Eel growth promoting method, eel production method, and eel growth promoting agent

Abstract

[Problem] To provide a means for promoting growth of eels and the like that grow slowly in culture of edible eels. [Problem] To provide a method for promoting the growth of eels, and the like, whereby eel fries (glass eels and small eels longer than the glass eels) are allowed to ingest an eel feed containing soy isoflavone. In the culture of edible eels, eel fry take in soy isoflavone and the like, so that the growth of the eels can be promoted. Especially, the eels growing slowly can be promoted to grow, so that the production efficiency of the eels can be improved, the breeding period can be shortened, and labor force and cost required by breeding can be reduced.

Description

[0001] This application claims the benefit of Japanese application No. 2023-023872, filed on February 17, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present invention relates to a method for promoting eel growth by allowing fry to ingest eel feed containing soy isoflavones, an eel production method including the above steps, and an eel growth promoter containing soy isoflavones as an active ingredient. Background Art

[0003] Eel is a general term for fish belonging to the genus Anguilla in the family Anguilla. There are 19 known species worldwide, including the Japanese eel, the flower eel, and the European eel. Eel has been used as food throughout Europe and East Asia since ancient times, and is particularly popular in Japan, where it has become a traditional ingredient.

[0004] For example, the Japanese eel (Anguilla japonica), which inhabits various parts of East Asia, including Japan, is believed to spawn near the Mariana Islands in the Pacific Ocean. After spawning as a juvenile eel (Leptocephalus), it metamorphoses into nearly transparent glass eels, measuring 5 to 6 cm in length and weighing 0.2 to 0.3 grams. They then ride the Kuroshio Current to coastal areas of East Asia. It is then presumed that these eels settle in inland waters, coastal areas, and brackish waters, growing into young eels (such as black fry). After further maturation after five to ten years, they return to their spawning grounds. Other eels follow a similar lifestyle, although spawning and settlement locations may vary.

[0005] Due to high demand and high profits, eels are widely farmed, accounting for over 99% of total consumption. Currently, the mainstream eel farming method involves capturing naturally occurring glass eels that migrate to coastal areas as seedlings, then stocking them in aquaculture ponds and raising them to mature. While there are reports of successful laboratory trials of artificially hatched fry, raising them into parent eels and subsequently producing the next generation of fry, commercial production has yet to be achieved.

[0006] If glass eels are raised in captivity during their transition from young to young, they will mostly become males. Therefore, farmed eels for human consumption are bred and shipped as males. Male eels typically grow slower after exceeding 300g, resulting in tougher flesh and a reduced quality and flavor. Therefore, farmed eels for human consumption are typically shipped before reaching 250g. Furthermore, female eels have a greater growth limit than males, maintaining their tender flesh even after exceeding 300g, resulting in a higher commercial value than males. However, as mentioned above, most eels in captivity become males, making females difficult to obtain.

[0007] Here, soybean isoflavones, which are a relevant matter of the present invention, will be described below.

[0008] Soy isoflavones are a general term for flavonoid compounds with isoflavone as the basic skeleton, and are mainly found in large quantities in soybean germ, etc. Soy isoflavones are divided into four types: glycoside type (glycoside; structure covalently bound to sugar), aglycone type (non-glycoside; structure with the sugar portion of the glycoside detached), acetylated form of the glycoside, and malonylated form of the glycoside. Three compounds are known in each category, for a total of 12 known soy isoflavone compounds. Among them, the three glycoside-type soy isoflavones are genistin, daidzein, and glycitein, while their aglycone types (with the sugar portion detached) are genistein, daidzein, and glycitein, respectively. The composition and content of each compound vary depending on the type of raw soybean, etc., and the extraction, purification, and processing methods. In addition, as a method for isolating specific compounds from soy isoflavones, for example, Patent Document 1 discloses a method for separating high-purity genistein from an isoflavone mixture using a solvent.

[0009] Soy isoflavones have a chemical structure similar to that of female hormones (estrogen), and are also known as phytoestrogens. Furthermore, soy isoflavones in the aglycone form have estrogen-like effects and are said to be effective in preventing heart disease, menopausal disorders, osteoporosis, and breast cancer.

