Method for farming salmonid fish

Abstract

To provide a method for farming salmonid fish that is capable of promoting growth at low cost.SOLUTION: A method for farming salmonid fish includes: an acclimation step in which salmonid fish, previously reared for a predetermined or longer period in freshwater rearing water, are acclimated in rearing water with a prescribed salt concentration for a predetermined or longer period; and a return-rearing step in which the acclimated fish are returned to freshwater rearing water following the acclimation step and then reared.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a method for cultivating salmonid fish.

[0002] Salmonid fish are the fish species with the highest seafood consumption in Japan, and in recent years, the farmed production of salmonid fish has been increasing the most in the world. It is known that the growth of salmonid fish, which are born in freshwater, is dramatically promoted by marine aquaculture during the period when they can transition to seawater. In Norway and Chile, where salmonid fish farming is popular, it is possible to use areas with seawater temperatures below 18°C ​​for farming, and it is possible to farm salmonid fish, which are cold-water fish species, throughout the year during the period when they can transition to seawater. However, in Japan, seawater temperatures exceed 20°C in summer even in the northernmost sea area, so salmonid fish cannot be farmed throughout the year, and the shipping size is smaller than that of foreign fish. In addition, it is difficult to expand farms for marine aquaculture due to issues such as fishing rights, and the farmed production of salmonid fish in Japan has remained flat.

[0003] Therefore, attempts have been made to improve the efficiency of salmonid fish farming in Japan. For example, in Patent Document 1, focusing on the characteristics of deep ocean water such as cleanliness and low water temperature, deep ocean water (30 psu or more) pumped up from a depth of 100 m or more in the ocean is pumped into an aquarium, circulated or pumped and stored, and rainbow trout, which are normally farmed in freshwater, are farmed in deep ocean water to promote their growth. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2007-97518 A (pages 4 to 8, Figures 1 to 3) Summary of the Invention [Problem to be solved by the invention]

[0005] However, in Patent Document 1, when transferring from freshwater rearing to deep seawater rearing, if individuals with a fork length of 11 cm or more are not used, the survival rate will decrease due to low seawater adaptability, and it is necessary to grow the rainbow trout fry to a sufficient size in advance during freshwater rearing. Therefore, there is a risk that the transition cycle to rearing in deep seawater will slow down due to the prolonged freshwater rearing period, and the efficiency of aquaculture will decrease. In addition, in Patent Document 1, rainbow trout are grown to shipping size by rearing in deep seawater, so there is a risk that the introduction cost of equipment for pumping a large amount of deep seawater into a large aquarium for rearing will also be high. A method that can promote the growth of salmonid fish at low cost is desired.

[0006] The present invention has been made in light of these problems, and has an object to provide a method for cultivating salmonid fish that can promote growth at low cost. [Means for solving the problem]

[0007] In order to solve the above problems, the method for cultivating salmonid fish of the present invention comprises: an acclimation step of acclimating the salmonid fish, which has been reared in freshwater rearing water for a predetermined period of time or more, to rearing water having a predetermined salinity for a predetermined period of time or more; a return breeding process in which the fish is returned to freshwater breeding water after the acclimation process; The present invention is characterized by the following. According to this feature, when salmonid fish are temporarily acclimatized to rearing water containing seawater and then returned to freshwater, their growth can be promoted compared to when they are reared in freshwater only. In other words, as long as the rearing water can be prepared to have the salinity required for the acclimation process, the growth of salmonid fish can be promoted at low cost.

[0008] The acclimation process is characterized in that salmonid fish that have been reared in freshwater rearing water for a predetermined period of time or more are acclimated to rearing water with a predetermined salinity for a predetermined period of time or more, and then are acclimated to rearing water with a predetermined salinity higher than the predetermined salinity for a predetermined period of time or more. According to this characteristic, when salmonid fish are acclimatized from freshwater to rearing water with gradually increasing salinity and then returned to freshwater, their growth can be further promoted compared to when they are reared only in freshwater.

[0009] The acclimation step is characterized in that it is carried out for a period of one week or more in rearing water having a predetermined salinity. According to this feature, when the acclimation process is carried out for one week or more in rearing water of each salinity concentration, the seawater adaptability is improved, and the growth of salmonid fish can be efficiently promoted especially in the subsequent return rearing process.

