DESCRIPTION

FEEDSTUFF CONVERSION METHOD FOR BENTHOS, CLEANING METHOD FOR CULTIVATION POND OR SHALLOW OCEAN WATER ON TIDELAND, AND FEEDSTUFF CONVERSION AGENT FOR BENTHOS

Technical Field

The present invention is directed to improving a biological soil environment such as a cultivation pond or shallow ocean water on tideland. Particularly, the present invention can solve a worldwide problem, that is, destruction of the natural environment involved in prawn aquaculture business. The present invention relates to a method for converting organic matter into feedstuff for benthos living in the bottom mud of a cultivation pond or shallow ocean water on tideland, a method for cleaning a cultivation pond or shallow ocean water on tideland, and an agent for converting organic matter into feedstuff for benthos.

Background Art Prawn aquaculture has been performed worldwide by repeatedly changing a production area with destruction of the natural environment. Black Tiger Prawn, which is positioned in the mainstream of prawn aquaculture, has been cultivated by developing and constructing a pond in the deposited soil of a brackish water region mainly in the tropical or

subtropical zone and constructing a pond therein. Such an aquaculture has a drawback. Immediately after a cultivation pond is constructed, a high productivity is obtained; however, the productivity decreases as the pond is continuously used. At last, no prawn is produced in the pond. Such a pond is regarded as being decayed and exhausted and finally abandoned. Aquaculture traders then move to a new land for seeking deposited soil. As a consequence, they have destroyed the natural environment more and more. A cultivation pond decays and exhausts at earliest in 2 years and at latest in about 5 years. This is a repeating cultivation failure which occurs for unknown reasons and which is a problem impossible to solve.

Then, the present inventors have investigated a cause of the repeating cultivation failure. As a result, they obtained the following findings. In the mainstream aquaculture, that is, aquaculture for Black

Tiger prawn, the soil productivity of a pond is essential as a condition. The "soil has productivity" means that the soil serves as a medium for a microbial process. If the soil loses the function as a medium, prawn is no longer cultivated. The productivity of soil, that is, microbial process, includes not only decomposing residual prawn feedstuff, excrement of prawn, and deposits such as organic matter produced in a pond, thereby maintaining a biological environment but also consuming the organic deposits to convert them into feed efficient and variable for prawn. The bottom soil of a pond must have these two roles. When these roles

disappeared or deteriorated, the pond was abandoned by the reason that a prawn did not grow occurred for unknown reasons. (the repeating cultivation failure)

In an event of dealing with the aforementioned problems involved in prawn aquaculture business, a water stream generator is provided in the cultivation pond for prawn to circulate water of the pond. In this event, organic matter deposited on a non- circulate water region of the cultivation pond is eliminated as sludge or directly decomposed by a solid chlorine tablet (see Patent Literature l). However, this method is not sufficient. This is because deposited organic matter cannot be utilized positively as feedstuff even if oxygen consumption of the mud deposited at the bottom and ammonia are reduced. Alternatively, in an attempt to utilize floating or precipitated organic matter as feed, a specific solution for plankton propagation is scattered to propagate plankton in other events (see Patent Literature 2). However, this method uses a specific extract derived from a fermentation product. Thus, this method is irrelevant to the technical field of the present invention, that is, conversion of deposited organic matter into feedstuff by use of amorphous ferric hydroxide. Furthermore, there is a technique for cultivating young fish by arranging members of inhibiting tidal current at bottom of the sea with a space interposed between them, closing the whole spaces with a net to use the closed space as a cultivation area, and scattering chlorella to cultivate young fish and the like (see Patent Literature 3). However, this technique is used for inhibiting young fish

and chlorella from flowing out of the space by tidal current and not used for improving the quality of bottom mud that has deteriorated due to feces and the residual feed. Furthermore, a technique for applying specific bacteria onto seaweed to obtain feedstuff is known (see Patent Literature 4). Moreover, a method of supplying specific feed to detoxify poisoned bivalve is described (see Patent Literature 5). However, these techniques are not concerned with use of the bottom mud as a medium for a microorganism to convert deposited organic matter into feedstuff. There is a description where an iron component is converted into amorphous ferric hydroxide by aeration to remove the iron component for water treatment (see Patent Literature 6). There is another description where amorphous ferric hydroxide is obtained by blowing air into an aqueous sulfate solution containing an alkaline metal and ferrous sulfate, which is used as an adsorbent or deodorizer for sulfur oxide, hydrogen sulfide or nitrogen oxide in the air or a gas (see Patent Literature 7).

