were achieved even with the minimum amount (1.3 kg) of Artemia, and that increasing the amount of Artemia to 17.7 kg resulted in no increase in prawn production.

In an experiment done in three smaller containers, the concentration of the larvae was kept constant (10 post-larvae per liter), and Artemia was added at the rate of 250, 500 and 1,000 nauplii per 1 per day. The results showed that there was no difference in the growth and survival rate, in spite of the fact that the post-larvae consumed all of the varying quantities of Artemia nauplii which were added to the different containers (Kittaka, unpublished data).

The experiments indicated that there is no significant difference in the growth and survival rate of the post-larva within the range of 0.75 to 3.0 kg of Artemia per culture tank per day. Furthermore, it is a significant fact that almost the same production level was attained even in the culture tanks which were fed with lesser amounts of Artemia, except three culture tanks in the initial experiments.

Fig. 2. Relation between the amount of Artemia given and the number of young prawns produced per tank.

Artemia used in this experiment is a Canadian product and its hatching rate is about 40 percent.

Apparently, as mentioned above, zooplankton such as copepods, etc., which feed on the micro-diatoms which in turn multiply in the waters enriched with N and P, make up an important part of the food of the early stage of post-larvae.

3.1.3.5 Feeding of baby neck clam Baby neck clam is one of the most suitable foods for the post-larva during the bottom living period (Hudinaga and Miyamura, 1962; Hudinaga and Kittaka, 1966). The clam meat is also suitable for planktonic post-larva because when it is crushed into small pieces it has a certain degree of bouyancy (Hudinaga and Kittaka, 1966). In this experiment, clam meat was used throughout the entire post-larval period.

The amount of baby neck clam used for feeding is shown in Table III, and the relationship between the total amount of food and the number of larvae produced up to the P₂₁ stage is shown in Fig. 3. The production of a million larvae was obtained by adding only 18 kg up to the P₂₁ stage, but the production did not increase as expected when the amount of food was increased to 100 kg.

Some of the diatoms produced by fertilization with N and P were not consumed by the larvae or the zooplankton and gradually precipitated along with some undigested food on the bottom of the tank. The amount of sedimentation gradually increased as the larvae grew, and by the time they entered into the post-larva stage many protozoa and micro-organisms, which propagated secondarily among the sediment, and many benthic macro-organisms, such as polychaets, barnacles, bivalves, etc. which also entered the tank as plankton, created an environment on the bottom of the culture tank which was vital to the feeding of the larvae. This sediment and the number of benthic organisms in the bottom of the tank began to decrease about the time the post-larvae entered the P₁₀ stage, which is a bottom-living stage. After the P₂₀ stage, only larger sized polychaets and mature barnacles remained on the bottom of the culture tank. This is clear indication that the bottom-living post-larvae were feeding on the sediment and the benthic organisms. Table IV shows the relationship between the total weight of the young prawns in each individual culture tank (W), which is based on the harvested numbers and an average weight of larvae, and the amount of the clam meat fed up to the time of harvesting (F).

Fig. 3. Relation between the amount of baby neck clam meat given and the number of young prawns produced per tank.

The comparatively low yield of 200 × 10³ young prawns per tank was produced for an F/W value of only 2.4. It seems likely that a similar production could be obtained relying entirely upon the natural supply of food.

3.1.4 Discussion In the conventional method, cultured diatom, Skeletonema costatum, was fed to the larvae in the zoea stages and Artemia after the mysis stage, especially in the post-larva stage. However, through the results of our experiments in supplying the larvae with various foods in each stage of development, we have found many kinds of food which can be used as substitutes (Hudinaga and Kittaka, 1966).

After finding that untreated sea water and direct sunlight produced a fairly good crop of diatoms, we have done further experiments as follows. We have used untreated sea water for the culture tank, increased illumination for the indoor type of culture tank, and we have reared the entire developmental stages in the outdoor type of culture tank (Hudinaga and Kittaka, 1966). Each experiment was successful. As a result of these series of experiments, we now conduct tests in a larger culture tank. It has proved possible to produce an average of 1,000 × 10³ young prawns in a tank of dimensions 10 m × 10 m × 2 m (depth).

TABLE IV

Relation between total weight of fry produced and amount of baby neck clam meat given

The new method and the conventional method of rearing the larvae in the culture tank differ in several ways.