[0010] The acclimation step is characterized in that it is carried out for a period of one week in rearing water having a predetermined salinity. According to this feature, when the acclimation process is carried out for one week in rearing water of each salinity concentration, the growth of salmonid fish can be efficiently promoted in the shortest period of time.

[0011] The rearing water used in the acclimation process is a mixture of seawater and freshwater, and the rearing water to which the fish are initially transferred from the freshwater rearing water is characterized by containing up to 50% seawater by volume. According to this feature, by using rearing water containing up to 50% seawater as the first rearing water to which the fish are transferred from freshwater, the survival rate of salmonid fish during the acclimation process can be increased.

[0012] The rearing water used in the acclimation process is a mixture of seawater and freshwater, and the rearing water immediately before being transferred to the return rearing process is characterized in that it contains up to 70% of its total volume as seawater. According to this feature, by using rearing water containing up to 70% seawater by volume as the rearing water immediately before being transferred to the return rearing process, the survival rate of salmonid fish during the acclimation process can be increased.

[0013] The rearing water, the salinity of which has been gradually increased in the acclimation process, is characterized in that the difference in the proportion of seawater to the total volume is within 25%. This feature reduces the stress caused by the sudden change in salinity of the breeding water when the fish are transferred to breeding water with gradually increasing salinity during the acclimation process, thereby increasing the survival rate of salmonid fish.

[0014] The seawater is characterized in that it is artificial seawater. This feature makes it easier to keep the water temperature below 18°C, which also reduces the risk of disease.

[0015] The acclimation step is characterized in that it is carried out on fry weighing 0.3 g to 0.5 g after completion of yolk sac absorption. According to this feature, the mortality rate due to stress during the acclimation process is low, and the growth rate per period is relatively high, so the effect of growth promotion is the highest. In addition, by targeting the smallest individual stage, the acclimation process can be carried out on a large number of fish in a minimum space, making aquaculture more efficient. [Brief description of the drawings]

[0016] [Figure 1] 1 is a graph showing changes in survival rate of cherry salmon before yolk sac absorption during the experimental period of the salinity tolerance experiment in Example 1. [Diagram 2] 1 is a graph showing changes in survival rate of cherry salmon after yolk sac absorption during the experimental period of the salinity tolerance experiment in Example 1. [Diagram 3] 1 is a graph showing the change in plasma osmotic pressure of cherry salmon after yolk sac absorption during the experimental period of the salinity tolerance experiment in Example 1. [Figure 4] 1 is a graph showing the results of growth experiment 1 using the salmonid fish farming method of Example 1, showing the change in average body weight of cherry salmon after yolk sac absorption during the experimental period. [Diagram 5] 1 is a graph showing the results of growth experiment 2 using the salmonid fish farming method of Example 1, showing the change in average body weight of cherry salmon after yolk sac absorption during the experimental period. [Figure 6]1 is a graph showing the results of growth experiment 3 using the salmonid fish farming method of Example 1, showing the change in average body weight of cherry salmon after yolk sac absorption during the experimental period. [Figure 7] 13 is a graph showing the change in plasma osmotic pressure when the cherry salmon of the seawater-experienced and inexperienced groups reared in growth experiment 3 were transferred to rearing water containing 100% seawater. [Figure 8] 13 is a graph showing changes in growth hormone mRNA in salmon in the seawater-experienced and seawater-naive groups during rearing in growth experiment 1. [Figure 9] Graph (a) shows the results of measuring MGH1 in individuals from the seawater-experienced and naive groups, graph (b) shows the results of measuring NPY in individuals from the seawater-experienced and naive groups, and graph (c) shows the results of measuring AgRP in individuals from the seawater-experienced and naive groups. [Figure 10] 13 is a graph showing the results of growth experiment 4 using the salmonid fish farming method of Example 2, in which (a) shows the change in average body weight of Nagano-strain rainbow trout after yolk sac absorption during the experimental period, and (b) shows the change in average body weight of Donastigma-strain rainbow trout after yolk sac absorption during the experimental period. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The method for cultivating salmonid fish according to the present invention will be described below with reference to examples. EXAMPLES

[0018] In this example, we investigated the effect of seawater acclimation on the growth of cherry salmon, a salmonid fish, and found that it is significant for promoting growth and improving seawater adaptability. Seawater acclimation (hereinafter sometimes simply referred to as "acclimation") is also called seawater experience, and refers to artificially and temporarily exposing young fish to seawater during the period when they are primarily growing in a freshwater environment.

