This invention relates to a sac-~ry poncl alld a methc)cl Eo~ culturing fish, particularly a metl1od in whict1 s.1c~ry are reared from an eyecl or hatching stage to an adult or releasincl stage ancl in which the growth rate oE the sac-Ery is controlled. Rearing of Eish in ponds has become increasingly popular in order to replenish fish resources. Thls practice is particu- larly prevalent in areas having rivers containing, or capable of containing, salmon and trout~ However, in many areas, the practice of rearing sac-fry in ponds has problems. In particular, maintain- ing the temperature oE water in the pond at an optimum temperaturefor rearing fry is a problem. Research shows that fry grow more slowly in water having a temperature other than an optimum temperature, particularly if the water is some degrees colder than the optimum The growth pro- cess of the salmon and trout, from the stage at which the spawn is collected to the stage at which the fry swim-up, may be expressed on the basis of a daily cumulative water temperature. The daily cumulative water temperature is obtained by multiplying the number of days the fry are reared, counted from the day of fertilization, by the average water temperature (C) for these days. Using the daily cumulative water temperature as basisl salmon and trout gene- rally require about 450 to 710 days C to proceed from fertiliza- tion to hatching and about 960 to 1100 days C to proceed from fer- tilization to swimming-up. Feeding of the fry is generally started when they begin to swim-up and, at this stage, the fry are reared in a sac-fry pond of a larger depth until they can be released. The optimum water temperature for rearing fry is approxi- mately 8C. However, salmon and trout spawn in autumn and are released in the following spring. Therefore, the fry are reared in the ponds over the cold season. For example, salmon and -trout are extensively reared in ponds in the Hokkaido and Tohoku districts in Japan. In some cases, the temperature of spring water in Hokkaido is about 8C during winter but this water is found only in certain '7'7'~i locallties and is availahLe only in limited quantities. In other areas, river water is generally used. The temperat~1re oE the river water d~lrillg the winter period may be as low as 1C to ~C. There- Eore, assuming the mean daily cumulative water temperature to rear fry to swimming-up is 1000 daysC, it would take 250 days to reach this stage at 4C but only 125 days at 8C. Therefore, there is a clear disadvantage in rearing the fry in water colder than 8~C. Apart from the increase in time required to rear fry at lower temperatures, the estimated rate of return of juveniles reared at the optimum temperature is about 3% while that at lower temperatures is about 1.5%. Also, in cold water, the death rate of the fry is increased and the fry have a habit of swimming against the flow of water to seek an environment more agreeable to them. Therefore, they tend to gather in the vicinity of the water feed part of the pond or water tank. However, in their natural environ- ment, salmon and trout fry grow from the hatching stage to the swimming-~p stage while remaining substantially motionless. There- fore, it is preferable that the salmon and trout fry are reared in the pond under conditions similar to those which would occur in their natural environment. In order to overcome these problems, it has been proposed that the water is heated to maintain its temperature at a suitable level for culturing fish. Fry of salmon must be reared in flowing water, therefore large quantities of thermal energy are required to heat the water. This causes a large increase in the operating costs of the fish rearing pond. Another problem resides in that heating the water collected from a river causes a decrease in the conten-t of the dissolved oxygen of the water. Heating also disturbs the ecosystem of micro-organisms in the collected water. An object of this invention is to provide a fish cultur- ing method capable of having a suitable environment for rearing fish despite the use of water of temperature lower than a suitable level, so as to rear lively fry, and to control the rate of growth ~377~6 _ 3 ~- of tlle Ery. Also lt i5 an object to provide a sac-Ery pond suit- able for practi 5 ing this method. ~ ccording to one aspect oE this invention, there is pro- vided a fish culturing method in which long wavelength infrarecl rays are irradiated into substantially the en-tire Ery rearing por- tion of the sac-fry pond. According to another aspect, there is provided a sac-fry pond having radiation means for irradiating long wavelength infrared rays into substantially the entire fry rearing portion of the sac-fry pond. The application of long wavelength infrared rays to a sac-fry pond causes the fry to be heated without substantially increasing the temperature of the water , and thus promotes the growth of the fry. The term "fry" includes sac-fry in the stage of their growth which is between the time at which eyes appear clearly on the bodies created in the spawn or the time at which the spawn are hatched and the time at which feeding