Eel feminization induction method, eel breeding method, eel feminizer, and eel feed

Abstract

[Problem] To provide a means for inducing the feminization of an eel reliably and highly efficiently while further reducing concerns about safety and environmental issues.　[Solution] Provided are: a method for inducing the feminization of an eel, the method comprising feeding an eel feed to the eel during the stage of a glass eel and / or a small eel, wherein the eel feed contains soybean isoflavone in an amount of 0.05% by weight or more in terms of dried content; and others. The feminization of an eel can be induced reliably and highly efficiently by feeding the eel feed to the eel for a predetermined period during the stage where the eel grows from a glass eel to a small eel. In this case, the concerns about safety and environmental issues can be further reduced compared with the case where the feminization is induced using estradiol 17β or the like.

Description

[Technical Field]

[0001] The present invention relates to a method for inducing eel feminization by feeding eel feed containing soybean isoflavones to eels in the stage from glass eels to small eels, an eel rearing method by feeding soybean isoflavones to eels, an eel feminizer, eel feed, etc. [Background technology]

[0002] Eel is a general term for fish belonging to the genus Anguilla and family Anguillidaceae, and there are 19 known species in the world, including the Japanese eel, giant mottled eel, and European eel. Eels have been eaten since ancient times in Europe and East Asia, and are a traditional and popular food ingredient in Japan in particular.

[0003] For example, the Japanese eel (scientific name: Anguilla japonica), which lives throughout East Asia, including Japan, is thought to spawn near the Mariana Islands in the Pacific Ocean, develop into a larva called a leptocephalus, and then metamorphose into a nearly transparent glass eel measuring 5-6 cm in length and weighing 0.2-0.3 g. It is believed that these eels then ride the Kuroshio Current to reach the coastal areas of East Asia. They then settle and grow in inland, coastal, or brackish waters, becoming small eels (such as black eels), which then mature over a period of 5-10 years before migrating back to their spawning grounds. Other eel species also follow similar lifestyles, although their spawning and settlement locations may differ.

[0004] Due to the large demand and high profits, eel farming is widespread, and it is said that more than 99% of all eels consumed are farmed eels. The current mainstream method of eel farming involves capturing wild glass eels that migrate to coastal areas as seedlings for farming, and then releasing them into farm ponds to raise and develop them. There have been reports of success at the laboratory level in fully farming eels, in which fry obtained from artificial incubation are allowed to grow into adult eels and then the next generation of fry is obtained, but this has not yet been achieved on a commercial scale.

[0005] Generally, male eels begin to slow down in growth once they exceed 300g in weight, and their flesh becomes tougher, resulting in a decline in quality and flavor. Female eels, on the other hand, have a greater growth limit than male eels, and even after exceeding 300g in weight, their growth does not slow down, and their flesh remains soft, maintaining their quality and flavor. However, when eels are raised in aquaculture environments from the glass eel to small eel stage, most of them become male eels, making it difficult to obtain female eels.

[0006] It is known that feminization can be induced by feeding eels a compound feed containing a female hormone (estradiol 17β) during the period when glass eels grow into small eels. As another method for feminizing eels without using hormones, for example, Patent Document 1 describes a method for promoting feminization by rearing eels before sexual differentiation in an aquarium containing a hollow container that covers the entire body of the eel that enters the tank.

[0007] Here, soybean isoflavones will be explained as a matter relating to the present invention.

[0008] Soy isoflavones are a general term for flavonoid compounds with an isoflavone skeleton, found primarily in soybean germ. Soy isoflavones are classified into four types: glycosides (glycosides; structures covalently bonded to sugars), aglycones (non-glycosides; structures in which the sugar moiety of the glycoside has been removed), acetylated glycosides, and malonylated glycosides. Each type contains three known compounds, for a total of 12 known soy isoflavones. Of these, the three glycosides are genistin, daidzin, and glycitin, while their aglycone forms (with the sugar moiety removed) are genistein, daidzein, and glycitein, respectively. The composition and content of each compound vary depending on the type of soybean used and the extraction, purification, and processing methods. Furthermore, as a means for separating specific compounds in soy isoflavones, for example, Patent Document 2 discloses a means for separating highly purified genistein from an isoflavone mixture using a solvent.

