JP2008154459A - Method for accelerating maturation of eel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for accelerating the maturation of female eels, by which the female eel can be matured without having to administer exogenous hormone. <P>SOLUTION: The method for accelerating the maturation of the eel is the one for accelerating the maturation of the eel, by making the ovary of the female eel, whose ovary is not matured, mature to a state in which an egg can be laid. The method includes a step for maturing the ovary of the female eel by growing the female eel, while periodically changing elements constituting a cultivation environment in a cultivation pond according to a prescribed time cycle, and the elements to be periodically changed includes the temperature of sea water changing within a specific range. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to eel aquaculture technology, and more particularly to a ripening method for maturing a female eel ovary.

  The Japanese eel (scientific name Anguilla Japana) is one of the important fish species in the aquaculture industry and is widely cultivated in East Asia. However, since seed production technology has not yet been established, all seedlings for aquaculture are covered with white eels that are naturally captured. However, the catch of glass eels has declined over the last 30 years, and the price increase has had a serious impact on the eel aquaculture industry. For this reason, research aimed at artificially maturing eels began in Japan in the 1960s with the aim of establishing seedling production technology. Since eels do not mature under rearing conditions, methods have been developed to administer multiple exogenous hormones. As a result, in 1973, we succeeded in obtaining fertilized eggs and larvae for the first time in the world. Thereafter, the method of exogenous hormone administration has been improved, and now it is possible to obtain fertilized eggs and larvae constantly. Finally, in 2003, larvae obtained by the hormone administration method were successfully grown to glass eels.

  Summarizing the above-described methods for obtaining practical seedlings of eel, the first step is to obtain a parent fish for egg collection by feminizing natural white eels, and by laying the parent fish for egg laying by hormonal ripening. Stage 2 of obtaining an eel egg for aquaculture, Stage 3 of fertilizing, hatching and breeding the eel egg to obtain a glass eel for cultivation as a practical seedling, and Stage 4 of growing this practical seedling and obtaining a commercial eel adult It is divided into. Since eels all become male when they are randomly raised, in Step 1, in order to obtain female eggs for egg laying, a female hormone (for example, estradiol 17β) is mixed in the feed and administered as described above. A feminization treatment is required. The female fish thus obtained will not mature to a state where they can lay eggs if reared as they are. Therefore, in the next stage 2, gonad stimulating hormone (GTH) must be ripened to develop the ovary to a state where it can lay eggs. Eggs are collected from ripened female fish, mixed with semen collected from male fish, fertilized, and the resulting fertilized eggs are hatched and grown to obtain glass eel (practical seedlings) for commercial aquaculture. It is done.

  However, problems such as inferior fertilization rate and hatching rate and problems such as low survival rate of larvae have not been solved, and mass production of glass eels is still difficult. There are concerns that these problems are the result of exogenous hormones administered for ripening adversely affecting egg quality. One of the reasons for this is that during the administration of exogenous hormones, it was reported that the blood profile of steroid hormones involved in maturation is significantly different from that of other fish species in the process of maturation. It is mentioned. Furthermore, considering the consumer's image of hormone administration, there is concern about the commercial value of white eel produced by current hormone ripening technology. From the above, there is a strong demand to develop a technique for promoting eel maturation without using exogenous hormones.

  The present invention has been made in view of the above circumstances, and makes it possible to obtain high-quality fertilized eggs and larvae essential for eel farming by maturing female eels without administering exogenous hormones. It provides a method for ripening eel female fish.

  In order to achieve the above object, the method of ripening eels according to the present invention includes a female eel that is triggered to start maturation and has not matured to a state where the ovaries can lay eggs in the seawater pond environment. An element that periodically fluctuates elements constituting the aquaculture environment of the aquaculture pond while cultivating the female eel while periodically fluctuating according to a predetermined time cycle, and the element that is periodically fluctuated Includes the temperature of the seawater that changes within a predetermined temperature range.

  Preferably, the constant condition includes a light condition of the seawater pond environment in addition to the temperature of the seawater, and the light condition is a state in which light is shielded and light irradiation is not used according to a predetermined time cycle ( It includes changing between a dark state) and a predetermined light irradiation state (bright state).

  More preferably, the water flow in the seawater pond is controlled to simulate the migratory environment of the descending eel.