[0010] In addition, Patent Document 2 discloses a method for inducing eel feminization by taking soy isoflavones, etc. In addition, Non-Patent Document 1 records that 0, 25, 50, 100, 500, and 1000 mg of genistein were applied to carp per kilogram of feed, and the results showed that 100 mg and 500 mg of genistein significantly improved growth, body composition, lipid metabolism, and immunity; Non-Patent Document 2 records that 0, 10, 20, 40, 60, and 80 mg of soy isoflavones were applied to golden pomfret per kilogram of feed, and the results showed that 40 mg of soy isoflavones significantly improved growth and enhanced antioxidant capacity. and immunity; on the other hand, non-patent document 3 records that 2.5g of 40% soy isoflavones were applied to each kilogram of feed for yellow eel, and the results showed that the growth rate was reduced and the alkaline phosphatase in the blood was increased (abnormalities in the liver, etc.); non-patent document 4 records that 0.3% genistein was applied to the dry matter feed of rainbow trout, and the result was a decrease in food intake, but the protein utilization rate was improved, and finally the growth rate was obtained at the same level as the control group. Overall, it was judged that the addition of 0.3% genistein had no beneficial effect on the growth of rainbow trout.

[0011] Patent Document 1:

[0012] Japanese Patent Application Laid-Open No. 7-173148

[0013] Patent Document 2:

[0014] Japanese Patent No. 6970992

[0015] Non-patent document 1:

[0016] Liping Yang, et al "Evaluation of dietary genistein on the antioxidantcapacity, non-specific immune status, and fatty acid composition of common carp (Cyprinus carpio.L)" Aquaculture Volume 550,15March 2022,737822.

[0017] Non-patent document 2:

[0018] Chuanpeng Zhou, et al "The Effects of dietary soybean isoflavones ongrowth, innate immune responses, hepatic antioxidant abilities and diseaseresistance of juvenile golden pompano Trachinotus ovatus" Fish&ShellfishImmunology Volume 43, Issue 1, March 2015, Pages 158-166.

[0019] Non-patent document 3:

[0020] Yajun Hu, et al "Effects of dietary soy isoflavone and soy saponin ongrowth performance,intestinal structure,intestinal immunity and gutmicrobiota community on rice field eel(Monopterus albus)" Aquaculture Volume537,15May 2021,736506.

[0021] Non-patent document 4:

[0022] C.Torno, et al "Effects of resveratrol and genistein on growth, nutrient utilization and fatty acid composition of rainbow trout" Animal Volume 13, Issue5, 2019, Pages 933-940 Summary of the Invention

[0023] Problems to be solved by the invention

[0024] For example, when glass eels are stocked in aquaculture ponds, their growth rates often vary. Some individuals reach shipping size quickly, while others produce a large number of slow-growing individuals. A high number of slow-growing individuals reduces the farm's eel production efficiency and prolongs the breeding cycle, increasing labor and costs. Therefore, promoting the growth of slow-growing individuals is crucial, both for production efficiency and labor and cost considerations.

[0025] Therefore, an object of the present invention is to provide a method for promoting the growth of slow-growing edible eels in culture.

[0026] Solutions used to solve problems

[0027] The present invention provides a method for promoting the growth of eels, etc., by causing fry eels to ingest eel feed containing soybean isoflavones.

[0028] In the cultivation of edible eels, the growth of eels can be promoted by allowing eel fry to ingest soy isoflavones and the like.

[0029] The present invention is particularly effective in promoting the growth of slow-growing eels. In edible eel farming, the production of a large number of slow-growing individuals reduces eel production efficiency, prolongs the breeding cycle, and increases the labor and costs required for breeding. The present invention, however, can promote the growth of slow-growing eels, thereby improving eel production efficiency, shortening the breeding cycle, and reducing the labor and costs required for breeding.

[0030] In the present invention, "eel fry" refers to glass eels and larger eels. For convenience, eels weighing less than 0.5g are referred to as glass eels, and eels weighing 0.5g to 20g are referred to as small eels. Eel fry includes both types of eels.

[0031] Effects of the Invention

[0032] According to the present invention, the growth of edible eels can be promoted in the cultivation of edible eels, and it is particularly effective for promoting the growth of slow-growing eels in the cultivation of edible eels. DETAILED DESCRIPTION

[0033] <About the eel feed of the present invention>

[0034] The present invention broadly encompasses: eel feed containing 0.01% by weight or more of soy isoflavones based on a ratio in dry matter, eel feed containing 0.01% by weight or more of soy isoflavone aglycones based on a ratio in dry matter, and eel feed containing 0.002% by weight or more of genistein based on a ratio in dry matter.