There is still another description where a solution mixture of an aqueous solution of a ferrous salt and an alkaline aqueous solution is aerated to obtain ferric hydroxide for use in magnetic recording (see Patent Literature 8). However, these documents do not describe that ferrous chloride or ferrous sulfate is dissolved in seawater or salted fresh water and the solution is aerated to produce and separate amorphous ferric hydroxide as a precipitate. These documents are directed to ferric hydroxide for use in water treatment, gas treatment and magnetic recording and not for use in improving the bottom mud of a cultivation

pond and thus differ in the technical field and object.

Cultivation of Black Tiger prawn performed in the tropical or subtropical zone depends on the soil productivity of a cultivation pond. After the soil productivity is used up, a repeating cultivation failure occurs. However, an effective countermeasure against the repeating cultivation failure has not yet been developed.

[Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 08-298894

[Patent Literature 2] Japanese Patent Application Laid-Open (JP-A) No. 06-296444

[Patent Literature 3] Japanese Patent Application Laid-Open (JP-A) No. 11-289903

[Patent Literature 4] Japanese Patent (JP-B) No. 2772772

[Patent Literature 5] Japanese Patent Application Laid-Open (JP-A) No. 11-46617

[Patent Literature 6] Japanese Patent Application Laid-Open (JP-A) No. 06-114385

[Patent Literature 7] Japanese Patent Application Laid-Open (JP-A) No. 06-122519 [Patent Literature 8] Japanese Patent Application Laid-Open

(JP-A) No. 09-71422

Disclosure of the Invention

The present invention relates to a method for functioning the

bottom mud (utilizing soil productivity) of a cultivation pond or as a medium for a microbial process, maintaining and recovering the soil productivity. Objects of the present invention are to significantly improve the productivity of a prawn cultivation pond whose productivity has been low, prevent a repeating cultivation failure, recover the pond in which prawn has not grown due to a repeating cultivation failure, and improve the environment of shallow ocean water on tideland that has deteriorated due to organic burden.

Means for attaining the aforementioned objects are as follows. (l) A method for converting an organic matter into feedstuff for benthos, comprising- blending an amorphous ferric hydroxide with at least one of the organic matters A and B or scattering the amorphous ferric hydroxide over the bottom of either a cultivation pond or shallow ocean water on tideland, wherein the organic matter A is a deposited organic matter on a bottom and the organic matter B is in a soil on the bottom. (2) A method for cleaning one of a cultivation pond and shallow ocean water on tideland by improving a biological environment thereof, comprising: blending an amorphous ferric hydroxide with at least one of the organic matters A and B or scattering the amorphous ferric hydroxide over the bottom of either a cultivation pond or shallow ocean water on tideland on tideland;

converting an organic matter into feedstuff for benthos; wherein the organic matter A is a deposited organic matter on a bottom and the organic matter B is in a soil on the bottom.

(3) The method for converting an organic matter according to (l), the method further comprising: aerating water of either the cultivation pond or shallow ocean water on tideland from the bottom.

(4) An agent used in the method for converting an organic matter according to (l), the agent comprising an amorphous ferric hydroxide. (5) The agent according to (4), wherein method for forming the agent comprises ^" - dissolving at least one of ferrous chloride and ferrous sulfate in either seawater or salt containing-water, aerating the resulting solution, precipitating the resulting flock, and separating the resulting precipitate.

The present invention relates to a method for functioning the soil (utilizing soil productivity) of a cultivation pond or shallow ocean water on tideland as a medium for a microbial process, maintaining and recovering the soil productivity, and provides a method for improving the productivity of a prawn cultivation pond whose productivity has been low, simultaneously preventing a repeating cultivation failure and recovering the pond in which prawn has not grown due to a repeating cultivation failure, and also provides a method for improving the environment of

shallow ocean water on tideland that has deteriorated due to organic burden.

The present invention has been attained by investigating a cause of a repeating cultivation failure in a prawn pond as mentioned above and provides a method for not only preventing a repeating cultivation failure but also significantly improving the productivity in a prawn cultivation pond originally low in productivity. The method of the present invention also provides a means for solving a problem- the destruction of the natural environment with prawn cultivation currently in progress.