[0009] Soy isoflavones have a similar chemical structure to the female hormone estrogen and are also known as phytoestrogens. The aglycone form of soy isoflavones has estrogen-like effects and is believed to be effective in preventing heart disease, menopausal symptoms, osteoporosis, breast cancer, and other conditions.

[0010] Furthermore, Non-Patent Document 1 describes that when European eels were fed 2 mg / kg (dry feed) of genistein for 100 days, 55% became female, whereas when they were fed a higher dose of 20 mg / kg (dry feed), only 15% became female. [Patent Document 1] Japanese Patent Application Publication No. 2018-143182 [Patent Document 2] Japanese Patent Application Publication No. 7-173148 [Non-Patent Document 1] Itai Tzchori, et al, ``The influence of phytoestrogens and oestradiol-17β on growth and sex determination in the European eel (Anguilla Anguilla)'', Aquaculture Research, 2004, 35, 1213-1219. DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0011] As mentioned above, it is already known that adding estradiol-17β to formulated feed can induce feminization in eels. Compared to other feminization techniques, this technique remains the most reliable and efficient method for inducing feminization in eels.

[0012] However, estradiol 17β is known to have a very strong feminizing effect even in trace amounts. Aquatic organisms, in particular, are more sensitive to exogenous hormones than terrestrial animals, and many cases have been reported in which hormone-like substances in environmental waters disrupted reproductive function. Estradiol 17β has also been pointed out as a possible carcinogen and reproductive toxicant. Therefore, given the risk that estradiol 17β may be released into environmental waters when used in aquaculture, there are significant concerns about both the environmental impact and safety of using estradiol 17β in aquaculture.

[0013] For this reason, this technology is limited to use in research or laboratories, with the assumption that the eels will not be consumed, and the use of estradiol 17β to induce feminization is not being used in eel farming. As of now, no eels that have been feminized using estradiol 17β are available on the market.

[0014] On the other hand, as mentioned above, even large female eels weighing around 500g have tender flesh and good quality and flavor. Currently, eels weighing 200-250g are the norm on the market, but in a situation where we are 100% dependent on natural resources, there is a demand for larger distribution sizes to make effective use of eel resources. Therefore, there is likely to be a high potential market demand for female eels that can be easily grown to a large size and maintain high quality. If reliable and highly efficient feminization induction technology can be developed that alleviates safety and environmental concerns, it may be possible to ship feminized farmed eels as food.

[0015] Therefore, an object of the present invention is to provide a novel method for inducing feminization in eels reliably and efficiently, while minimizing safety and environmental concerns. [Means for solving the problem]

[0016] The present inventors have newly discovered that feminization of eels can be induced by feeding eel feed containing soy isoflavones to glass eels and baby eels.

[0017] Therefore, the present invention provides a method for inducing feminization in eels by feeding eel feed containing soybean isoflavones at a dry matter ratio of 0.05% by weight or more to glass eels and / or baby eels.

[0018] By feeding eel feed supplemented with soy isoflavones to eels for a certain period during the period when they grow from glass eels to young eels, for example, the period when sex is not yet determined or when they are sexually plastic, it is possible to induce feminization in eels reliably and efficiently.

[0019] A wide variety of soy foods routinely consumed by humans contain soy isoflavones. Therefore, even if trace amounts of soy isoflavones remain in eels that have ingested soy isoflavones (or one or more specific compounds thereof) during growth, there are few safety concerns when humans eat those eels. Furthermore, because soy isoflavones are a group of plant-derived compounds, even if they are released into the environment, they are likely to have less impact on the environment than estradiol 17β and the like. Therefore, the present invention may potentially reduce safety and environmental concerns compared to feminization induction using estradiol 17β and the like.

[0020] Furthermore, by reducing safety and environmental concerns regarding the induction of feminization in eels, it may become possible to ship feminized farmed eels as food. As mentioned above, female eels tend to grow large and have good quality and flavor, so it is presumed that there is a high potential demand for female eels in the market. The present invention makes it possible to farm and produce female eels and supply them to the market in response to that demand.