[0035] By making eel feed contain soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein, etc.), feeding these feeds for a certain period of time during the eel fry (for example, the class weighing 0.2g to 20g) can promote the growth of eels.

[0036] Formulated feed used as eel feed is often prepared by adding water, feed oil (fish oil), etc., kneading it into a paste, and then feeding it to eels.

[0037] In the present invention, the soy isoflavone content is preferably 0.01% by weight or more, more preferably 0.05% by weight or more, and most preferably 0.1% by weight or more, based on the dry matter (dry feed, the same below) weight of the feed before preparation (addition of water, etc.) at the time of use as 100%. There is no particular upper limit, but if the soy isoflavone content is high, depending on the time of ingestion, almost all eels will be induced to feminize. Therefore, for example, it is preferably 4.0% by weight or less.

[0038] Soy isoflavones can be widely used and are not particularly limited. For example, ready-made products can be used, or soy isoflavones prepared by extraction, purification, and treatment from raw materials such as soybeans by known methods can be used.

[0039] From another perspective, based on the ratio in dry matter, that is, when the weight of the feed before preparation (addition of water, etc.) is set as 100%, it is preferred that the content of soybean isoflavone aglycones is 0.01% by weight or more, more preferably 0.05% by weight or more, and most preferably 0.1% by weight or more. The upper limit is not particularly limited, but if the content ratio of soybean isoflavone aglycones is high, almost all eels will be induced to feminize depending on the time of ingestion, so, for example, it is preferably 4.0% by weight or less. In addition, "soybean isoflavone aglycones" are the non-sugar parts of soybean isoflavones (the same below), and their content weight can be obtained, for example, by converting the molecular weight ratio of glycosides to aglycones based on the weight of the added soybean isoflavones, or by analyzing using known test methods. In addition, the isoflavone aglycones of the present invention also widely include components that have been chemically treated with isoflavone aglycones, as long as they are derived from soybeans and maintain their growth-promoting effect on eels, such as hydroxylated isoflavones (dihydroxyisoflavones) as hydroxylated isoflavone aglycones.

[0040] From another perspective, based on the ratio in dry matter, that is, when the weight of the feed before preparation (addition of water, etc.) is set to 100%, it is preferred that genistein be contained in an amount of 0.002% by weight or more, more preferably 0.01% by weight or more, and most preferably 0.02% by weight or more. The upper limit is not particularly limited, but if the genistein content ratio is high, almost all eels will be induced to feminize depending on the time of ingestion, and therefore, for example, it is preferably 0.8% by weight or less.

[0041] There are no particular limitations on the method for adding genistein to eel feed. For example, genistein can be added to eel feed by adding soy isoflavones containing genistein to eel feed. Genistein can also be added directly to eel feed. Soy isoflavones or genistein can also be pre-added to eel feed during the production stage.

[0042] Genistein can widely adopt known ones, is not particularly limited.For example, can use ready-made products, also can directly use the soybean isoflavones that contain genistein, can also use by known method from soybean isoflavones etc., extract, purify, process and separate or highly purified genistein, can also use by known method from beans or its processed products, separate, extract, ferment etc. and the genistein that obtains, can also use by known method and synthetic genistein.For example, when described soybean isoflavones contains the genistein more than 10 weight %, have following advantage: be easier to obtain, make, prepare, and can effectively promote eel growth.

[0043] Furthermore, the genistein of the present invention also broadly encompasses pharmacologically acceptable salts thereof and genistein derivatives as long as the growth-promoting effect on eels is maintained.

[0044] The eel feed of the present invention only needs to contain soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein), and is not narrowly limited to other formulas, component compositions, etc.

[0045] For example, the formula and composition of eel feed, other than the soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof), can be the same as those of commonly used formulated feeds. Formulated feeds are feeds that contain nutrients necessary for eel growth in appropriate proportions, such as fish meal (50% by weight or more on a dry matter basis), and may further contain starch, calcium phosphate, salt, yeast, herbal extracts, and the like.

[0046] The fish meal in the formulated feed can be widely used and is not particularly limited. For example, it can be powder obtained by processing sardines, mackerel, herring, horse mackerel, etc. Furthermore, it can be fish meal from other fish species, or a mixed powder of fish meal from multiple fish species. The appropriate choice can be made based on the type of eel, growth status, cost, and other factors.