[0019] Many of the salmonid fish hatcheries in Japan are located in mountainous areas in order to utilize spring water or the upper reaches of rivers where the water temperature is low. In this embodiment, taking into consideration such geographical issues and the risk of disease when using seawater directly from the sea, seawater acclimation is performed using artificial seawater. Furthermore, the inventors' experiments and research have clarified the minimum individual stage at which seawater acclimation is possible. This makes it possible to acclimate large quantities of salmon to seawater at low cost in a minimum space. In the following embodiment, cherry salmon hatched from eggs collected at the Inland Water Branch of the Miyazaki Prefectural Fisheries Experimental Station are used.

[0020] [Salinity tolerance experiment 1 (cherry salmon)] First, we prepared 50% seawater breeding water (17 psu) by mixing equal amounts of seawater (34 psu) and freshwater, and 75% seawater breeding water (26 psu) by mixing seawater and freshwater in a 75:25 ratio. The seawater referred to here is made by dissolving artificial seawater in freshwater to create a salinity condition of 34 psu. In this embodiment, when the 100% seawater condition is 1000 mOsm, the 50% seawater condition can be replaced with 500 mOsm, and the 75% seawater condition can be replaced with 750 mOsm.

[0021] In order to investigate the time when cherry salmon can be acclimatized to seawater and the seawater concentration (salinity) at which they can be acclimatized, cherry salmon fry before absorbing the yolk sac that had been reared in freshwater were transferred to tanks containing rearing water with 50% seawater conditions, 75% seawater conditions, and 100% seawater conditions for a specified period of time or longer, specifically 7 days, 14 days, and 28 days after hatching, and the changes in survival rate were investigated. The results are shown in Figure 1.

[0022] [result] As shown in Figure 1(a), 7-day-old cherry salmon larvae were transferred from freshwater to rearing water with 50% seawater, 75% seawater, and 100% seawater, respectively. The survival rate of the group transferred from freshwater to rearing water with 50% seawater was 0% within 72 hours after the seawater transfer, the survival rate of the group transferred from freshwater to rearing water with 75% seawater was 0% within 60 hours after the seawater transfer, and the survival rate of the group transferred from freshwater to rearing water with 100% seawater was 0% within 8 hours after the seawater transfer. In addition, the survival rate of the control group of the seawater-experienced group raised in this experiment 1, which was not transferred to seawater and continued to be raised in freshwater, was 100%.

[0023] As shown in Figure 1(b), 14-day-old cherry salmon larvae were transferred from freshwater to rearing water with 50% seawater, 75% seawater, and 100% seawater, respectively. In group A, which was transferred from freshwater to rearing water with 50% seawater, the survival rate dropped to about 75% 72 hours after the transfer to seawater, and remained at about 75% until 96 hours after the transfer to seawater. In the group transferred from freshwater to rearing water with 70% seawater, the survival rate was 0% within 60 hours after the transfer to seawater, and in the group transferred from freshwater to rearing water with 100% seawater, the survival rate was 0% within 11 hours after the transfer to seawater.

[0024] As shown in Figure 1(c), 28-day-old cherry salmon larvae were transferred from freshwater to rearing water with 50% seawater, 75% seawater, and 100% seawater conditions. The survival rate of the group transferred from freshwater to rearing water with 50% seawater was 100% up until 96 hours after the transfer to seawater. The survival rate of the group transferred from freshwater to rearing water with 75% seawater was 0% within 48 hours after the transfer to seawater, and the survival rate of the group transferred from freshwater to rearing water with 100% seawater was 0% within 11 hours after the transfer to seawater.