[0021] In the present invention, the term "small eels" refers to young eels that have grown larger than glass eels. For convenience, eels weighing less than 0.5 g will be referred to as glass eels, and eels weighing 0.5 g or more will be referred to as small eels. [Effects of the Invention]

[0022] The present invention makes it possible to induce feminization in eels reliably and efficiently, with fewer safety and environmental concerns. BEST MODE FOR CARRYING OUT THE INVENTION

[0023] <About the eel feed according to the present invention> The present invention broadly encompasses eel feed containing 0.05% by weight or more of soybean isoflavones based on dry matter, eel feed containing 0.05% by weight or more of soybean isoflavone aglycones based on dry matter, and eel feed containing 0.006% by weight or more of genistein based on dry matter.

[0024] Feminization of eels can be induced by adding soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) to eel feed and feeding this feed to glass eels and / or baby eels for a certain period of time.

[0025] Formula feed used for eels is often made by adding water, feed oil (fish oil), etc., kneading it into a paste, and then feeding it to the eels.

[0026] In the present invention, the soy isoflavone content, expressed as a percentage of dry matter (dry feed, the same applies hereinafter), i.e., when the weight of the feed before preparation at the time of use (addition of water, etc.) is taken as 100%, is preferably 0.05% by weight or more, more preferably 0.2% by weight or more, and most preferably 1.0% by weight or more. There are no particular restrictions on the upper limit of the soy isoflavone content, but from the viewpoint of feed efficiency, 20% by weight or less is preferred, 8.0% by weight or less is more preferred, and 4.0% by weight or less is most preferred.

[0027] Soy isoflavones are not particularly limited and can be a wide variety of known soy isoflavones. For example, ready-made products may be used, or products prepared by extracting, purifying, and treating soybeans or other raw materials using known methods may be used.

[0028] From another perspective, the soy isoflavone aglycone content, expressed as a percentage of dry matter, i.e., when the weight of the feed before preparation (addition of water, etc.) is taken as 100%, is preferably 0.05% by weight or more, more preferably 0.2% by weight or more, and most preferably 1.0% by weight or more. There are no particular limitations on the upper limit of the soy isoflavone aglycone content, but from the standpoint of feed efficiency, it is preferably 20% by weight or less, more preferably 8.0% by weight or less, and most preferably 4.0% by weight or less. Note that "soy isoflavone aglycone" refers to the non-sugar portion of soy isoflavones (hereinafter the same), and its content weight can be obtained, for example, by converting the weight of added soy isoflavones into the ratio of the molecular weights of glycosides and aglycones, or by analysis using known test methods.

[0029] From another perspective, the genistein content in the dry matter, i.e., when the weight of the feed before preparation (addition of water, etc.) is taken as 100%, is preferably 0.006% by weight or more, more preferably 0.02% by weight or more, and most preferably 0.1% by weight or more. There are no particular restrictions on the upper limit of the genistein content, but from the viewpoint of feed efficiency, it is preferably 5.0% by weight or less, more preferably 2.0% by weight or less, and most preferably 1.0% by weight or less.

[0030] There are no particular limitations on the means for incorporating genistein into eel feed. For example, genistein may be incorporated into eel feed by adding soybean isoflavones containing genistein to the eel feed, genistein may be added directly to the eel feed, or soybean isoflavones or genistein may be incorporated into the eel feed in advance during the feed production stage.

[0031] Genistein can be a wide variety of known genisteins and is not particularly limited. For example, a ready-made product may be used, or soy isoflavones containing genistein may be used as is, or genistein-isolated or highly purified genistein obtained by extraction, purification, or processing from soy isoflavones using known methods may be used, or genistein-isolated or highly purified genistein obtained by separation, extraction, fermentation, or other known methods from beans or processed beans, or genistein-isolated or synthesized using known methods. For example, soy isoflavones containing 10% or more genistein by weight have the advantages of being relatively easy to obtain, manufacture, and prepare, and of being able to reliably and efficiently induce feminization in eels.

[0032] In addition, the genistein according to the present invention also broadly encompasses its pharmacologically acceptable salts and derivatives of genistein, so long as they retain their feminization-inducing activity in eels.