is started, and fry in the stage of growth which is between the time at t~lich feeding is started and the time at which the fry have grown into juveniles and are released or the time at which the fry have grown into adults. The term "sac-fry pond" includes a sac-fry pond used to rear fry in their stage of growth which is between the time at which eyes appear on the bodies in the spawn or the time at which the spawn are hatched and the time at which feeding is started, and a sac-fry pond used to rear the fry in their stage of growth which is between the time at which feeding is started and the time at which the fry are released or the time at which the fry have grown into adults. Embodiments of the invention are described, by way of example only, with reference to the drawings in which: Fig. 1 is a schematic illustration of the apparatus of Example 1; ~ ~77~; -- 4 Fig. 2 is a sectional view oE a long wavelencJth in~rared ray radiator used in Fig. l; Fig. 3 is a sectional view oE a principal portion oE a sac-fry pond in Example 2, ~ ig~ ~ is a schematic view in longitudinal section of the sac-fry pond in example 2, and Fig. 5 is a partially cutaway, perspective view9 of a heating unit of Fig. 3. A long wavelength infrared ray radiator, which is inclu- ded in the heating means used in the present invention, will now bedescribed. Generally, the term l'long wavelength" infrared rays means infrared rays having a wavelength of ~ to 1000 ~m. These long wavelength infrared rays may include other thermic rays and visible rays, and may also include other thermic rays and visible rays which are radiated simultaneously with the radiation of the long wavelength infrared rays. However, the amount of visible light must be kept to a minimum as young fish, from hatchlings to fry which have just swum up, should not be irradiated with visible rays. A long wavelength infrared ray radiator means preferably comprises means for radiating the infrared rays from the surface of the water in the pond, and means for radiating the rays from the bottom of the pond. The long wavelength infrared ray radiator (which will hereinafter be referred to as an infrared heater) may be any radiator capable of radiating long wavelength infrared rays. Preferably, the infrared heater radiates rays having a wavelength of about 5 to 50 ~m, and more preferably about 15 to 50 ~mO The fry of salmon and trout, when in a stage of growth in which eyes ~377~j appear on the spawn or a stage oE growth in which th~ spawn are hatched, and up to a stage of growth in which the fry swim up, rnust be reared in the dark. ThereEore, a means for raclia~ing the in~ra- red rays at a lo~ temperature is preferably used. In order to radiate the infrared rays from a position above the pon~, it is preferable that the infrared heater is fixed at a position suitably distant from the surEace of the water for ease of handling. In order to generate the infrared rays at a low tempera- ture, an electric current is applied to, for example, a layer of carbon particles to generate Joule heat at a low temperature of, for example, not more than 100C. The Joule heat is then radiated from a radiating surface which is preferably roughened with ceramic particles, such as silica sand. Thus/ long wavelength infrared rays containing no visible rays are radiated. An example of the infrared heater provided above the upper surface of the pond is formed as follows. A gypsum plaster board is coated on one of its surfaces with a layer obtained by dispersing carbon particles into a binder of a synthetic resin. Electrode wires are then arranged at both end portions of the resultant product to form a heating layer. A radiating layer, which is obtained by dispersing ceramic particles into a binder of a synthetic resin, is formed on the heating layer or on the other surface of the gypsum plaster board, or black paint is applied to the same surface. The other or rear surface of the heating layer is covered with an insulating material. A modified example of this infrared heater is formed by substituting the gypsum plaster board by a metal sheet. Metal-sheet infrared heaters are capable of radiating long wavelength infrared rays containing no visible rays and may be used for heating fry in the pond. In use the tempera- ture of the radiating layer is usually maintained at, for example,substantially not more than 100 C and preferably at 20 to B0C. A waterproofed heating unit which has its temperature regulated to a high level and capable of radiating long wavelength ~ ~37~ inErared rays havinc~ no visible rays may be placed orl the hottom o~ the pond to provide a radiator on the pond bed sur~ace. A more practical heatin~ means is obtained by Einishing the bottom portion oE the pond with mortar and heating the surface oE the rnortar to a low temperature. In order to heat this mortar layer, a suitable heat source such as an electric heat source, a hot water supply source or some other heating medium is used. If this mortar layer is formed to a thickness of 4 to 5 cm, cracking of the mortar is prevented and variations in temperature of the heating unit are smoothed. This arrangement