[0047] Furthermore, for example, components that improve the growth efficiency of eels, such as lactic acid bacteria, butyric acid bacteria, digestive enzymes, and dried vegetables, may be appropriately contained.

[0048] <About the eel growth promoter of the present invention>

[0049] The present invention broadly encompasses: (1) an eel growth promoter containing soy isoflavones as an active ingredient that allows eel fry to take in 5 mg / kg (eel body weight) or more of soy isoflavones per day; (2) an eel growth promoter containing soy isoflavone aglycones as an active ingredient that allows eel fry to take in 5 mg / kg (eel body weight) or more of soy isoflavones per day; and (3) an eel growth promoter containing genistein as an active ingredient that allows eel fry to take in 0.625 mg / kg (eel body weight) or more of genistein per day.

[0050] The eel growth promoter of the present invention contains soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) as active ingredients. By feeding these growth promoters to eel fry (e.g., weighing 0.2g to 20g) for a certain period of time, the growth of the eels can be promoted.

[0051] In the case of a formulated feed, feeding is usually performed on a full stomach. In this case, the feeding amount is, for example, about 5 to 10% of the body weight of eel fry (for example, weighing 0.2 g to 20 g).

[0052] In this case, for example, during the eel fry period (e.g., those weighing 0.2g to 20g), the formulated feed can be fed at a rate of 5-10% of their body weight. The daily intake of soy isoflavones is preferably at least 5 mg / kg (eel body weight), more preferably at least 25 mg / kg, and most preferably at least 50 mg / kg. While there is no particular upper limit, if the soy isoflavone content is high, feminization will be induced in almost all eels depending on the time of ingestion. Therefore, for example, it is preferably at or below 2,000 mg / kg.

[0053] From another perspective, for example, when fry (eels weighing 0.2g to 20g, for example) are in the early stages of feeding, the formulated feed can be fed at a rate of 5-10% of their body weight. The daily intake of soy isoflavone aglycones is preferably at least 5 mg / kg (eel body weight), more preferably at least 25 mg / kg, and most preferably at least 50 mg / kg. While there is no particular upper limit, a high soy isoflavone aglycone content can induce feminization in almost all eels depending on the timing of ingestion. Therefore, for example, a lower intake of 2,000 mg / kg is preferred.

[0054] From another perspective, for example, during the period of eel fry (e.g., those weighing 0.2g to 20g), the feed intake of the formula feed can be 5 to 10% of the body weight, and the daily intake of genistein is preferably 0.625 mg / kg (eel body weight) or more, more preferably 3.125 mg / kg or more, and most preferably 6.25 mg / kg or more. The upper limit is not particularly limited, but if the genistein content ratio is high, almost all eels will be induced to feminize depending on the intake period, so, for example, it is preferably 250 mg / kg or less.

[0055] The eel growth promoter of the present invention may contain excipients, lubricants, binders, disintegrants, solvents, solubilizers, suspending agents, buffers, isotonicity agents, preservatives, antibacterial agents, antioxidants, pH adjusters, dispersants, colorants, defoaming agents, and the like, depending on the purpose, application, dosage form, and the like.

[0056] Preferred examples of the excipient include lactose, granulated sugar, D-mannitol, starch, crystalline cellulose, and light anhydrous silicic acid.

[0057] Preferred examples of lubricants include magnesium stearate, calcium stearate, talc, and colloidal silicon dioxide.

[0058] Preferred examples of the binder include crystalline cellulose, granulated sugar, D-mannitol, dextrin, hydroxypropyl cellulose, hydroxypropyl methylcellulose, and polyvinyl pyrrolidone.

[0059] Preferred examples of disintegrants include starch, carboxymethyl cellulose, carboxymethyl cellulose calcium, croscarmellose sodium, and sodium carboxymethyl starch.

[0060] Preferable examples of the solvent include water for injection, alcohol, propylene glycol, polyethylene glycol (MACROGOL), sesame oil, corn oil and the like.

[0061] Preferred examples of the solubilizing agent include polyethylene glycol, propylene glycol, D-mannitol, benzyl benzoate, ethanol, trisaminomethane, cholesterol, triethanolamine, sodium carbonate, sodium citrate, and the like.