  According to the present invention, a female eel can be matured without administering an exogenous hormone as in the prior art, so that it is possible to obtain healthy eggs, healthy larvae, and healthy white eels. Contributes to the improvement of eel aquaculture production efficiency.

  It has been clarified that the spawning ground for Japanese eels is in the western Mariana Islands (15 degrees north latitude and 140 degrees east longitude). And the larvae born here (Leptocephalus) were transported to the west on the North Equatorial Current, then transformed into the white eel in the process of switching to the Kuroshio Current and coming to East Asia. It is considered. After that, eels that have grown in rivers and ponds and marshes for five to fifteen years fall from the autumn to early winter to spawn and begin migratory. Eels captured at this time are called descending eels (or silver eels) and have already been triggered to mature, and in most individuals the developmental stage of the oocyte reaches the primary yolk stage. ing. At present, the laying migratory route and swimming depth of descending eels have not been elucidated, and no matured eels have been found beyond being caught in the coastal area. It is completely unknown whether maturity is progressing. This is one of the causes that makes research on artificial ripening of female eels difficult.

  The inventors have found that in the process of studying the effects of water temperature and / or light on the breeding of female eels in a seawater pond environment, the periodic fluctuations of these environmental factors affect the ovary development of female eels. The present invention has been determined and based on the findings.

  An object to which the ripening method according to the present invention is applied, that is, a female eel that has been triggered to start maturation and has not matured to a state where the ovaries can lay eggs, is that the oocyte is from the primary stage to the primary yolk It is a female eel in the stage of the ball stage. The eel egg formation stage (maturity stage) is divided into the following 8 stages (A to H) by observation of ovarian tissue specimens. In the case of descending eels, the oogenesis stage is from the oilball phase to the primary yolk bulb phase (mostly the primary yolk bulb phase).

1) First growth period
A. Dyeing stage
B. Peripheral period 2) Second growth period
C. Oil ball period: Diameter of about 70 to 200 μm
D. Primary yolk bulb phase: about 200-400 μm in diameter
E. Secondary yolk bulb phase: diameter of about 400-600 μm
F. The third yolk sphere stage: Diameter of about 600 to 800 μm
3) Maturity
G. Nuclear migration period
H. Maturity Period In the present invention, the seawater aquaculture pond environment means a culture environment filled with seawater having a normal composition, regardless of the type and size of ponds, ginger, tanks, corals, and the like. However, as described in detail below, it is necessary to provide means for periodically changing the environmental conditions. The aquaculture pond is preferably shaped so that eels can move around smoothly, and is preferably cylindrical. A particularly preferable culture pond is a cylindrical tank having a depth of 2.4 m and a diameter of 3.0 m.

  In the present invention, elements included in the constant periodic variation condition include temperature, light, and the like. Among them, a critical environmental condition is temperature. That is, the most important requirement in the present invention is to periodically change the water temperature of the culture pond environment. The variation range of the water temperature is preferably such that the difference between the high temperature and the low temperature is about 8 ° C. to about 12 ° C., and preferably the temperature difference is about 10 ° C. In addition, the fluctuation cycle is normally set to one cycle of 24 hours, but is not limited thereto. From the viewpoint of cost, a period that is as long as possible within the range where ripening is achieved is preferable. Such water temperature control may be performed by combining, for example, a heating device having a temperature control function well known to those skilled in the art and a temperature control method by adding cold water.

  It was confirmed that female eels reared in the periodic temperature changes as described above were promoted to mature and the ovaries matured. However, it was found that in breeding at a temperature exceeding 15 ° C., maturation was not promoted and the oocyte regressed. On the other hand, it was found that when rearing at a low water temperature of 5 ° C. or lower, the appearance of degenerated eggs was suppressed, but maturation did not progress. Therefore, it is considered that 5 to 15 ° C. is optimal for the temperature range that is periodically changed. In addition, a degenerated egg means the egg of the process in which the oocyte which accumulated egg yolk retreats, collapses, and is absorbed by the surrounding somatic cell.