First of all, the new method does not involve culturing the planktonic food nor its collection in a separate culture tank but relies on the direct application of fertilizer into the culture tank. In the new method, the propagation of phytoplankton takes place first then the zooplankton (copepods) follows, and among the sediment on the bottom the increase and growth of the benthic organisms takes place.

Secondly, the new method employs fewer female prawns and in turn it provides a considerably lower density of early stage larvae. Regarding the ten tanks which produced more than 800 × 10³ larvae, the average figures per tank were:

In the conventional small culture tank, the capacity of which is 1/100 that of the larger tank, the average figures per tank are:

Multiplying these figures by 100 for comparison with those for the larger tank gives 200–500, 20,000–40,000 × 10³ and 1,000 × 10³. That is to say, the new method, in comparison with the conventional method, requires 1/5 to 1/10 the number of female prawns per unit volume of the water, and produces 1/5 to 1/10 the number of nauplii, yet the survival rate from nauplii to young prawns is 24%, which is far better than the conventional figure of 2.5 to 5%. In both cases, the resulting number of young prawns per unit volume of water is about the same.

Thirdly, in the new method, the larvae are more or less dependent on natural food, which is produced by the addition of fertilizers all through the rearing period, therefore the addition of artificially prepared foods such as Artemia and clam meat can be reduced drastically. In some cases it may be possible to rear young prawns entirely on natural foods but some addition of food is advisable for higher densities of larvae. Thus to produce 1,000 × 10³ young prawns 5 kg of Artemia eggs (Canadian product with 40% hatching rate) and 80 kg of baby neck clam meat will be satisfactory. In the conventional method, the standard procedure was to provide 5 g of Artemia eggs (Californian product with 80% hatching rate) each day for seven days for every 10,000 early stage post-larvae. Since the survival of post-larvae averages 40%, the necessary amount of Artemia to produce 1,000 × 10³ young prawns when the Californian product is used is 8.75 kg. The equivalent amount of Canadian Artemia eggs is 17.5 kg.

According to the conventional method, the amount of the baby neck clam per day to produce 10,000 young prawns averages 70 g for the period from P₄ to P₁₁ and 140 g for the period from P₁₂ to P₂₀. Therefore, the total amount of the clam meat given up to the 20th day as post-larvae is approximately (70 × 8) + (140 × 9)-1,820 g, and the amount of clam meat necessary for every 1,000 × 10³ young prawns would be 180 kg. Therefore, in the new method, the necessary amount of Artemia to produce a unit number of young prawns is reduced to 28% and baby neck clam is reduced to 44%.

Fourthly, the new method eliminates such processes as the cultivation and collection of diatoms, the frequent feedings and the stirring which was required for even distribution of food, etc.

According to Dr. Kittaka's experience, the personnel necessary to produce 10,000 × 10³ of young prawns per month is one technician, one male laborer and two female helpers.

The above mentioned series of experiments has succeeded in reproducing the natural development of larvae, as in the ocean, within the environment of the artificial culture tank. It has achieved true mass production of young prawns in the culture tank and reduction of production costs.

One great disadvantage of the new method is that the quality of water in the culture tank tends to be lowered due to the use of raw sea water passed through only a coarse filter.

As shown in Table I, two out of the eight culture tanks in which the experiments were discontinued (excluding No. 9–12 culture tanks) showed a poor hatching rate to the nauplius stage, and the other six culture tanks had a high mortality rate in the early zoea stage. In both cases, the quality of water affected the results. The use of sand filtered sea water, as used in the conventional method, could obtain a more consistent quality of water, but the propagation of natural foods by fertilizing the water with N and P could not then be obtained and the advantages of the new method would be reduced to half.

In any case, this would have little influence on the total production of young prawns, since the culture tank which shows poor production can usually be detected in an early zoea stage, within a short period of stocking it.

The outdoor culture tanks used for these experiments were exposed to direct sunshine in summer, with resulting increases of temperature, and they experienced decreases of specific gravity due to rainfall, and we were concerned about the effects on the larvae. There were no records of critical cases which might have been a direct result of these conditions.

Since the new method introduces the maximum use of natural conditions, local and seasonal influences upon the cultivation of prawns will be unavoidable. The combined use of the new and the conventional methods could be recommended in some local cases. At present we are conducting experiments on the new method of hatching and raising of young prawns at sites other than the coastal area of Seto Inland Sea. After gathering all the materials and data of the experiments, the problems presented above will be re-evaluated.