enables long wavelength infrared rays to be radiated stably from the bottom of the pond and into the water through a comparatively simple temerature control operation. The intensity of the long wavelength infrared rays used is not specifically limited, and may vary depending upon the conditions of use of the infrared heater. This intensity is determined experimentally in a suitable manner before practising the invention~ The infrared heater radiates long wavelength infrared rays, which contain no visible rays, uniformly to substantially all of the fry rearing regions in the pond. Any conventional sac-fry pond may be used. The sac-fry pond is preferably suitable for controlling fry from the stage of growth in which eyes appear on the spawn or the spawn are hatched, to the stage of growth in which the fry swim up. The pond is also of size to maintain the depth of the water at a level suitable for rearing fry up to a stage of growth in which the fry are released. Furthermore, the pond is preferably provided with a water feed and discharge arrangement which enables uniform and calm water currents to be generated in all parts of its interior, and for it to be kept clean easily. In a pond used to rear fry until they have swum up, it is usually necessary to lay pebbles on its bottom, among which the fry hide. The interior of the pond is then similar to the natural environment of the fry. In the place of pebbles, rings of ~3~ 7~i a suitable length obtained by cutting a ~plastic pipe crosswLse into pieces or some other suitable modiEied products may be used. A sac-fry pond for use in rearing salmon and trout is now further described. A water tank having a flat bottom is construc- ted so that the depth of water in it is maintained at a suitable level of several to ten centimetres. Pebbles are then laid on the flat bottom surface in a layer of around 3 to 4 cm. A weir is provided to carry out feeding and discharging of water effectively to obtain a uniform distribution of water currents in the pond. The water tank is shut off from direct sunlight and is kept dark. It is also protected against any excessively large vibrations. A suitable number of fry for example, not more than 15,000 fry per 1 m3 are held in the water tank. Over population of fry in the tan~ is to be avoided. Once the fry have reached a stage of growth in which they have already swum up and, in the later stages, are released, the depth of water in the tank is set to a level not less than 40 cm so as to prevent excoriation of the fry. Preferably, the new inflow water exchange rate is not less than 1. The water in the pond is controlled so that exchanging of the water in the whole of the interior of the pond is done uniformly. In a sac-fry pond in which the depth of the water is 40 cm, the population density of fry is set to, for example, 10 kg/m2, and water is fed at not more than 1~/min per 1 kg of fry, the feediny conditions being controlled suitably. The type of fish reared in the sac-fry pond is not speci- fically limited. Accordingly, this sac-fry pond may be used for rearing various kinds of young fish and shellfish, such as flat- fish, young yellowtail, sea bream, eel, scallop, ear shell, sea urchin and shrimp in addition to salmon and trout referred to above. ~3777~ Example 1 __ The invention is now clescribed with regard to rearing of Ery (sac-fry) oE salmon from the stage of growth in which the spawn are hatched to the stage of growth in which the fry swim up. Fig. 1 is a schematic diagram illustrating the fry rear- ing pond used in this Example. In Fig. 1, a sac-fry pond 1 compri- sing a water tank of about 0.5 m in width and about 2 m in length is illustrated. Pebbles 2, the diameter of each of which is around 3.5 cm, are laid on the bottom and the depth of the water in the tank is maintained at about 5 cm. A weir is positioned at each lengthwise end portion of the pond 1 to provide a water feed weir 4 and a water discharge weir 5. Both of the weirs 4 and 5 have a plurality of pipes 3, of about 2 cm in diameter and made of hard vinyl chloride, spaced at intervals of 10 cm apart and positioned below the water level~ An infrared heater Hl is suspended with strings a so that it is positioned above the sac-fry pond 1 (usually at a distance from the water level of up to about 40 cm). The position of the infrared heater is adjustable in that the distance between the heater and the water level may be regulated freely so as to irra- diate the surface of the water uniformly with long wavelength infrared rays and to expose as necessary the upper surface of the sac-fry pond 1 to the open air. The distance between the infrared heater and the surface of water is in the range of 10 to 200 cm. The infrared heater H1 is constructed as follows. As shown in the sectional view of Fig. 2, a heating layer 7 consisting of carbon particles, and a binder of a synthetic resin into which the carbon particles are dispersed is provided on a surface of a flexible board 6. Metal electrode wires 8 are arranged in the vicinity of both of the longer edges of the resultant product~ An infrared ray radiating layer 9 consisting of