[0025] Thus, the results of salinity tolerance experiment 1 confirmed that cherry salmon larvae up to 28 days after hatching, before absorption of the yolk sac, could not be habituated to conditions of 75% seawater or higher, but that cherry salmon larvae 28 days after hatching could be habituated with a survival rate of 100% if they were in 50% seawater.

[0026] [Salinity tolerance experiment 2 (cherry salmon)] Next, we prepared 50% seawater rearing water (17 psu) by mixing equal amounts of seawater and freshwater, 70% seawater rearing water (24 psu) by mixing seawater and freshwater in a 7:3 ratio, 80% seawater rearing water (27 psu) by mixing seawater and freshwater in an 8:2 ratio, and 90% seawater rearing water (31 psu) by mixing seawater and freshwater in a 9:1 ratio.

[0027] In order to investigate the time when cherry salmon fry after absorbing the yolk sac can become acclimated to seawater and the upper limit of the seawater concentration (salinity) at which they can become acclimated to seawater, cherry salmon fry 35 days after hatching and after absorbing the yolk sac were transferred from freshwater to aquariums containing breeding water with 50% seawater conditions and allowed to acclimate for 7 days, after which the cherry salmon fry were transferred from the 50% seawater breeding water to tanks containing 70% seawater conditions, 80% seawater conditions, 90% seawater conditions, or 100% seawater conditions, and the changes in survival rate were investigated. The results are shown in Figure 2.

[0028] [result] As shown in Figure 2, 35-day-old cherry salmon fry were acclimated for 7 days in 50% seawater rearing water, and then transferred from 50% seawater to 70%, 80%, 90%, and 100% seawater. In the group transferred to 70% seawater, the survival rate dropped to about 90% 60 hours after the seawater transfer, and remained at about 90% until 168 hours (1 week) after the seawater transfer. In the group transferred to 80% seawater, the survival rate began to drop 36 hours after the seawater transfer, and was about 10% until 168 hours after the seawater transfer. In addition, in the group transferred to breeding water containing 90% seawater, the survival rate was 0% within 36 hours after the transfer, and in the group transferred to breeding water containing 100% seawater, the survival rate was 0% within 24 hours after the transfer.

[0029] Thus, from the results of Salinity Tolerance Experiment 2, it was confirmed that cherry salmon fry after absorbing the yolk sac 35 days after hatching can be acclimatized with a high survival rate (about 90%) if they are acclimated to rearing water with 50% seawater conditions for 7 days and then transferred to rearing water with 70% seawater conditions. The inventors confirmed that the minimum individual stage for cherry salmon is a fry 35 days after hatching (i.e., weighing 0.3 to 0.5 g).

[0030] Furthermore, the results of salinity tolerance experiment 2 showed that the survival rate of cherry salmon fry, after absorbing the yolk sac 35 days after hatching, was less than 10% when they were transferred from freshwater to rearing water with 50% seawater conditions, and then from 50% seawater conditions to rearing water with 80% seawater or higher conditions. These results confirmed that even when the salinity is increased in stages, stress caused by a sudden change in the salinity of the rearing water is a factor that reduces the survival rate of cherry salmon fry.

[0031] [Salinity tolerance experiment 3 (cherry salmon)] Next, based on the results of the above salinity tolerance experiments 1 and 2, the change in plasma osmotic pressure was measured when cherry salmon fry after absorbing the egg yolk sac 35 days after hatching was transferred in the order of freshwater to 50% seawater, and then from 50% seawater to 70% seawater. The results are shown in Figure 3. In addition, Figure 3 shows a graph of the change in plasma osmotic pressure of the inexperienced group (freshwater group) of cherry salmon fry after absorbing the egg yolk sac 35 days after hatching that was reared only in freshwater as a control group for the seawater-experienced group reared in this experiment 3, and the results of the significant difference judgment (Student's t-test). In addition, plasma osmotic pressure was measured using an osmotic pressure measuring device (Wesco 5520).

[0032] [result] As shown in Figure 3(a), the plasma osmolality of the seawater-experienced group increased significantly during the 7 days of rearing in rearing water with 50% seawater conditions compared to the freshwater group. In detail, the plasma osmolality of the seawater-experienced group increased to approximately 360-385 mOsm / kg H2O during rearing in rearing water with 50% seawater conditions. In addition, during rearing in rearing water with 50% seawater conditions, the plasma osmolality of the seawater-experienced group gradually increased as time passed after the transition to seawater.