[0062] Preferred examples of suspending agents include surfactants (stearyl triethanolamine, sodium lauryl sulfate, lauryl aminopropionic acid, lecithin, benzalkonium chloride, benzethonium chloride, glyceryl monostearate, etc.), hydrophilic polymers (polyvinyl alcohol, polyvinyl pyrrolidone, sodium carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, etc.), and the like.

[0063] Preferred examples of the buffer include buffers such as phosphate, acetate, carbonate, citrate, tartrate, Tris, and HEPES.

[0064] Preferred examples of the isotonic agent include sodium chloride, glycerol, and D-mannitol.

[0065] Preferred examples of drugs for the purpose of preservation include thimerosal, parabens, phenoxyethanol, chlorobutanol, benzyl alcohol, phenylethyl alcohol, dehydroacetic acid, sorbic acid, other various preservatives, antibiotics, synthetic antibacterial agents, and the like.

[0066] Preferred examples of antioxidants include sulfites and ascorbic acid.

[0067] Preferred examples of pH adjusters include acids such as hydrochloric acid, carbonic acid, acetic acid, citric acid, phosphoric acid, boric acid, and sulfuric acid; alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, calcium hydroxide, and magnesium hydroxide; alkali metal carbonates or hydrogencarbonates such as sodium carbonate; alkali metal acetates such as sodium acetate; alkali metal citrates such as sodium citrate; bases such as trometamol; monoethanolamine; and diisopropanolamine.

[0068] Preferred examples of the dispersant include sodium carboxymethylcellulose, hydroxypropylmethylcellulose, polyvinylpyrrolidone, and polysorbate 80.

[0069] Preferred examples of colorants include caramel color, gardenia pigment, anthocyanin pigment, annatto pigment, paprika pigment, safflower pigment, monascus pigment, carotene pigment, carotenoid pigment, flavonoid pigment, cochineal pigment, amaranth (Red No. 2), erythrosine (Red No. 3), allura red AC (Red No. 40), carmine (Red No. 102), fluorescent pink (Red No. 104), rose Bengal (Red No. 105), acid red (Red No. 106), tartrate (Yellow No. 4), sunset yellow FCF (Yellow No. 5), fast green FCF (Green No. 3), brilliant blue FCF (Blue No. 1), indigo carmine (Blue No. 2), copper chlorophyllin, and sodium copper chlorophyllin.

[0070] Preferred examples of the defoaming agent include dimethicone, simethicone, silicone emulsion, sorbitan sesquioleate, and nonionic substances.

[0071] In addition to the above, this agent may also contain auxiliary ingredients as appropriate, such as light-absorbing pigments (riboflavin, adenine, adenosine, etc.) as preservation and efficacy aids, chelating agents and reducing agents for stabilization (vitamin C, citric acid, etc.), carbohydrates (sorbitol, lactose, mannitol, starch, sucrose, glucose, dextran, etc.), casein digests, various vitamins, lactic acid bacteria, butyric acid bacteria, digestive enzymes, dried vegetables, etc.

[0072] <About the eel growth promotion method of the present invention>

[0073] The present invention includes all of the following contents: (1) a method for promoting the growth of eels, comprising causing eel fry to ingest an eel feed containing 0.01% by weight or more of soy isoflavones based on a ratio of dry matter; (2) a method for promoting the growth of eels, comprising causing eel fry to ingest an eel feed containing 0.01% by weight or more of soy isoflavone aglycones based on a ratio of dry matter; (3) a method for promoting the growth of eels, comprising causing eel fry to ingest an eel feed containing 0.002% by weight or more of genistein based on a ratio of dry matter; (4) a method for promoting the growth of eels, comprising causing eel fry to ingest 5 mg / kg (eel body weight) or more of soy isoflavones per day; (5) a method for promoting the growth of eels, comprising causing eel fry to ingest 5 mg / kg (eel body weight) or more of soy isoflavone aglycones per day; and (6) a method for promoting the growth of eels, comprising causing eel fry to ingest 0.625 mg / kg (eel body weight) or more of genistein per day.

[0074] For example, during the period of eel fry (for example, those weighing 0.2g to 20g), by allowing the eels to ingest eel feed containing soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) at the above-mentioned ratio for a certain period of time, from another perspective, by allowing the eels to ingest soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) within the above-mentioned intake range for a certain period of time, the growth of the eels can be promoted.