  In the present invention, in addition to periodically changing the breeding temperature as described above, it is preferable to periodically change the illumination conditions with light. Such a change in light conditions is achieved by shielding the seawater aquaculture pond environment and periodically turning on an appropriate light source provided in the light shielding area, thereby turning on (bright) and off (dark). Can be achieved by switching between the two. The bright state and the dark state here do not necessarily need to be synchronized with daytime and nighttime due to the solar cycle. More preferably, the periodic fluctuations in temperature and light conditions are synchronized so that low temperatures are experienced in the bright state and high temperatures are experienced in the dark state. For example, the light is turned on during the daytime to be in a bright state, and the nighttime is turned off to be in a dark state. The illumination in this case is preferably performed using a blue light source (wavelength 400 to 500 nm), and the brightness is controlled by a dimmer. The brightness is preferably 200 lux or less at the brightest place in the aquarium (just below the light source). Such lighting devices are well known to those skilled in the art. The brightness here is a value measured using a commercially available illuminometer (Topcon Corporation, model IM-5).

  Furthermore, in the present invention, it is preferable to simulate the migratory environment of the descending eel by controlling the water flow in the seawater pond. In this case, the water flow is preferably maintained at a constant flow rate of 10 to 20 cm / second without being periodically changed.

  As will be described in the following examples, the method of the present invention can promote maturation of female eels when viewed at the gonad index (GSI) and the oogenesis stage (maturity stage).

  Hereinafter, the present invention will be described in more detail with reference to examples.

<Materials and methods>
As a test fish, 108 female descending eels (silver eels) captured in a stationary net in Mikawa Bay, Aichi Prefecture in November to December 2005 were used. Of these 108 eels, 15 were dissected on the day of capture in order to grasp the mature state at the time of capture (untreated section). These individuals were anesthetized with 2-phenoxyethanol and then weighed. The ovaries were then removed and weighed to calculate the gonad index (GSI: gonad weight / fish body weight × 100). A part of the ovary was used for measurement of oocyte diameter (hereinafter referred to as egg diameter) and preparation of a tissue specimen for determining the maturation stage of the oocyte.

  The remaining 93 eels, 16-20, were divided into 5 experimental zones as shown in FIG. 1, and were reared under the water temperature and day length conditions in Mikawa Bay until the start of the experiment. An indoor circular aquarium (diameter 3 m; depth 2.4 m) was used for breeding, and no forced water flow was intentionally formed in the aquarium. However, the inevitable water flow on the system of temperature control with a heat pump was not excluded. In addition, no feeding was performed both before and during the experiment. The experiment started on January 20, 2006. The temperature and photoperiod (illumination conditions) of seawater (salt concentration: 30 to 31 ‰) during the experiment period in each experimental group are as shown in FIGS. The numbers given to the curves in the figure indicate the numbers of the corresponding experimental sections, among which 3 to 5 (a) indicate the experimental sections 3 to 5 from January 20, 2006 to February 16, 2006. 3 to 5 (b) show experimental sections 3 to 5 from February 17, 2006 to June 20, 2006.

  The temperature control was performed using a heat pump (model FDCSP140HD3) manufactured by Mitsubishi Heavy Industries, Ltd. In addition, LBH150SB (100V, 1.6A) manufactured by Sumitomo 3M Limited was used for lighting. The vertical axis in FIG. 3 shows the illumination intensity expressed as a relative numerical value, assuming that the irradiation at the maximum capacity of the used light source is 100%.

  As shown in FIG. 2, in Experimental Group 1 (Curve 1) and Experimental Group 2 (Curve 2), water temperature fluctuations of 5 to 15 ° C. and 10 to 20 ° C. were given in a 24-hour cycle, respectively. In experimental groups 3 to 5 (curves 3 to 5), the animals were reared under constant water temperature conditions of 5, 15 and 20 ° C, respectively. In addition, as shown in FIG. 3, the lighting was turned on during the daytime and darkness was performed at nighttime. In the experimental groups 3 to 5, the lighting on and off times were changed on February 17 (3 to 5 (a) and 3 to 5 (b) in FIG. 3). The illumination was controlled using a blue light source (wavelength: 400 to 500 nm) in all sections, and the brightness was controlled with a dimmer. At 3 months (April 20), 4th month (May 18), and 5th month (June 20) after the start of the experiment, 4-10 fish were randomly taken from each experimental section and dissected The presence or absence of maturation was examined based on the measured values of GSI (gonad index) and egg diameter.

  GSI and egg diameter data are shown as mean ± standard deviation. Since all the data of the experimental plots to be compared did not show normality and equivariance, the difference in mean values was tested using the Kruskal-Wallis test. When a significant difference was recognized in the average value, the untreated section and each experimental section were compared by a multiple comparison method (Steel test). In all statistical analyses, the significance level was 5%.