The problem of the prawn pond is a matter as to how to manage an organic burden and how to control soil and water quality against the organic burden. A common problem resides in environmental deterioration of shallow ocean water on tideland currently on issue. The technique of the present invention is also an effective means for improving shallow ocean water on tideland environment.

As is in the present invention, the technique for adding amorphous ferric hydroxide to soil to convert precipitated organic matter positively to feedstuff for benthos; at the same time, maintaining and improving the water quality and a biological environment of the bottom mud with the progress of cultivation has not yet been developed. It is no invention other than the present invention that provides a technique for solving aforementioned problems.

Brief Description of Drawings

Figure 1 is a graph showing improvement of FCR by addition of amorphous ferric hydroxide.

Figure 2 is a graph showing improvement of prawn in size by addition of amorphous ferric hydroxide.

Figure 3 is a graph showing the productivity of prawn versus concentration (change) of amorphous ferric hydroxide.

Figure 4 is a graph showing an improvement of oxidation reduction potential and a decrease of ammonia concentration by addition of amorphous ferric hydroxide.

Figure 5 is a graph showing the effect of amorphous ferric hydroxide in decreasing oxygen consumption of the bottom mud of a cultivation pond.

Figure 6 is a graph showing an improvement of photosynthesis rate by aeration.

Figure 7 is a graph showing an improvement of detritus productivity by aeration.

Figure 8 is a graph showing sedimentation rates of ferric hydroxide in seawater, salted fresh water and fresh water. Figure 9 is a view showing arrangement of aeration nozzles.

Best Mode for Carrying out the Invention

<Conversion into feedstuff for benthos in cultivation pond or shallow ocean water on tideland >

According to a first aspect of the present invention, there is provided a method for converting an organic matter into feedstuff for benthos, comprising: blending an amorphous ferric hydroxide with at least one of the organic matters A and B or scattering the amorphous ferric hydroxide over the bottom of either a cultivation pond or shallow ocean water on tideland, wherein the organic matter A is a deposited organic matter on a bottom and the organic matter B is in a soil on the bottom. The "cultivation pond" refers to a pond for use in cultivating shellfish such as prawn and crab, fish or analogous fishery products. Either a natural pond or an artificial pond is acceptable as the cultivation pond. The "feedstuff for benthos" refers to feedstuff for shellfish such as prawn and crab, fish or analogous fishery products living near the bottom surface. The "amorphous ferric hydroxide" refers to ferric oxide of an amorphous state containing hydroxide. As a method of " blending an amorphous ferric hydroxide or scattering the amorphous ferric hydroxide ", amorphous ferric hydroxide reduced in moisture by a dewaterer or suspended in water may be added onto the bottom surface or top surface of a pond through a pipe by using a pump or through gravity fall.

The present invention was attained on the basis of the finding the propagation of microorganisms capable of improving the bottom mud of a pond by adding amorphous ferric hydroxide at each of the initial

stage soon after construction of a cultivation pond, the middle stage and terminal stage. Since the microorganisms decompose organic matter deposited on the bottom of a pond, the organic matter deposited on the surface of the soil or contained in the soil of a cultivation pond or shallow ocean water on tideland can be converted into feedstuff.

The present inventors added amorphous ferric hydroxide to a prawn cultivation pond, which initially exhibited a productivity of a certain level and thereafter exhausted, at each of the initial stage, middle stage and terminal stage of the pond, and maintained the concentration of amorphous ferric hydroxide in the surface (3 mm in depth) of the bottom mud to 0.25%. As a result, they found not only that high productivity and large-size prawn were obtained compared to those before exhaustion of the cultivation pond but also that the amount of requisite feedstuff decreased. Microorganisms can easily utilize amorphous ferric hydroxide as an iron dissolved in water. As a result, the flora of microorganisms can be maintained healthy by addition of amorphous ferric hydroxide. Consequently, deposits such as residual prawn feed, excrement of prawn and organic matter produced by a pond are decomposed and converted into feedstuff useful for prawn. The biological environment results in being improved. Even if a pond whose productivity is low, if amorphous ferric hydroxide is added and its concentration of the soil is controlled, the productivity of a prawn pond can be improved. The principles can be also applied to shallow ocean water on tideland.