[0033] The eel feed of the present invention is not limited narrowly by other ingredients or composition, as long as it contains at least soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein).

[0034] For example, the composition and ingredients of eel feed other than soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof) may be the same as those of commonly used compound feed. Compound feed is a mixture of nutrients for eel growth in appropriate proportions, and may contain, for example, 50% or more by weight of fish meal on a dry matter basis, as well as starch, calcium phosphate, salt, yeast, herbal extracts, etc.

[0035] The fish meal in the compound feed can be a wide variety of known fish meal, and is not particularly limited, but may be, for example, powder obtained by processing sardines, mackerel, herring, horse mackerel, etc. Fish meal from other fish species may also be used, or a mixture of fish meal from multiple fish species. The fish meal can be selected appropriately taking into consideration the type of eel, growth conditions, cost, etc.

[0036] Other ingredients that improve the growth efficiency of eels, such as lactic acid bacteria, butyric acid bacteria, digestive enzymes, and dried vegetables, may also be included as appropriate.

[0037] <About the eel feminization inducer according to the present invention> The present invention broadly encompasses eel feminization inducers containing soy isoflavone as an active ingredient at a daily intake of 10 mg / kg or more (of eel body weight; the same applies hereinafter), eel feminization inducers containing soy isoflavone aglycone as an active ingredient at a daily intake of 10 mg / kg or more, and eel feminization inducers containing genistein as an active ingredient at a daily intake of 1.2 mg / kg or more.

[0038] The eel feminization inducer of the present invention contains soybean isoflavones (or soybean isoflavone aglycones, or one or more specific compounds thereof, such as genistein) as an active ingredient. By feeding soybean isoflavones to eels at the glass eel and / or baby eel stage, feminization can be induced in eels reliably and efficiently.

[0039] In the case of compound feed, the general guideline for feeding amounts is 5-8% of body weight for glass eels to small eels (e.g., weighing approximately 0.2-20g), and 2-3% of body weight for small eels (e.g., weighing approximately 20-30g) depending on growth. With a feeding amount of 2% of body weight, it is preferable for the eel to consume 10 mg / kg or more of soy isoflavones per day, more preferably 40 mg / kg or more, and most preferably 200 mg / kg or more. Furthermore, from the perspective of feed efficiency, with a feeding amount of 8% of body weight, it is preferable for the soy isoflavone intake to be 16 g / kg or less per day, more preferably 6.4 g / kg or less, and most preferably 3.2 g / kg or less.

[0040] From another perspective, with a feed amount of 2% of body weight, it is preferable for the rats to ingest 10 mg / kg or more of soy isoflavone aglycones per day, more preferably 40 mg / kg or more, and most preferably 200 mg / kg or more. Also, from the viewpoint of feed efficiency, with a feed amount of 8% of body weight, it is preferable for the rats to ingest 16 g / kg or less of soy isoflavone aglycones per day, more preferably 6.4 g / kg or less, and most preferably 3.2 g / kg or less.

[0041] From another perspective, when the feeding amount is 2% of body weight, the genistein intake is preferably 1.2 mg / kg or more per day, more preferably 4.0 mg / kg or more, and most preferably 20 mg / kg or more. Also, from the viewpoint of feed efficiency, when the feeding amount is 8% of body weight, the genistein intake is preferably 4.0 g / kg or less per day, more preferably 1.6 g / kg or less, and most preferably 0.8 g / kg or less.

[0042] The eel feminization inducer 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, antifoaming agents, and the like, as appropriate, depending on the purpose, use, dosage form, etc.

[0043] Suitable examples of excipients that can be used include lactose, sucrose, D-mannitol, starch, crystalline cellulose, and light anhydrous silicic acid.

[0044] Suitable examples of lubricants that can be used include magnesium stearate, calcium stearate, talc, and colloidal silica.

[0045] Suitable examples of binders that can be used include crystalline cellulose, sucrose, D-mannitol, dextrin, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, and polyvinylpyrrolidone.