4 RAISING SHRIMP FROM FRY TO ADULT STAGE

In Japan, shrimp ponds for culturing from fry to adult are now mostly in disused salt farms. The area of one culture pond is usually 10,000 m² to 30,000 m², and ranges from 2,000 m² to 100,000 m². The depth of the pond is about 1 to 2 meters. The pond is connected to the sea, so that it is partly drained during the ebb tide and floods during the flood tide. In the Seto Inland Sea there are regular high and low tides twice a day, so the pond is flooded and drained twice a day. The exchange of water is greatest at spring tides.

The best season for releasing fry in the culture pond is from early May to mid-June, and the fry at that time weigh 0.01 – 0.02 g. After about 6 months, fry grow into shrimp of 15 – 25 g in weight, which is their marketable weight. Weight of shrimp per 1 m² is 200 – 300 g, that is 10 – 20 shrimp per 1 m².

It is, of course, impossible for all the fry released in the pond to grow into adult shrimp. The fry in the pond decrease in number either by their feeding on each other or by predation, and their survival rate during the 6 months is 30 to 50 percent. Therefore the number of fry to be released should be decided by taking the above survival rate into consideration.

The foods for shrimp culture in Japan, as already mentioned, are short-necked clams, shrimps, fish and so on. As these animal protein sources are potential human food, Kuruma shrimp culture in Japan cannot be said to be very productive, but we may be proud of the invention of the mass production method of shrimp fry.

Now the study on food for shrimp culture is going on and I expect this food problem will soon be solved. At that time the method of Kuruma shrimp culture in Japan will undergo a complete change. I hope to explain the rearing method from fry to adult in detail after the food problem has been solved. At present, only the outline method is described in this report.

I should like to report experiments on the effect of the difference of food quality on the survival rate of shrimp and on the potential increase of production per unit area. It is to be noted that these were only small scale experiments and not on a commercial basis. These experiments were mainly conducted by Mr. Isamu Ando and Mr. Isao Ishida.

4.1 Comparative experiments with a variety of foods

In order to determine differences in growth and survival rates by feeding a variety of foods, the following experiments were conducted, testing the following foods:

1. Frozen small shrimp;

2. Crushed neck clam meat which was washed to separate the shell and meat;

3. Crushed neck clam meat without washing before separation of shell and meat;

4. A mixture of 80 percent frozen small shrimp and 20 percent by weight of crushed neck clam.

The experiments were conducted in a tank of 10 m × 10 m × 2 m (depth) and of concrete structure. One tenth of the total bottom area of the tank was covered with sand of 10 cm thickness, and a sufficient amount of aeration to keep a good oxygen content was applied. Excess food was provided.

The young shrimp which were used in the experiment were all offspring from the same female shrimp. The average weight of the young shrimp was 0.11 g at the beginning of the experiments. Each experimental tank contained 4,000 young shrimp; therefore the total weight of young shrimp in each tank was about 0.44 kg, with the young shrimp distributed about 40 per square meter.

The experiments were started on 10 June 1966 and completed on 9 November 1966 (five months). The data are summarized in Table V and water temperatures are given in Table VI.

TABLE V

Results of comparative experiments with a variety of foods

* Total shrimp production in five months per unit area
** Food consumption per gram of shrimp during five months

TABLE VI

Average water temperature (°C) for each ten-day period during feeding experiments¹

¹ In June and July the morning temperature reading was taken at 08.00, in August-November at 05.00. The evening temperature reading was taken at 20.00 throughout.

The results shown in Table V were largely unexpected.

The food used in tank 1 consisted of small frozen shrimp which were thought to be suitable for the shrimp, but the results were unexpectedly poor. Frozen food alone appears to be nutritionally inadequate. In this case, the shrimp became cannibalistic to compensate for the deficiency in nutrition and the resultant survival rate was only 16.8%. Tanks 2 and 3 both were fed with identical clam meat. Results were far superior to those in tank 1. However, in the case of tank 2, the clam meat was washed to separate the meat and the shell. Therefore, some of the soluble protein of the clam meat apparently was washed away, producing a lower quality food than that in tank 3. In tank 4, by using only 20% of freshly crushed clam meat and 80% of frozen small shrimp, results were better even than in tanks 2 and 3.