A process for converting organic matter into feedstuff is still unknown," however, when amorphous ferric hydroxide is blended with and scattered onto the bottom soil of a pond where feces, residual feed, planktons and other organic matter precipitate and deposit, ammonia can be reduced and the oxidation-reduction potential, can be increased. This is conceivably because bacterial decomposition of organic matter is accelerated, thereby facilitating conversion of the organic matter into feedstuff.

<Cleaning of cultivation pond or shallow ocean water on tideland > According to a second aspect of the present invention, there is provided a method for cleaning one of a cultivation pond and shallow ocean water on tideland by improving a biological environment thereof,comprising: blending an amorphous ferric hydroxide with at least one of the organic matters A and B or scattering the amorphous ferric hydroxide over the bottom of either a cultivation pond or shallow ocean water on tideland, ^" converting an organic matter into feedstuff for benthos," wherein the organic matter A is a deposited organic matter on a bottom and the organic matter B is in a soil on the bottom.

The "cleaning" refers to improving the quality of the soil of the bottom surface and water by improving the oxidation-reduction potential of the bottom-surface soil in a cultivation pond or shallow ocean water on tideland and reducing the ammonia concentration of the soil, with the

result that a biological environment is improved.

When feces and residual feed deposit on the bottom mud in a cultivation pond, and when a burden of organic matter increases in shallow ocean water on tideland, the oxygen consumption in the water and bottom mud increases, frequently causing shortage of oxygen.

Furthermore, since the environment leads to a reduction state more and more, oxidative decomposition of organic matter does not proceed and a harmful ammonia generation takes place. As a result, the state of soil and water quality of the cultivation pond or shallow ocean water on tideland deteriorate. In the present invention, the oxidation-reduction potential of the bottom mud can be increased and the ammonia concentration of the soil can be decreased by adding amorphous ferric hydroxide. It was confirmed that the biological environment and water quality were improved. <Conversion of organic matter into feedstuff for benthos by aerating water from the bottom of cultivation pond and shallow ocean water on tideland>

According to a third aspect of the present invention, there is provided a method for converting an organic matter according to claim 1, the method further comp rising- aerating water of either the cultivation pond or shallow ocean water on tideland from the bottom.

The "aerating" refers to introducing air into water of a cultivation pond or shallow ocean water on tideland. As a method for introducing

air, for example, air is pressurized by a blower, supplied through a pipe to the bottom of a pond and ejected from the outlets of aeration nozzles, which are attached to the pipe provided at the bottom of the pond so as to face upward, thereby blowing the air into the solution. The effect of the present invention according to the first aspect can be further improved. An aeration system is an efficient and energy saving method for supplying oxygen into the water of a cultivation pond, compared to a conventional water-wheel method. In this manner, oxygen is supplied and dissolved in seawater. Besides this, the present inventors found that vertical current circulation is produced by air bubbles moving up through water and it exerts a great effect. Vertical current circulation, which is produced by aeration in the daytime, promotes photosynthesis sterically in seawater. Actually, dissolved oxygen concentration showed an excessive saturation of 200% by carrying out aeration for about 3 hours under the daytime. In the cultivation pond having no vertical current circulation, the concentration of plankton is high and thus photosynthesis is ruled by light permeability. Therefore, the depth for effective photosynthesis is about 50 cm from the surface. In consideration that a general depth of a cultivation pond is about 1.2 m, the dissolved oxygen concentration of the pond merely reaches an excessive saturation of about 120% on an average. In addition, the concentration of oxygen at the bottom of the pond is low, suggesting that it has already been not preferable as a biological environment.

By virtue of this method, a high oxygen concentration can be maintained uniformly to the bottom of the pond and thus a large amount of oxygen can be supplied from the top to the bottom mud. Therefore, a normal flora of microorganisms can be efficiently formed in couple with the method according to the invention of the first aspect.