[0033] As shown in Figure 3(b), the plasma osmolality of the seawater-experienced group increased significantly over the 28 days after they were transferred from 50% seawater to 70% seawater compared to the freshwater group. In detail, the plasma osmolality of the seawater-experienced group increased to approximately 375-460 mOsm / kg H2O during rearing in 70% seawater. During rearing in 70% seawater, the plasma osmolality of the seawater-experienced group reached its maximum (approximately 460 mOsm / kg H2O) one day after the transfer to seawater, and gradually decreased over time.

[0034] Thus, from the results of salinity tolerance experiments 1 to 3, it was confirmed that cherry salmon fry after absorbing the yolk sac 35 days after hatching could acclimate with a survival rate of 90% or more by increasing the salinity of rearing water from freshwater to 50% seawater, and then from 50% seawater to 70% seawater. It was also confirmed that the change in plasma osmotic pressure caused by the salinity, i.e., osmotic pressure adjustment, was suppressed, and the adaptability to seawater was increased by gradually increasing the salinity of rearing water from freshwater to 50% seawater, and then from 50% seawater to 70% seawater.

[0035] That is, in seawater acclimation treatment of salmonid fish, acclimation with a high survival rate is possible by gradually increasing the salinity from freshwater to 50% seawater, and then from 50% seawater to 70% seawater. In particular, for cherry salmon after absorbing the yolk sac 35 days after hatching, in the first acclimation step in which they are transferred from freshwater rearing directly to seawater, they can be acclimated for more than one week in rearing water with a 50% seawater condition, and in the second acclimation step in which they are transferred from the first acclimation step to rearing in seawater with a higher salinity, they can be acclimated for more than one week in rearing water with a 70% seawater condition. The salinity concentration at which the fish can be acclimated for one week or more refers to a salinity concentration at which the fish can be acclimated for one week or more with a survival rate of 90% or more in the first and second acclimation steps. In the first acclimation step, the rearing water can be acclimated with a high survival rate if the rearing water is up to 55% seawater conditions, and in the second acclimation step, the rearing water can be acclimated with a high survival rate if the rearing water is up to 75% seawater conditions.

[0036] Furthermore, the difference in seawater conditions, i.e., the ratio of seawater to the total volume of the rearing water, between the first and second acclimation processes is preferably within 25%, and more preferably within 20%, thereby reducing stress caused by sudden changes in the salinity concentration of the rearing water and increasing the survival rate of the salmonid fish.

[0037] [Growth experiment 1] Hereinafter, growth experiments carried out on the method for cultivating salmonid fish (hereinafter referred to as "this method") will be described. In this experiment 1, cherry salmon after absorbing the yolk sac 35 days after hatching, which is the minimum stage for individuals to be able to acclimate to seawater, are the subjects. In this example, breeding water is introduced into a breeding pond for aquaculture and circulated, allowed to flow, or pumped and left to stand, and salmonid fish are housed in the breeding pond for breeding. Breeding ponds range from small ones of around 0.1 ton to large tanks of over 200 ton.

[0038] First, in a farm adjacent to the hatchery, the salmon are raised in a tank filled with freshwater breeding water (hereinafter, for the sake of distinction, this tank will be referred to as the "first tank") for 35 days after hatching, until they become cherry salmon fry (weighing 0.3 to 0.5 g) after absorbing the yolk sac. Hereinafter, for the sake of distinction, the raising in this breeding water will be referred to as the "freshwater breeding process."

[0039] Next, the cherry salmon fry are transferred from the first tank to a second tank. The second tank is filled with breeding water with 50% seawater conditions, which is a mixture of equal amounts of seawater and freshwater. In this method, breeding in the breeding water with 50% seawater conditions filled in the second tank is called the "first acclimation process." The breeding period in the second tank in the first acclimation process was one week.