ceramic particles and the binder of a synthetic resin is formed on the surface of the 77~; heating layer 7. The temperature oE the heating Layer 7 and thal: o the radiating layer 9 are maintainecl at about 50 C and about 40 C, respectively in use. This inErared heater 2 consists oE a heating unit capable of radiating energy containing a high percen- tage of long wavelength infrared rays and no visible rays. Refer- ence number 9' denotes a pockmarked layer consistiny of a ceramic material and a binder of a synthetic resin into which the ceramic material is dispersed. In use, river-bed water was fed into the sac~fry pond 1 under regular management conditions, and about 10,000 fry (sac-fry) of salmon having a daily cumulative water temperature of 480 day C were placed in the water. The sac-fry were managed under such con- ditions as required by an ordinary salmon-rearing management, until the sac-fry swam up. The average temperature of the river-bed water during this time was 4.8 C. During the sac-fry rearing operation, the distance be- tween the infrared heater H1 and the surface of the water was regulated in accordance with the growing condition of the fry and the temperature of the water. Without radiation, it took 110 days to rear the fry up to the stage of growth in which the fry swam up while, in this Example, this number of days could be reduced to 80, and the fry could be reared to lively juveniles. The condition of growth of the fry during this time was found by observing the fry frequently. During this fry-rearing period, the temperature was maintained at a certain level by the radiation from the infrared heater 2 and measured with a regular thermometer and did not vary. The consumption of energy was redu- ced to a much lower level than in the conventional method in which the water flowing into the sac-fry pond is heated and, moreover, the fry were reared to lively juveniles. Example 2 Fig. 3 is a sectional view of principal portions of ~3~7'7~; 1 o sac-fry pond used in Example 2, and Fig. ~ is a longitudlnal section illustrating the construction oE the sac-Ery pond. ReEerring to Fig. 4, a sac-fry pond 1 consisl:s of a rectanguLar water tank of about 1.8 m in width ancl about 50 m in length. Pebbles 2, having a diameter oE around 3.5 cm are laid on the bottom oE the tank. The depth of water in the tank may be regulated in the range of up to substantially 50 cm. A portion of the interior of the water tank was partitioned off by a water feed weir 4 and a water discharge weir 5 to provide a test section of about 12 m in length. Each weir 4,5 has pipes 3 of about 2 cm in diameter, made of hard vinyl chloride and spaced at regular inter- vals of 10 cm, positioned below the surface of the water. The test section was formed in parallel with the remaining parts of the water tank to provide a water feed passage 16 and a water discharge passage 17 on the front and rear sides of the water tank. A heating member 20 was provided on substantially the whole surface of the bottom 18 of the test section. As shown on an enlarged scale in Fig~ 3, the heating member 20 wa formed by laying a sheet of an insulating material 21 of 235 mm in thickness on the bottom 18, an electric heater H2 (commercially obtainable under the trademark, Plaheat) thereon, a mortar layer 23 and an agricultural polyethylene sheet 24 for waterproofing the heater H2 on the heater H2. The mortar layer 23 was formed to a thickness of about 4 cm. Its upper surface is the bottom surface 18' of the sac-fry pond 1. The high thermal capacity of this mortar layer 23 serves to smooth temperature variations. The thickness of the mortar layer was set to 4 cm not only to increase its thermal capacity but also to pre- vent the occurrence of cracks, i.e., for obtaining other technical effects. The sac-fry 10 were seen to settle calmly behind pebbles 2. The heating unit in this Example will now be described in detail with reference to Fig. 5. As shown in Fig. 5, the Plaheat heater H2 is formed by burying electrode wires 8, each of which 9 ~ 3777~i consists oE copper wires plainly woven like a ribbon, in both end portions of a sheet type conductive layer 27. 'L'his Jayer 27 con~ sists of a carbon particle-mixed thermoplastic resin~ The heatiny body thus obtained is covered with an insulating layer 29 consist- ing of an electrically insulating flexible resin. rrhe Plaheat heater H2 was obtained by cutting an extrusion-molded elongated product of such construction into pieces of a suitable length. The heater has a width of 23 cm. Temperature variation of the Plaheat heater H2 was controlled to within ~1 C by a temperature controller using a thermister. 