Furthermore, aeration is highly effective in increasing oxygen concentration and stirring water homogeneously, thereby accelerating decomposition of organic matter suspended therein. As a result, water quality is improved and a large amount of planktons are newly produced by virtue of inorganic matter increased through the decomposition. The planktons and floating suspended matters are effectively coagulated and precipitated by stirring. Therefore, raw materials for prawn feed can be supplied to a microbial medium at the bottom of a pond. The supplied matters are converted into feed for benthos in the microbial medium of the bottom mud. The feed is generally called detritus. Such an effect of coagulation/precipitation by stirring further increases if a water-wheel, which forms a horizontal current circulation, is used in combination. <Agent for converting organic matter into feedstuff for benthos , containing amorphous ferric hydroxide> According to a fourth aspect of the present invention, there is provided an agent used in the method for converting an organic matter according to claim 1, the agent comprising an amorphous ferric hydroxide. According to a fifth aspect of the present invention, there is provided an agent according to claim 4, wherein method for forming the agent

comprises^ dissolving at least one of ferrous chloride and ferrous sulfate in either seawater or salt containing-water, aerating the resulting solution, precipitating the resulting flock, and separating the resulting precipitate.

The "agent for converting organic matter into feedstuff for benthos" is one containing amorphous ferric hydroxide. The agent may be reduced in moisture content by a dewaterer or suspended in water. It is an economically excellent method that amorphous ferric hydroxide is produced from ferrous chloride (FeCb) and/or ferrous sulfate (FeSO ₄ ) as a raw material. It is also possible to produce amorphous ferric hydroxide from ferric chloride or ferric sulfate ; however, this method is improper since such a raw material is expensive. An example of the method for producing amorphous ferric hydroxide according to the present invention will be described below. Ferrous chloride or ferrous sulfate is added to and dissolved in seawater or NaCl-containing water. The solution is aerated to obtain amorphous ferric hydroxide. Since the produced amorphous ferric hydroxide is positively charged, when it is produced in general fresh water, coagulation does not proceed due to charge repulsion and thus no precipitation takes place. However, since seawater or NaCl containing water has a high electric conductivity, electric neutralization is found to easily take place. Thus, amorphous ferric hydroxide can be easily

coagulated, precipitated and separated.

Generally, the amount of amorphous ferric hydroxide required for prawn cultivation is not so large. Consequently, the amount of ferrous chloride or ferrous sulfate to be added is small. Since the pH of the pond does not change too acidic to damage cultivation of prawn, so,ferrous chloride or ferrous sulfate may be added directly to a prawn cultivation pond to produce and precipitate/add amorphous ferric hydroxide to the bottom soil of the pond. However, this case is not preferable from a prawn-cultivation point of view. This is because the gill of the prawn under cultivation is clogged with floating micro- particles of ferric hydroxide. The present inventors confirmed that if amorphous ferric hydroxide is once precipitated in the form of flock, it can be added satisfactorily without causing such a problem.

Examples

Examples will be explained below; however, the present invention is not particularly limited by these examples.

<Addition of ferric hydroxide to cultivation pond and Feed conversion ratio (FCR) of prawn> The relationship between addition of ferric hydroxide to a cultivation pond and FCR of prawn is shown in Figure 1. Condition A shows an Example in which ferric hydroxide was not added to a cultivation pond, whereas condition B shows an Example in which ferric hydroxide was added and cultivation was performed for 120 days to 135

days. Condition A shows an average value of 15 samples whereas Condition B shows an average value of 8 samples. The transverse axis of Figure 1 represents the concentration of amorphous ferric hydroxide in the bottom mud. The average value of Condition A in which no amorphous ferric hydroxide was added was 0.04%, whereas the average value of Condition B in which amorphous ferric hydroxide was added increased to 0.25%. The vertical axis represents FCR, which is the weight of feed required for gaining body weight (l kg) of prawn. FCR is generally called "conversion coefficient" in Japan and expressed as follows^

FCR = feed weight/the weight of prawn cultivated FCR in Period A is 1.75, whereas FCR in Period B decreases to 1.50, suggesting that the requisite amount of feed decreases. This means that planktons, residual feed and other organic matter in the bottom mud are efficiently converted into feed in the pond ecosystem.

<Addition of amorphous ferric hydroxide and prawn production>

The weights of prawn in Condition A and Condition B investigated above are compared and shown in Figure 2. An increase of weight per single prawn produces an economically significant effect. It is demonstrated that a large-size prawn can be cultivated by adding amorphous ferric hydroxide.

Figure 3 shows changes of productivity of prawn in a cultivation pond used in the test according to the present invention over a long period of time. In the pond used in the test according to the present

invention, initial productivity was high (Group A). Thereafter, exhaustion of the pond, proceeded with time and the productivity decreased (Group B indicated by □). The productivity of prawn was improved by addition of amorphous ferric hydroxide (Group C indicated by ■).