[0040] Next, the cherry salmon fry that have completed the first acclimation process are transferred from the second tank to a separate third tank. The third tank is filled with breeding water with 70% seawater conditions, which is a mixture of seawater and freshwater in a 7:3 ratio. In this method, breeding in the breeding water with 70% seawater conditions that fills the third tank is called the "second acclimation process." The breeding period in the third tank in the second acclimation process was four weeks.

[0041] Next, the cherry salmon fry that have completed the second acclimation process are transferred from the third tank to the fourth tank. The fourth tank is filled with freshwater breeding water. In this method, the breeding in the freshwater breeding water in the fourth tank is the "return breeding process." In actual salmonid fish farming, the individuals are grown to shipping size in this return breeding process.

[0042] At all stages, the salmon were fed a diet consisting of 1.5-5.0% of the total biomass. During the experiment, survival status was checked at appropriate times, and body weight was measured.

[0043] In addition, for the same period as the present method, a conventional culture method was also carried out using cherry salmon fry with approximately the same body weight at the start of culture as the present method, in which all the fish were cultured in freshwater, for comparison. The group cultured using the present method was designated the seawater-experienced group, and the control group cultured using the conventional freshwater culture method was designated the inexperienced group (freshwater group).

[0044] [result] The changes in average body weight of the cherry salmon in the seawater-experienced and naive groups are shown in Figure 4. As shown in Figure 4(a), there was no significant difference in the changes in average body weight between the seawater-experienced and naive groups during the first and second acclimation processes, and even three weeks after moving to the return-breeding process, there continued to be no significant difference in the changes in average body weight between the seawater-experienced and naive groups.

[0045] Four weeks after the transition to the reversion rearing process, the seawater-experienced group began to gain weight on average compared to the naive group, and about two months after the transition to the reversion rearing process, the average weight increased significantly by 20.9-24.2% compared to the naive group. The significance of the difference was determined by Student's t-test.

[0046] Furthermore, as shown in Figure 4(b), three months after the transition to the reversion rearing process, the seawater-experienced group continued to show a significant increase in average body weight compared to the naive group, and at about seven months after the transition to the reversion rearing process, the average body weight had increased significantly by 37.6% compared to the naive group. In other words, it was confirmed that the early growth of cherry salmon is greatly promoted by the seawater acclimation treatment after yolk sac absorption 35 days after hatching, which is the minimum individual stage.

[0047] [Growth experiment 2] Next, based on the results of the above growth experiment 1, in order to shorten the period of the acclimation process, the salmon were directly transferred from freshwater to rearing water with 70% seawater conditions, i.e., the first acclimation process in the above growth experiment 1 was not carried out, only the second acclimation process was carried out, and the rearing period in the third tank in the second acclimation process was shortened from 4 weeks to 3 days or 1 week, and the change in the average weight of the cherry salmon was examined. The other conditions were the same as those in the above growth experiment 1.

[0048] [result] The changes in average body weight of the cherry salmon in the seawater-experienced and naive groups are shown in Figure 5. As shown in Figure 5, in the group in which the first acclimation step was not performed and the second acclimation step was shortened to three days, there was not much difference in the changes in average body weight between the seawater-experienced and naive groups, confirming that no growth-promoting effect was observed.

[0049] On the other hand, in the group in which the first acclimation process was not carried out and the second acclimation process was shortened to one week, the average body weight had increased by 10.3% compared to the inexperienced group about four months after moving on to the reintroduction process, confirming the growth-promoting effect.

[0050] [Growth experiment 3] Next, based on the results of the above growth experiments 1 and 2, in order to shorten the period of the acclimation process, the rearing period in the second tank in the first acclimation process was shortened to one week, and the rearing period in the third tank in the second acclimation process was shortened from four weeks to one week, and the change in average weight of the cherry salmon was examined. The other conditions were the same as those of the above growth experiment 1.

[0051] [result] The changes in average body weight of cherry salmon in the seawater-experienced and naive groups are shown in Figure 6. As shown in Figure 6, in the group in which the first acclimation process was carried out for one week and the second acclimation process was shortened to one week, the average body weight began to increase significantly compared to the naive group about one and a half months after the transition to the return-rearing process, and from three months after the transition to the return-rearing process, the average body weight had increased significantly by 18.4-20.7% compared to the naive group, confirming a growth promotion effect comparable to that of growth experiment 1, in which the second acclimation process was carried out for four weeks.