[0046] Suitable examples of disintegrants that can be used include starch, carboxymethylcellulose, carboxymethylcellulose calcium, croscarmellose sodium, and carboxymethylstarch sodium.

[0047] Suitable examples of the solvent include water for injection, alcohol, propylene glycol, macrogol, sesame oil, corn oil, and the like.

[0048] Suitable examples of the solubilizing agent include polyethylene glycol, propylene glycol, D-mannitol, benzyl benzoate, ethanol, trisaminomethane, cholesterol, triethanolamine, sodium carbonate, and sodium citrate.

[0049] Suitable examples of suspending agents that can be used include surfactants (stearyltriethanolamine, sodium lauryl sulfate, laurylaminopropionic acid, lecithin, benzalkonium chloride, benzethonium chloride, glycerin monostearate, etc.), hydrophilic polymers (polyvinyl alcohol, polyvinylpyrrolidone, sodium carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, etc.), and the like.

[0050] Suitable examples of the buffering agent include buffer solutions such as phosphate, acetate, carbonate, citrate, tartrate, trishydroxymethylaminomethane, and HEPES.

[0051] Suitable examples of the isotonic agent include sodium chloride, glycerin, and D-mannitol.

[0052] Suitable examples of agents for preservative purposes include thimerosal, parahydroxybenzoic acid esters, phenoxyethanol, chlorobutanol, benzyl alcohol, phenethyl alcohol, dehydroacetic acid, sorbic acid, and various other preservatives, antibiotics, and synthetic antibacterial agents.

[0053] Suitable examples of antioxidants that can be used include sulfites and ascorbic acid.

[0054] Suitable examples of pH adjusters that can be used 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 hydrogen carbonates 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.

[0055] Suitable examples of dispersants that can be used include sodium carboxymethylcellulose, hydroxypropylmethylcellulose, polyvinylpyrrolidone, polysorbate 80, and the like.

[0056] Suitable examples of colorants that can be used include caramel color, gardenia color, anthocyanin color, annatto color, paprika color, safflower color, monascus color, carotene color, carotenoid color, flavonoid color, cochineal color, amaranth (Red No. 2), erythrosine (Red No. 3), Allura Red AC (Red No. 40), New Coccine (Red No. 102), Phloxine (Red No. 104), Rose Bengal (Red No. 105), Acid Red (Red No. 106), tartrazine (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 chlorophyll, and sodium copper chlorophyllin.

[0057] Suitable examples of the antifoaming agent include dimethicone, simethicone, silicone emulsion, sorbitan sesquioleate, and nonionic substances.

[0058] In addition to the above, this preparation may contain auxiliary ingredients, such as light-absorbing pigments (riboflavin, adenine, adenosine, etc.) that aid in preservation and efficacy, chelating agents and reducing agents (vitamin C, citric acid, etc.) for stabilization, carbohydrates (sorbitol, lactose, mannitol, starch, sucrose, glucose, dextran, etc.), casein digests, various vitamins, lactic acid bacteria, butyric acid bacteria, digestive enzymes, dried vegetables, etc.

[0059] <Method for inducing eel feminization according to the present invention> The present invention encompasses a method for inducing feminization in eels by feeding eel feed containing soybean isoflavones at a dry matter rate of 0.05% or more, when the eels are glass eels and / or small eels, and a method for inducing feminization in eels by feeding eel feed containing soybean isoflavones at a dry matter rate of 10 mg / kg or more per day when the eels are glass eels and / or small eels.The present invention also encompasses a method for inducing feminization in eels by feeding eel feed containing soybean isoflavone aglycones at a dry matter rate of 0.05% or more, when the eels are glass eels and / or small eels, and a method for inducing feminization in eels by feeding eel feed containing soybean isoflavone aglycones at a dry matter rate of 10 mg / kg or more per day when the eels are glass eels and / or small eels. Furthermore, the present invention encompasses a method for inducing feminization in eels by feeding eel feed containing 0.006% or more genistein by weight on a dry matter basis to glass eels and / or baby eels, as well as a method for inducing feminization in eels by feeding 1.2 mg / kg or more of genistein per day to glass eels and / or baby eels.