<Result>
Dissection was performed at the 3rd month (April), 4th month (May), and 5th month (June) from the start of the experiment, and the maturity of eels in each experimental group was examined. As shown in Table 1 below, there was no experimental group in which maturation was significantly advanced (that is, the values of GSI and egg diameter were increased) compared to the time of capture (no treatment group).

  However, as shown in FIG. 4 and FIG. 6, in experimental group 1 (fluctuation between 5 and 15 ° C.), there was one individual whose GSI was 8.49 and the egg diameter reached 412 μm, but 3 It was confirmed at the time of dissection of the month. Further, when the developmental stage of the oocyte of this individual was examined by histological observation of the ovary, the secondary yolk bulb stage was reached. The applicant's laboratory dissects approximately 700 female descending eels captured in Mikawa Bay from October 1997 to January 2006. The average values of their GSI and egg diameter are approximately 2.5 (1.0-5.2) and 230 [mu] m (150-340 [mu] m), and the developmental stage of the oocyte is from the oil bulb phase to the primary yolk phase (mostly the primary yolk phase) )Met. From these facts, it is considered that the breeding conditions in Experimental Zone 1 brought a maturation promoting effect, and the above-mentioned individuals appeared.

Furthermore, when the ovarian tissue specimens of all the dissected individuals were observed, as shown in Table 2 below, in the experimental group 1, at the time of dissection at the 3rd and 4th months, regressed eggs (water temperature, photoperiod) It was known that it was caused by fluctuations in environmental conditions such as the amount of dissolved oxygen), but it was as low as 20%, but increased to 80% at the 5th month (Table 2).

  Experiment Group 2 (10-20 ° C fluctuation) was reared by changing the water temperature in the range of 10 ° C as in Experiment Group 1, but it was because the water temperature zone was set higher by 5 ° C. At this point, 80% of the ovaries were observed in the ovaries of individuals.

  In experimental group 4 (constant at 15 ° C.) and experimental group 5 (constant at 20 ° C.) reared at high water temperature, regressed eggs were observed in the ovaries of all individuals at the time of dissection at the third month. The same was true for the month. On the other hand, in the experimental group 3 (constant at 5 ° C.), only one egg was observed in the 4th and 5th month. From this, maturation is not promoted in breeding at a constant water temperature of 15 ° C. or higher, and the oocyte regresses. However, breeding at low water temperature as in Experimental Zone 3 suppresses and delays the appearance of degenerated eggs. I found out that However, maturation did not progress under constant low water temperature conditions.

  From the above results, it can be concluded that periodic water temperature change is a necessary condition for promoting eel maturation, and that the water temperature zone is also important for suppressing oocyte regression.

FIG. 1 is a diagram showing the classification of each experimental section in the eel ripening experiment described in the examples. FIG. 2 is a diagram showing periodic fluctuations in seawater temperature in each experimental section. FIG. 3 is a diagram showing illumination control in each experimental section. FIG. 4 is a diagram showing the results of measuring the gonad index (GSI) in each eel individual in each experimental section. FIG. 5 is a diagram showing GSI (average value ± standard deviation) of eel individuals in each experimental group. FIG. 6 is a diagram showing the measurement results of the egg diameter of each eel individual in each experimental section. FIG. 7 is a diagram showing the egg diameter (average value ± standard deviation) in each eel individual in each experimental section.

Claims (4)

An eel ripening method for maturing an ovary of a female eel that has been triggered to start maturation and has not matured to the point where the ovaries can lay eggs:
In the seawater aquaculture pond environment, including the step of maturing the ovary of the female eel by breeding the female eel while periodically changing the elements constituting the aquaculture environment of the aquaculture pond according to a predetermined time cycle, The cyclically varying element includes the temperature of the seawater that varies in a predetermined temperature range.   In addition to including light and dark conditions in the periodically fluctuating elements, periodic fluctuations in the light and dark conditions and the temperature conditions of the seawater are such that the female eel experiences low temperatures in the light state and high temperatures in the dark state. The method according to claim 1, wherein synchronization is performed.   The method according to claim 2, wherein the temperature of the seawater is changed between 5 ° C. and 15 ° C.   The method according to any one of claims 1 to 3, wherein a constant water flow having a flow rate of 10 to 20 cm / sec is maintained in the seawater aquaculture pond environment.

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