[0052] [Physiological growth assessment] This promotion of early growth of cherry salmon was evaluated from a physiological aspect. First, the first acclimation step (50% seawater) was performed for one week by this method (see Growth Experiment 3), and the second acclimation step (70% seawater) was performed for one week. In the return rearing step, the seawater-experienced group was reared until the time when they could transfer to seawater (one month after hatching), and the naive group (freshwater group) was reared in freshwater only for the same period as the seawater-experienced group until the time when they could transfer to seawater (one month after hatching). The results of examining the change in plasma osmolality are shown in Figure 7. At the time of the transfer to rearing water with 100% seawater conditions, the average body weight of the seawater-experienced group was 89.4±5.7g, and the average body weight of the naive group was 75.5±5.7g.

[0053] As shown in Figure 7, the change in plasma osmotic pressure in the seawater-experienced group reared using this method (see Growth Experiment 3) upon transfer to rearing water with 100% seawater conditions was significantly lower 1 to 3 days after transfer to seawater compared to the naive group, confirming that the seawater-experienced group had improved seawater adaptability. Furthermore, due to improved seawater adaptability, fewer individuals died in the seawater-experienced group after transfer to seawater compared to the naive group.

[0054] Furthermore, as shown in Figure 8, it was confirmed that the seawater-experienced group reared using this method (see Growth Experiment 1) had a significant increase in growth hormone mRNA within one month of moving to the reintroduction rearing process, compared to the naive group. Growth hormone is secreted from the pituitary gland and promotes the differentiation of salt cells in the gills that actively excrete excess salts from the body, thereby developing seawater-type salt cells and improving adaptability to seawater. Growth hormone is also known to promote growth.

[0055] Furthermore, as shown in FIG. 9, no significant changes were observed in melanin-concentrating hormone 1 (MCH1), neuropeptide Y (NPY), and agouti-related peptide (AgRP), which are known as feeding factors.

[0056] In other words, based on the results of these physiological growth evaluations, it is speculated that the promotion of early growth in salmonid fish by the present method is due to the induction of the growth hormone system during the rebreeding process.

[0057] Based on the above, when juvenile salmonid fish were acclimatized from freshwater to rearing water with gradually increasing salinity and then returned to freshwater, the initial growth of the smallest individual stage cherry salmon was found to be promoted by 1.3 to 1.4 times compared to when they were reared in freshwater only.

[0058] In other words, the inventors have established a farming method that can promote the growth of salmonid fish at low cost, comprising a first acclimation step of acclimating salmonid fish that have been raised in freshwater breeding water for a predetermined period of time or more to breeding water with 50% seawater conditions for a period of one week or more, a second acclimation step of acclimating the salmonid fish to breeding water with 70% seawater conditions higher than those of the first acclimation step for a period of one week or more after the first acclimation step, and a return breeding step of returning the salmonid fish to the freshwater breeding water for breeding after the second acclimation step. Furthermore, the inventors have established a culture method for salmonid fish that can achieve growth promotion effects in the shortest time, comprising a first acclimation step in which cherry salmon after absorbing a yolk sac 35 days after hatching and which have been cultured in freshwater culture water, i.e., cherry salmon at the smallest individual stage (body weight 0.3 to 0.5 g), are acclimated to culture water up to 50% seawater conditions for a period of one week, a second acclimation step, after the first acclimation step, in which the salmon are acclimated to culture water up to 70% seawater conditions, which are higher than the first acclimation step, for a period of one week, and a return culture step in which the salmon are returned to the freshwater culture water for culture after the second acclimation step.

[0059] In the salmonid fish farming method of the present invention, the fish may be acclimated in breeding water up to 70% seawater condition, for example, and then moved directly to a return breeding process in freshwater. However, it is preferable to acclimate the fish from freshwater to breeding water with gradually increasing salinity in the first and second acclimation processes as described above, thereby obtaining a greater growth promotion effect and making salmonid fish farming more efficient.

[0060] In addition, by using artificial seawater, it is easy to prepare breeding water at 18°C ​​or less using cold water in mountainous areas where many hatcheries are located, which not only promotes the growth of salmonid fish, but also reduces the risk of disease compared to using seawater directly from the ocean.