[0075] For example, fry eels are allowed to ingest 5 to 2,000 mg / kg (eel body weight) of soy isoflavones daily for a certain period, more preferably 25 to 2,000 mg / kg, and most preferably 50 to 2,000 mg / kg.

[0076] From another perspective, fry eels are allowed to ingest 5 mg / kg to 2,000 mg / kg (eel body weight) of soy isoflavone aglycones daily for a certain period of time, more preferably 25 mg / kg to 2,000 mg / kg daily, and most preferably 50 mg / kg to 2,000 mg / kg daily.

[0077] From another perspective, the eel fry are allowed to ingest 0.625mg / kg to 250mg / kg (eel body weight) of genistein per day for a certain period of time, more preferably 3.125mg / kg to 250mg / kg per day, and most preferably 6.25mg / kg to 250mg / kg per day.

[0078] The method for ingesting soy isoflavones and the like into eels is not particularly limited. For example, eels can be fed with feed containing soy isoflavones and the like, eels can ingest soy isoflavones and the like separately from the feed, or preparations containing soy isoflavones and the like can be directly administered to individual eels.

[0079] The duration of soy isoflavone ingestion is not particularly limited, as long as the eels are ingested over a specific period. This period can be adjusted appropriately based on observation of the eel's growth, etc. For example, during the eel fry period (e.g., weighing 0.2g to 20g), the eels can be ingested with soy isoflavone continuously for 10 days to 6 months while observing their growth.

[0080] There are no particular restrictions on how often eels are fed soy isoflavones, etc., as long as they can be fed continuously without interruption. For example, feeding eels soy isoflavones, etc. 3 to 7 days a week can effectively promote their growth. Furthermore, the daily intake can be administered all at once or divided into 2 to 3 doses.

[0081] The present invention encompasses all methods for promoting growth by ingesting soy isoflavones and other substances for a certain period of time in eel fry that are either undifferentiated / unclear in sex, or after differentiation. The subject matter is not narrowly limited to sex differentiation. However, as mentioned above, it is known that ingesting soy isoflavones and other substances in eels can induce feminization (Patent Document 2). For individuals induced to feminize, it is difficult to clearly determine whether the feminization-inducing mechanism or the growth-promoting mechanism is more effective through ingestion of soy isoflavones and other substances. On the other hand, for male individuals (excluding those identified as female or feminized after growth), ingestion of soy isoflavones and other substances solely contributes to the growth promotion of eels, making it possible to clearly determine the growth promotion in the present invention. Therefore, for example, by limiting the subject matter to eel fry and excluding those identified as female or feminized after growth, it can be determined whether the present invention has been utilized. It is speculated that the growth-promoting mechanism in the present invention is different from the feminization-inducing mechanism, and the fact that growth is promoted and male individuals (non-feminized individuals) exist after growth indicates that the growth-promoting mechanism is more effective than the feminization-inducing mechanism in these individuals. For example, in a specific farm, after one year and six months of introducing glass eels, the number of eels that had not yet reached marketable size (e.g., 200g or more) was reduced to less than 50% of previous years, and males were present among these eels. This phenomenon is outside the scope of conventional technology and would not have occurred unless the present invention was utilized. Therefore, the actual occurrence of this phenomenon serves as evidence of the utilization of the present invention. Furthermore, while sexing eels is difficult until they reach a certain size, sexing can be achieved after they have grown using conventional technology.

[0082] <About the eel production method of the present invention>

[0083] The present invention includes all of the following contents: (1) a method for producing eels, wherein the eel fry is caused to ingest an eel feed containing 0.01% by weight or more of soybean isoflavones based on a ratio of dry matter; (2) a method for producing eels, wherein the eel fry is caused to ingest an eel feed containing 0.01% by weight or more of soybean isoflavone aglycones based on a ratio of dry matter; (3) a method for producing eels, wherein the eel fry is caused to ingest an eel feed containing 0.002% by weight or more of genistein based on a ratio of dry matter; (4) a method for producing eels, wherein the eel fry is caused to ingest 5 mg / kg (eel body weight) or more of soybean isoflavones per day; (5) a method for producing eels, wherein the eel fry is caused to ingest 5 mg / kg (eel body weight) or more of soybean isoflavone aglycones per day; (6) a method for producing eels, wherein the eel fry is caused to ingest 0.625 mg / kg (eel body weight) or more of genistein per day, etc.