The concentration of amorphous ferric hydroxide was checked by use of iron extraction with ethylenediaminetetraacetic acid(EDTA). The iron extracted was quantified in accordance with phenanthroline colorimetry (Methods of Soil Nutrition Analysis, Yokendo (2002), 15th edition, p.301-302)). In this manner, the concentration of amorphous ferric hydroxide in the bottom soil of a pond was obtained. A calibration curve was obtained from a colored iron standard solution. The EDTA extraction solution was subjected to colorimetry performed at 510 nm (Spectrophotometer U- 1500, manufactured by Hitachi Ltd.) to determine the iron concentration (mg/L) of the EDTA extraction solution. <Sampling of the bottom soil of pond>

A cylindrical transparent container (5.5 cm in diameter and 12 cm in height) was inserted in the bottom soil of a cultivation pond for prawn (5 points/hectare area). The container was covered with a lid and gently moved up so as not to drop the soil. This method enables to collect soil without disturbing the state of the bottom of the pond. The soil thus taken was separated into a surface (upper) layer and a lower layer depending upon the color of the soil. A small container (1.5 cm in diameter and 9.5 cm in height) was charged with the soil of the surface

layer so as not to introduce air. The soil was used for each analysis. <Measurement of oxidation-reduction potential, ammonia concentration and oxygen consumption of bottom mud when amorphous ferric hydroxide is added> The oxygen-reduction potential was improved and ammonia concentration of soil was decreased by adding amorphous ferric hydroxide. These are shown in Figure 4. Furthermore, the oxygen ' consumption of bottom mud decreased as shown in Figure 5. Both of these data indicating improvement of a biological environment were obtained by adding amorphous ferric hydroxide, which was produced by the method described later, in an amount of 0.2% by weight based on the bottom surface-layer soil of a pond of 3 mm in thickness during cultivation of prawn.

In Figure 4, at the time point of 120 days after initiation of prawn cultivation, the biological environment of the pond before addition (of amorphous ferric hydroxide) reached almost a limit on cultivation. The environment was recovered by addition (of amorphous ferric hydroxide) and prawn was able to be harvested in a certain period of cultivation. Figure 5 shows examples in which oxygen concentration was improved from a cultivation limit value of 0.3 ppm at nighttime because of high oxygen consumption at the bottom soil of a pond.

The oxidation-reduction potential, ammonia concentration and oxygen consumption of the bottom mud herein were measured by the following methods.

The oxidation-reduction potential was obtained by measuring ORP of the surface "layer of the soil taken by a soil collection method. A small container charged with the soil was filled with oxygen-free water. Then, an ORP electrode (manufactured by METTLER TOLEDO, type: INLAB501, REDOX (PLATINUM)) was inserted up to a depth of 2 cm. Five hours after the insertion, an ORP value was determined by an ORP meter (manufactured by WTW, type: MultiLine P4) (Soil nutrient analysis, Yokendo (2002), 15th edition, p.53-69).

The concentration of ammonia in soil was measured in accordance with potassium chloride solution leaching — indophenol blue absorptiometry (Soil environment analysis, Hakuyu-Sha (2000) 2nd Edition, p.241-245). The soil (wet soil) of the surface layer (2 cm) was taken by the soil collection method. From the soil, ammonium ions were eluted by a potassium chloride solution leaching method. The amount of ammonium ions was quantified by the indophenol blue absorptiometry and determined as the amount of ammonia contained in soil.

The bottom-mud oxygen consumption was measured by taking the bottom mud of a cultivation pond, culturing the mud in a brown glass bottle together with seawater of the pond, and determining the consumption of dissolved oxygen in the seawater. More specifically, soil (wet soil) was taken from the surface layer (l mm). The soil (4 g) was placed in a 530 ml (in volume) brown glass bottle. The bottle was filled with seawater. A magnetic stirrer was placed and a dissolved oxygen concentration measuring electrode (manufactured by WTW, type: CeIIOx