[0060] Eels undergo sex differentiation at a specific time as they grow from glass eels to small eels (for example, during the growth period up to a body weight of 0.2 g to 30 g), and once they reach a body weight of 30 g or more, they can be sexed by morphological observation of their gonads. By feeding eel feed containing the above-mentioned proportions of soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) around the time of sex differentiation, for example, at the glass eel and / or small eel stage, or, from another perspective, by feeding soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) within the above-mentioned intake range, it is possible to reliably and efficiently induce feminization in eels.

[0061] Depending on the growth stage of the eels, for example, when they are glass eels to small eels (for example, weighing approximately 0.2g or more but less than approximately 20g), they should be given 25mg / kg to 16g / kg (eel body weight) of soy isoflavones per day, more preferably 100mg / kg to 6.4g / kg, and most preferably 500mg / kg to 3.2g / kg; when they are small eels (for example, weighing approximately 20g to approximately 30g), they should be given 10mg / kg to 16g / kg (eel body weight) of soy isoflavones per day, more preferably 40mg / kg to 6.4g / kg, and most preferably 200mg / kg to 3.2g / kg.

[0062] From another perspective, depending on the growth stage of the eels, for example, when they are glass eels to small eels (for example, weighing approximately 0.2 g or more but less than approximately 20 g), they may be given 25 mg / kg to 16 g / kg (eel body weight) of soy isoflavone aglycone per day, more preferably 100 mg / kg to 12 g / kg, and most preferably 500 mg / kg to 8 g / kg; and when they are small eels (for example, weighing approximately 20 g to approximately 30 g), they may be given 10 mg / kg to 16 g / kg (eel body weight) of soy isoflavone aglycone per day, more preferably 40 mg / kg to 6.4 g / kg, and most preferably 200 mg / kg to 3.2 g / kg.

[0063] From yet another perspective, depending on the growth stage of the eels, for example, when they are glass eels to small eels (for example, weighing approximately 0.2 g or more but less than approximately 20 g), they should be given 3 mg / kg to 4.0 g / kg (eel body weight) of genistein per day, more preferably 10 mg / kg to 1.6 g / kg, and most preferably 50 mg / kg to 800 mg / kg; and when they are small eels (for example, weighing approximately 20 g to approximately 30 g), they should be given 1.2 mg / kg to 1.5 g / kg (eel body weight) of genistein per day, more preferably 4 mg to 600 mg / kg, and most preferably 20 mg / kg to 300 mg / kg.

[0064] There are no particular limitations on the means by which eels are ingested soy isoflavones, etc. For example, eels may be fed eel feed containing soy isoflavones, or the eels may be ingested soy isoflavones, etc. separately from the feed, or a preparation containing soy isoflavones, etc. may be administered directly to individual eels.

[0065] The period for which soy isoflavones, etc. are ingested is not particularly limited, as long as it includes the period before, during, and after the sex differentiation stage. For example, continuous ingestion of soy isoflavones, etc. for 2 to 6 months during the glass eel and / or baby eel stage is likely to enable reliable and highly efficient induction of feminization in eels.

[0066] The frequency of feeding soy isoflavones and the like is not particularly limited, as long as the feeding is continued without interruption throughout the feeding period. For example, feeding soy isoflavones and the like 3 to 7 days a week is likely to reliably and efficiently induce feminization in eels. Furthermore, depending on the growth stage of the eels, for example, when they are glass eels to small eels (e.g., weighing 0.2 g to 20 g), the daily amount to be fed can be divided into 2 to 3 feedings, and when they are small eels (e.g., weighing 20 g to 30 g), the daily amount to be fed can be fed once.

[0067] <Eel rearing method according to the present invention> The present invention encompasses all of the following: a method for raising eels in which glass eels and / or small eels are fed eel feed containing 0.05% or more by dry matter of soy isoflavones; a method for raising eels in which glass eels and / or small eels are fed 10 mg / kg or more by day of soy isoflavones; a method for raising eels in which glass eels and / or small eels are fed eel feed containing 0.05% or more by dry matter of soy isoflavone aglycones; and a method for raising eels in which glass eels and / or small eels are fed 10 mg / kg or more by day of soy isoflavone aglycones. Furthermore, the present invention encompasses a method for raising eels in which eels at the glass eel and / or baby eel stage are fed eel feed containing 0.006% or more by weight of genistein on a dry matter basis, and a method for raising eels in which glass eels and / or baby eels are fed 1.2 mg / kg or more of genistein per day.