[0084] As described above, for example, by including the step of causing eel fry to ingest eel feed containing soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) at the above-mentioned ratio, from another perspective, by including the step of causing eels to ingest soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) within the above-mentioned intake range, the growth of eels can be promoted, and as a result, the production efficiency of eels can be improved.

[0085] The content ratio or intake amount of soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) is such that the method, period, frequency, and timing of eel ingestion are the same as those described above.

[0086] Furthermore, the eel species targeted by the present invention are not particularly limited, as long as they belong to the genus Anguilla of the family Anguillidae. For example, the Japanese eel (scientific name: Anguilla japonica), the flower eel (scientific name: Anguilla marmorata), the European eel (scientific name: Anguilla anguilla), the American eel (scientific name: Anguilla rostrata), and the bicolor eel (scientific name: Anguilla bicolor bicolor) are also targeted. In the present invention, the most preferred eel is the Japanese eel.

[0087] [Example 1]

[0088] In Example 1, the growth performance of glass eels was studied when they were given soy isoflavones.

[0089] Glass eels (total weight 20.0 kg, average body weight 0.20 g) were introduced into an outdoor freshwater eel aquaculture pond and acclimated. After growing to an average body weight of 0.33 g, the eels were fed a diet supplemented with soy isoflavones for 40 days. For the next 6 to 8 months, they continued to be fed a standard diet. In a control group, the same number of glass eels (total weight 20.0 kg, average body weight 0.20 g) were fed only a diet without soy isoflavones and maintained for the same period and under the same conditions.

[0090] The formula feed is fed once a day, and the feeding amount is adjusted to the degree of fullness of the glass eel. In the early stage of breeding, 5 to 10% of the weight of the glass eel is fed. The feeding amount is gradually reduced according to the fullness, and it is about 1% by weight at the end of breeding.

[0091] In the formula feed to which soy isoflavones were added, the amount of soy isoflavones added was 0.4% by weight, based on the weight of the formula feed before the addition of soy isoflavones and before use as 100%. Furthermore, the soy isoflavones used contained approximately 40% by weight of genistein, so in the formula feed to which soy isoflavones were added, the amount of genistein was approximately 0.16% by weight, based on the weight of the formula feed before the addition of soy isoflavones and before use as 100%.

[0092] In this experiment, when glass eels were fed a formulated diet without soy isoflavones, their total weight at the time of introduction increased from 20.0 kg to 12,115 kg over the course of the rearing period, a weight gain of 606 times. In contrast, when fed a formulated diet supplemented with soy isoflavones, their total weight increased to 15,227 kg over the course of the rearing period, a weight gain of 761 times. Furthermore, visual observation of eels fed the formulated diet supplemented with soy isoflavones revealed a decrease in the number of individuals with exceptionally slow growth, a reduction in growth variability, and a more uniform size. The number of individuals weighing over 200 g that did not reach shipping standards within a certain period also decreased.

[0093] Furthermore, 10 randomly captured glass eels raised on a diet without soy isoflavones were sexed, and the female ratio was 0%. Meanwhile, 50 randomly captured glass eels raised on a diet with soy isoflavones were sexed, and the female ratio was 16%.

[0094] These results indicate that the intake of soy isoflavones by glass eels increases the weight gain rate, significantly promoting the growth of glass eels, and can also promote the growth of individuals that grow slower than other individuals.

[0095] In addition, based on the calculation that the amount of each feeding during the period of feeding the formulated feed with added soy isoflavones is 5 to 10 weight% of the glass eel's body weight, and the amount of soy isoflavones added in the formulated feed is 0.4 weight%, the soy isoflavone intake of an individual in this experiment is about 200 to 400 mg / kg body weight, and the genistein intake of an individual is about 50 to 160 mg / kg body weight.

[0096] [Example 2]

[0097] As disclosed in Patent Document 2, it is known that feminization can be induced in glass eels when they are fed appropriate amounts of soy isoflavones. Therefore, in Example 2, the growth promotion of glass eels when fed soy isoflavones as described in Example 1 was examined in relation to the induction of feminization in eels.