325) was inserted in the bottle. The concentration of dissolved oxygen in the brown glass bottle (O2 mg/L) was measured by a dissolved oxygen meter (manufactured by WTW, type- Oxi 33Oi). The seawater in the brown glass bottle was allowed to stand in a thermal insulation machine (dark place) controlled at 28°C while gently stirring the seawater at 60 rpm. The concentration of dissolved oxygen in the brown glass bottle was measured at the initial time, 2 hours, 4 hours and 6 hours after initiation of stand-still. Separately, a brown glass bottle (blank) containing seawater alone and no soil was prepared. The concentration of dissolved oxygen of the blank was measured in the same manner. Based on the dissolved oxygen concentration measured at each of the time points with respect to the brown glass bottle (test sample) and the brown glass bottle (blank) containing only seawater, dissolved oxygen consumption of the soil added therein per hour (O2 mg/h) was calculated. The obtained oxygen consumption of the soil (4 g) of the surface layer (l mm) thus obtained was converted into that of the soil ^: 1 mm layer x 1 m ² (specific gravity- 1.5, wet soil of about 1,500 g). In this way, the oxygen consumption (O2 rαg/m ² -mm-h) of the bottom mud was obtained. <Photosynthesis efficiency by aeration> Figure 6 shows a profile of dissolved oxygen concentration in the presence or absence of aeration obtained in an Example. Aeration was performed by a roots blower. Pipes equipped with air bubble generation nozzles so as to face upward were embedded in the bottom of a cultivation pond and air was supplied to be pond at a rate of 0.5

L/m ² -min. The arrangement of the aeration nozzles are shown in Figure 9. When aeration is not performed, the dissolved oxygen concentration of the surface of a pond shows a relative high value since photosynthesis effectively works, whereas the dissolved oxygen concentration of the bottom of the pond is low since light does not permeate to the bottom. When aeration is performed, the dissolved oxygen concentration of the entire pond increases, indicating that photosynthesis works highly efficiently. Since oxygen is supplied through aeration also to the bottom of the pond, the environment of the bottom mud is effectively improved. The concentration of dissolved oxygen was measured by the dissolved oxygen meter (manufactured by WTW, Type- Oxi330i). <Measurement of the production amount of detritus by aeration>

Figure 7 shows the production of detritus by aeration. The amount of detritus serving as feedstuff for prawn is increased by aeration.

The detritus was measured by setting a collection bottle at the bottom of a pond and capturing precipitated aggregate by the bottle. The amount of the captured detritus is determined as production of detritus. <Amorphous ferric hydroxide manufactured by dissolving ferrous sulfate (7 hydrates) in seawater or salted fresh water, aerating the solution, thereby separating as flock precipitates>

Five hundred mL of seawater containing 3.5% NaCl was placed in a 500 mL beaker. Ferrous sulfate (7 hydrates) was added up to 500

pprα in terms of iron. Aeration was performed by supplying air to produce flock of ferric hydroxide. The precipitation rate of the flock of ferric hydroxide was measured. Ferric hydroxide was produced in the same aeration manner except that diluted seawater containing 1.0% NaCl, salted fresh water containing 3.5% NaGl, salted fresh water containing 1.0% NaCl and fresh water were used in place of seawater containing 3.5% NaCl. The precipitation rate of the ferric hydroxide flock thus produced was measured. The results are shown in Figure 8. Amorphous ferric hydroxide flock can be easily and efficiently precipitated and separated by dissolving ferrous sulfate (7 hydrates) in seawater or salted fresh water and aerating. The environment of the bottom mud can be improved since the obtained amorphous ferric hydroxide flock precipitates on the bottom of a pond.

The precipitation rate of amorphous ferric hydroxide was measured by taking a solvent at a depth of 5 cm from the surface of water after aeration was terminated and measuring absorption of the solvent thus taken at 720 nm (by an absorptiometer (Spectrophotometer U-1500) manufactured by Hitachi Ltd). The time required for obtaining an absorbance of 0.1 was measured. Based on the measurement time, the precipitation rate (cm/h) of the amorphous ferric hydroxide generated per hour was calculated.

Industrial Applicability

The present invention makes it possible to function the bottom mud

(utilize soil productivity) of a cultivation pond or shallow ocean water on tideland as a medium for a microbial process, maintain and recover the soil productivity. In this way, the productivity of a cultivation pond whose productivity has been low can be significantly improved. Besides this, a repeating cultivation failure can be prevented, a pond where no cultivation is made due to a repeating cultivation failure can be recovered and the environment of shallow ocean water on tideland a that has deteriorated due to organic burden can be improved.