4.2 Experiments to determine the maximum cultivation density of Kuruma shrimp

The tanks used in this experiment were of the same dimensions as those in the previous experiment (10 m × 10 m × 2 m). The experiment was started on 10 June 1966 and completed on 10 November 1966.

The young shrimp used in these experiments were all offspring from the same female shrimp. The initial average weight of the young shrimp was 0.075 g. Sufficient aeration was applied. Results shown in Table VII, indicate that a concentration of 280 shrimp per square meter is too high.

TABLE VII

Maximum cultivation density of shrimp per unit area

* Total shrimp production in five months per unit area

The food for tanks 5 to 7 was 52% frozen small shrimp and 48% neck clam meat with shell.

The numbers and concentrations of young shrimp introduced were as follows:

Tank 5 - 15,000 (150/m²) Total weight 1,125 kg

Tank 6 - 28,000 (280/m²) Total weight 2,100 kg

Tank 7 - 15,000 (150/m²) Total weight 1,125 kg

Tanks 5 and 6 each had a double bottom structure over 1/10 of the total bottom area of the tank. Tank 7 had no double bottom structure. One tenth of the total area consisted of a sand floor. The thickness of the sand floor was about 10 cm in all three tanks. Water temperature is indicated in Table VIII. Oxygen concentration (cc. per liter) is shown in Table IX.

Tank 5 produced almost a 100% survival rate, (98.9%), even though a fairly high density of 150/m² of young shrimp was used. Tank 7 produced an 80.7% survival rate. In Tank 6 the density of the young shrimp per unit area was 280/m², and the survival rate was only 53.6%, considerably inferior to those of Tanks 5 and 7. It appears that a good survival rate and a normal growth of shrimp can be obtained with a density of 150/m² of young shrimp.

TABLE VIII

Average water temperature (°C) for each ten-day period during density experiments

Table IX

Average oxygen concentration (cc./l) for each ten-day period during density experiment

5 PROSPECT OF PENAEID SHRIMP CULTURE

In Japan the price of shrimp is very high. That is why the Kuruma shrimp (P. japonicus) culture pays on a commercial basis even in waters of relatively low temperature like the Seto Inland Sea. In winter, head-on shrimp costs about $8 per kg. If the shrimp price fell to half the present price, the shrimp culture enterprises would become bankrupt. We should not expect the method of Karuma shrimp culture now practised in Japan to be commercially profitable in other temperate countries.

Generally speaking, penaeid shrimp are abundant in the coastal waters of tropical and subtropical zones. The best places for shrimp culture in future should be sought in tropical and subtropical regions. Vast and boundless marshes, swamps or jungles in the tropics or subtropics will be made available for suitable shrimp farms. These areas are at present not utilized for any production purpose and are thought useless, but they are the best places for penaeid shrimp culture. Abundant solar energy and warm water are really gifts of heaven. It is not impossible to harvest shrimp twice a year if the water temperature in winter does not fall under 20°C. Fortunately, the mass production technique of Kuruma shrimp fry has been developed. I believe this technique is applicable not only to the production of Kuruma shrimp but also to that of penaeid shrimp in general.

The feeding of shrimp remains a problem, but it will be solved before long. The technique of propogating natural food organisms for shrimp in the tropics and subtropics by fertilization will develop in the future.

In this way, the cultivation of the vast and boundless marshes, swamps or jungles in the tropics or subtropics as penaeid shrimp culture farms will greatly contribute toward the increased supply of animal protein to the human race.

6 REFERENCES

Hudinaga, M., 1935 The study of Penaeus. 1. The development of Penaeus japonicus Bate. Rep.Hayatomo Fish.Res.Lab., 1(1):1–51

Hudinaga, M., 1942 Reproduction, development and rearing of Penaeus japonicus Bate. Jap.J. Zool., 10:(2)305–93

Hudinaga, M. and J. Kittaka, 1966 Studies on food and growth of larval stage of a prawn, Penaeus japonicus, with reference to the application to practical mass culture. Inf.Bull.Planktol.Japan, (13):83–94

Hudinaga, M., 1967 The large scale production of the young Karuma prawn, Penaeus japonicus Bate. Inf.Bull.Planktol.Japan, December issue Commemoration No. of Dr. Y. Matsue: 35–46

Hudinaga, M. and M. Miyamura, 1962 Breeding of the Karuma prawn (Penaeus japonicus Bate). J.oceanogr.Soc.Japan, 20:694–706