[0068] As mentioned above, by raising eels while feeding them eel feed containing the above-mentioned proportions of soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) around the time of sex differentiation, for example when they are glass eels and / or baby eels, or, from another perspective, by raising eels while feeding them soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds thereof, such as genistein) within the above-mentioned intake range, it is possible to reliably and efficiently induce feminization in eels.

[0069] The content or intake amount of soy isoflavones, etc., as well as the means, duration, frequency, and timing of intake are the same as above.

[0070] <Method for producing female eels according to the present invention> The present invention encompasses all of the following: a method for producing female eels that includes a step of feeding eel feed containing soybean isoflavones at a dry matter rate of 0.05% or more by weight when the eels are at the glass and / or baby eel stage; a method for producing female eels that includes a step of feeding eels at the glass and / or baby eel stage with 10 mg / kg or more of soybean isoflavones per day; a method for producing female eels that includes a step of feeding eels at the glass and / or baby eel stage with eel feed containing soybean isoflavone aglycones at a dry matter rate of 0.05% or more by weight when the eels are at the glass and / or baby eel stage; a method for producing female eels that includes a step of feeding eels at the glass and / or baby eel stage with 10 mg / kg or more of soybean isoflavone aglycones per day; Furthermore, the present invention encompasses a method for producing female eels that includes a step of feeding eel feed containing 0.006% or more genistein by weight on a dry matter basis to glass eels and / or baby eels, as well as a method for producing female eels that includes a step of feeding 1.2 mg / kg or more of genistein per day to glass eels and / or baby eels.

[0071] The content or intake amount of soy isoflavones (or soy isoflavone aglycones, or one or more specific compounds therein, such as genistein), as well as the means, duration, frequency, and timing of intake, are the same as those described above.

[0072] In addition, the present invention broadly encompasses female eels created or produced by the above-mentioned methods for inducing eel feminization, eel farming methods, or female eel production methods. Identification of whether a female eel has been created or produced by the methods of the present invention can be determined, for example, by the frequency of female appearance and whether or not it has the characteristics of a farmed eel.

[0073] The type of eel that is the subject of the present invention is not particularly limited as long as it belongs to the genus Anguilla and the family Anguillidaceae. Examples of eels that are subject of the present invention include the Japanese eel (scientific name: Anguilla japonica), giant mottled eel (scientific name: Anguilla marmorata), European eel (scientific name: Anguilla anguilla), American eel (scientific name: Anguilla rostrata), and Bicolor eel (scientific name: Anguilla bicolor bicolor). In the present invention, it is most preferable that the eel is the Japanese eel. Example 1

[0074] In Example 1, an investigation was conducted into whether soy isoflavones could induce feminization in eels.

[0075] Seventy unsexed glass eels (average weight 0.4g) were placed in each group in a 150L FRP tank, fed ad libitum with a compound feed containing various proportions of soy isoflavones, and reared at a water temperature of 28°C.

[0076] The soy isoflavone-supplemented formula feed was fed five times per week, with the amount of feed per feeding being 7-8% of body weight for the first two months, and approximately 2.5% of body weight thereafter.

[0077] Approximately six months after the start of rearing, small eels weighing approximately 30 g or more were sampled as appropriate and sexed.

[0078] Sexing was determined by morphological observation of the gonads, with those with ovaries being identified as female and those with testes as male.