[0098] Glass eels with an average weight of 0.20g were introduced into an outdoor freshwater eel aquaculture pond for acclimation and maintained on a standard formula diet. Subsequently, glass eels grown to an average weight of 0.33g were fed a formula diet supplemented with soy isoflavones and maintained on a standard formula diet supplemented with soy isoflavones until they reached approximately 3g. The diet was then switched to a standard formula diet without soy isoflavones and maintained on a standard formula diet without soy isoflavones. The soy isoflavones used were the same as in Example 1. The soy isoflavones added to the formula diet supplemented with soy isoflavones were 0.25% by weight, based on the weight of the formula diet before and at the time of use being 100%. In a control group, the same glass eels were maintained under the same conditions and for the same period of time, fed only a standard formula diet without soy isoflavones.

[0099] The results showed that when glass eels were fed only a formulated diet without soy isoflavones (control group), the average weight of the eels was 24.0g (n=30) three months after being introduced to the culture pond. In contrast, when glass eels were fed a formulated diet supplemented with soy isoflavones for a certain period of time, the average weight of the eels was 29.2g (n=30) three months after being introduced to the culture pond, indicating approximately 1.22-fold growth enhancement compared to the control group. Furthermore, sex determination was performed, and the results showed that the female ratio of the glass eels fed only a formulated diet without soy isoflavones (control group) was 0% (all 30 examined were male). In contrast, among the glass eels fed a formulated diet supplemented with soy isoflavones for a certain period of time, 11 of the 30 examined were male, 14 were female, and 5 were other, for a female ratio of 33%.

[0100] The body weights of 11 males and 14 females of glass eels that were fed a formulated diet supplemented with soy isoflavones for a certain period were measured. The average body weight of males was 30.4 g (standard deviation 4.6 g), and the average body weight of females was 28.0 g (standard deviation 4.1 g).

[0101] As mentioned above, even when comparing only males, the average body weight increased from 24.0g to 30.4g when supplemented with soy isoflavones compared to when not supplemented, confirming that soy isoflavones promote growth. Furthermore, there was no significant growth difference between males and females when supplemented with soy isoflavones. These results suggest that the growth-promoting effect of soy isoflavones differs from the mechanism of feminization induction.

[0102] [Example 3]

[0103] As mentioned above, in eel farms, there are often many individuals (dead eels) that grow slowly and do not reach shipping size even after a year, reducing eel production efficiency. Therefore, in Example 3, the growth-promoting effect of soy isoflavones on these slow-growing individuals was studied.

[0104] Eels captured between December 2021 and April 2022 and raised in outdoor freshwater eel aquaculture ponds for approximately one year, but reaching an average weight of only 11.0 g, were fed a diet supplemented with soy isoflavones for 54 days. The soy isoflavones used were the same as those used in Example 1, and the amount of soy isoflavones added to the diet was 0.25% by weight, based on the weight of the diet before and after addition of the soy isoflavones as 100%.

[0105] The results showed that individuals that had reached an average weight of 11.0g (n=60) after approximately one year grew to an average weight of 19.3g (n=60) after 54 days of feeding on a formula diet supplemented with soy isoflavones, a roughly 1.8-fold increase. The feminization rate was 22%. These results suggest that soy isoflavones have a growth-promoting effect on slow-growing individuals.

Claims

1. A method for promoting the growth of eels, comprising: feeding eel fry with eel feed containing soy isoflavones.

2. A method for promoting the growth of eels, comprising allowing eel fry to ingest eel feed containing soybean isoflavone aglycones.

3. The method for promoting eel growth according to claim 1, wherein the eel growth promoting method comprises soybean isoflavones including genistein.

4. The method for promoting eel growth according to claim 3, wherein the eel contains 0.002% by weight or more of genistein in terms of dry matter.

5. The method for promoting eel growth according to claim 1, wherein the eel fry (excluding those that are determined to be female or feminized after growth) are fed an eel feed containing soy isoflavones. The method for promoting eel growth according to claim 1 , wherein the eel is an eel (Anguilla japonica).

7. A method for producing eels, comprising the step of allowing fry eels to ingest eel feed containing 0.01% by weight or more of soybean isoflavones in terms of dry matter.

8. An eel growth promoter comprising as an active ingredient soy isoflavones in an amount of 5 mg / kg (eel body weight) or more per day for eel fry.

9. An eel growth promoter comprising as an active ingredient an amount of genistein that allows eel fry to take in 0.625 mg / kg (eel body weight) or more per day.