10,000 eyed spawns were placed in a sieve type net, and the net was set about 1 cm below the water level in the sac~fry pond 1 in each of the above Examples and in a Comparative Example. The fry which broke out of the spawn swam through the net and hid in the pebble layer. River water of a temperature of 1 to 2 C was supplied as the fry-rearing water into each sac-fry pond during the test period. A water~low rate of 35~/min was maintained. When spring water of a sultable temperature of 8 C was used, the standard flow rate was 80Q/min. The number of days required by the fry to absorb their yolk sacs and swim up in each sac-fry pond is shown in the Table below. Example 3 and Comparative Example Two sac-fry ponds having the same shape as the sac-fry pond in Example 2 were set in parallel with the sac-fry pond in Example 2. A heating unit having the same specification as the heating unit used in Example 1 was suspended about 40 cm above the surface of the water in one of the two sac-fry ponds, and a fry- rearing operation was carried out with the temperature of the surface of the heating unit maintained at 30 C throughout the fry-rearing period (Example 3). In the other sac-fry pond~ the fry were reared in the same manner as in Examples 2 and 3 except that 776 heating equipment was not provided at all Eor th~ colnparison's sake (Comparative Example). While tests were conclucted in ExaopLe--. 2 and 3 and the Comparative Example, the average atmospheric tempera- ture was -12 C, the temperature of the water was 1 to 2 C, and the clry bulb temperature and black bulb temperature of the surface of the water in the test section in Example 3 were 0 to 1 C and 8 C, respectively. The results of Example 3 and the Comparative Example are shown together with those of the Example 2 in the following tableO Table ...._ Number of days Temperature counted from the Average of heater day of hatching temperature of spawns to that of water C of swimming-up of C . .. _. . Example 2 12 - 15 50 1 - 2 Example 3 30 52 .. Comparative Example _ 120 1 - 2 ., _ .. .__ __ __ (Reference) _ In general when sac-fry of salmon, which took 30 days from the day of fertilization to that of hatching, were reared in spring water (referred to as "Re~erence" in the above table) of a suitable temperature, they swum up normally in 60 days in total (at a daily cumulative water temperature of 480 days C) after the day of fertilization thereof. The total number of days in which the fry in Example 2 and the fry in Example 3 were reared from the day of fertilization to that of swimming-up are 80 days and 82 days, respectively. Therefore, the fry in these Examples grew at sub- stantially the same rate. The results show that the fry could bereared to the stage of growth, in which the fry swam up, in only 77~; - 13 - a~out 20 days more than in the case where sprin~ water o~: a suit-- able temperature was used. The ~esult ~i5 clespite kh~ r~ in ~t~se Examples bein~ reared under bad conditions, L.e., by u~Lng rlv~r water, the temperature of which was 6 to 7 C lower thcln a suitable level. On the other hand, 150 days were required in the Compara- tive Example, so that it is understood that the fry cannot be grown suEficiently before the release season. If the fry are reared by directly heating the water in the sac-fry pond, the required quan- tity of thermal energy is ten times larger than that in the aboveExamples using long wavelength infrared radiation. ~lso, the quality of the water deteriorates due to heating whlch adversely affects the growth of the fry. A result, which deserves special mention, of a comparison between Examples 2 and 3 and the Comparative Example will now be described. In Examples 2 and 3, movement of the fry was minimal until they swam up. The number oE fry which moved was smaller in Example 2 than in Example 3. In the comparative example, many fry moved toward the upstream side of the sac-fry pond. Since such movement of fry is usually observed even in spring water of a suit- able temperature it can be said that the results of Example 2 are surprising. It is considered that the cause of this phenomenon is as follows. The temperature of the bottom of the sac-fry pond is higher than that of the water, and the radiant thermal energy from the infrared rays warms the fry, to raise their temperature. Accordingly, the fry hide among the pebbles and stay relatively motionless. Consequently, the fry do not move against the flow of water, i.e., they do not act by instinct. Therefore, if spring water of a suitable temperature is used in the sac-fry pond in the present invention, the pond may be applied advantageously to the rearing of fry. The consumption of energy may be reduced to a far lower level (not more than 1/10) than in the case where water is directly heated when the temperature thereof is low, ancl, moreover, the ~ry can be reared to lively juveniles. As descrlbed above, the fry pond is constructed so that substantially the whole surface of the bottom of the pond serves as a radiating floor surface. Therefore, if this sac-fry pond is used to rear fry from the stage of growth in which eyes appear on the spawn or the stage oE growth in which spawn are hatched up to the stage of growth in which the fry swim up or are released or become adults, the growth of the fry is promoted even when water of a tem- perature which is not higher than a suitable level is usedO More- over, the habit oE the fry of moving toward the upstream side of a flow of water to see~ more suitable environment can be suppressed to enable the fry to stay relatively motionless in the water of a low temperature.