[0079] The results are shown in Tables 1 and 2. In each table, "number of tails" represents the number of small eels sampled, "soy isoflavone addition ratio" represents the ratio (wt%) of soy isoflavone addition when the weight of the formula feed before soy isoflavone addition and before preparation at the time of use is taken as 100%, "aglycone content ratio" represents the ratio (wt%) of soy isoflavone aglycone content weight when the weight of the formula feed before soy isoflavone addition and before preparation at the time of use is taken as 100%, "genistein content ratio" represents the ratio (wt%) of genistein content weight when the weight of the formula feed before soy isoflavone addition and before preparation at the time of use is taken as 100%, and "percentage of females" represents the ratio of females out of the number of individuals sampled. Note that "soy isoflavone aglycone content weight" represents the weight of the non-sugar portion of the added soy isoflavones, and an analytical value obtained based on a known test method was used. In addition, the "control" shows the results when the animals were fed a normal formula feed without soy isoflavones instead. [Table 1] [Table 2]

[0080] As shown in Table 1, feminization of eels could be induced by adding 1% or more by weight of soy isoflavones to eel feed when they were raised at the glass eel to small eel stage. Furthermore, as shown in Table 2, when eels were raised on eel feed with an aglycone content of 0 to 0.040% by weight relative to the diet weight, all individuals differentiated into males, whereas when the aglycone content was 0.100% or more by weight, 46% of individuals were induced to become male, when it was 0.336% by weight, 95% of individuals were induced to become male, and when it was 1.076% by weight or more, all individuals were induced to become male. From another perspective, when we focus on the genistein contained in the feed by adding soy isoflavones, when glass eels to small eels were raised on eel feed containing 0 to 0.004% genistein by weight, all individuals differentiated into males, whereas when the genistein content was 0.01% by weight, 46% of individuals were induced to become feminized, when it was 0.031% by weight, 95% of individuals were induced to become feminized, and when the genistein content was 0.155% by weight or higher, all individuals were induced to become feminized.

[0081] In this experiment, the amount of food given per feeding was 7-8% of body weight for the first two months, and approximately 2.5% thereafter. Converting this to the fact that if the soy isoflavone addition rate was 1% by weight, the soy isoflavone intake per feeding in the first two months would be approximately 700-800 mg / kg body weight, and the soy isoflavone intake per feeding thereafter would be approximately 250 mg / kg body weight. Similarly, when the aglycone content of the diet was 0.100% by weight, the aglycone intake per feeding in the first two months was approximately 70.0–80.0 mg / kg body weight, and thereafter approximately 25.0 mg / kg body weight. When the aglycone content of the diet was 0.336% by weight, the aglycone intake per feeding in the first two months was approximately 235.2–268.8 mg / kg body weight, and thereafter approximately 84.0 mg / kg body weight. When the aglycone content of the diet was 1.076% by weight, the aglycone intake per feeding in the first two months was approximately 753.2–860.8 mg / kg body weight, and thereafter approximately 269.0 mg / kg body weight. Furthermore, when the genistein content was 0.01% by weight, the genistein intake per serving in the first two months was approximately 7 to 8 mg / kg of body weight, and thereafter approximately 2.5 mg / kg of body weight; when the genistein content was 0.031% by weight, the genistein intake per serving in the first two months was approximately 21.70 to 24.8 mg / kg of body weight, and thereafter approximately 7.75 mg / kg of body weight; and when the genistein content was 0.155% by weight, the genistein intake per serving in the first two months was approximately 108.5 to 1,240 mg / kg of body weight, and thereafter approximately 38.75 mg / kg of body weight.

Claims

1. During the glass eel and / or small eel season, A method for inducing feminization in eels by feeding eel feed containing soy isoflavone aglycones at a dry matter ratio of 0.1% by weight or more.

2. A method for inducing feminization in eels as described in claim 1, wherein the eel feed contains 0.01% by weight or more of genistein in terms of dry matter.

3. 2. The method for inducing feminization in eels according to claim 1, wherein the eel is a Japanese eel.

4. A method for raising eels at the glass eel and / or baby eel stage, comprising: A method for raising eels by feeding eel feed containing soy isoflavone aglycones at a dry matter ratio of 0.1% by weight or more.

5. During the glass eel and / or small eel season, A method for producing female eels, comprising the step of feeding eel feed containing soybean isoflavone aglycones at a dry matter ratio of 0.1% by weight or more.

6. Eel feed containing more than 0.1% by weight of soy isoflavone aglycones based on dry matter.

7. 7. The eel feed according to claim 6, which contains 0.01% by weight or more of genistein in terms of dry matter.

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