[0061] In addition, the first acclimation process is performed on cherry salmon fry that have been reared in freshwater for a specified period of time and have grown to 0.3 to 0.5 g after absorbing the yolk sac, so there are few deaths due to stress during the acclimation process, and the growth rate per period is relatively high, so the effect of promoting growth is the greatest. Also, by targeting cherry salmon fry at the smallest individual stage, the acclimation process can be performed on a large number of them in a minimum space, making aquaculture more efficient.

[0062] Furthermore, cherry salmon raised using this method have suppressed increases in plasma osmotic pressure that would occur when they are transferred to 100% seawater conditions during the period when transfer to seawater is possible, and their ability to adapt to seawater is improved. This is expected to promote growth from the freshwater rearing period while reducing mortality when they are transferred to marine aquaculture. It is also possible to increase the final individual size within the limited culture period, making aquaculture more efficient. EXAMPLES

[0063] In this example, rainbow trout was used as the salmonid fish, and it was verified whether the salmonid fish farming method established for cherry salmon in Example 1 above could be applied to rainbow trout in the early juvenile stage (body weight 0.3 to 0.5 g).

[0064] [Growth experiment 4] In this experiment, the rainbow trout were Nagano Fisheries Research Institute rainbow trout (hereafter referred to as "Nagano strain") and Donaldson Steelhead strain rainbow trout (hereafter referred to as "Donast strain"), and changes in average body weight of rainbow trout were examined when the first acclimation process was carried out for one week in the second tank (breeding water with 50% seawater conditions) and the second acclimation process was carried out for one week in the third tank (breeding water with 70% seawater conditions) in the same manner as in growth experiment 3. In this experiment, the Nagano strain used a population with an average body weight of 0.44±0.08g after completion of yolk sac absorption, and the Donast strain used a population with an average body weight of 0.42±0.04g after completion of yolk sac absorption.

[0065] [result] The changes in average body weight of the Nagano rainbow trout in the seawater-experienced and naive groups are shown in Figure 10(a). As shown in Figure 10(a), there was no significant difference in the changes in average body weight between the seawater-experienced and naive groups in the first and second acclimation processes, and even about two weeks after moving to the return breeding process, there continued to be no significant difference in the changes in average body weight between the seawater-experienced and naive groups.

[0066] About one month after moving to the rebreeding process, the seawater-experienced group had a significant increase in average body weight of 5.0-15.8% compared to the inexperienced group. In other words, it was confirmed that the seawater acclimation process for Nagano strain rainbow trout at the smallest individual stage (body weight 0.3-0.5g) greatly promoted their initial growth.

[0067] The changes in average body weight of the seawater-experienced and naive Donasti rainbow trout are shown in Figure 10(b). As shown in Figure 10(b), there was no significant difference in the changes in average body weight between the seawater-experienced and naive groups during the first and second acclimation processes, and even after about two weeks of transition to the return breeding process, there continued to be no significant difference in the changes in average body weight between the seawater-experienced and naive groups.

[0068] About one month after moving to the rebreeding process, the seawater-experienced group showed a significant increase in average body weight of 9.0-28.8% compared to the inexperienced group. In other words, it was confirmed that the seawater acclimation treatment of Donastig rainbow trout at the smallest individual stage (body weight 0.3-0.5g) greatly promoted their initial growth. In addition, the growth promotion effect was particularly strong in Donastig rainbow trout compared to Nagano rainbow trout, confirming that there is a difference in responsiveness between the strains.

[0069] Although the embodiments and modified examples of the present invention have been described above with reference to the drawings, the specific configuration is not limited to these embodiments, and the present invention also includes modifications and additions that do not deviate from the gist of the present invention.

[0070] For example, in the above embodiment, the first acclimation step and the second acclimation step are each performed for one week, but the present invention is not limited to this, and each acclimation step may have a different period as long as it is one week or more.

[0071] In the above embodiment, the salt concentration is increased in two stages in the first and second acclimation steps, but the salt concentration may be increased in three or more stages. In this case, in order to reduce stress caused by a sudden change in the salt concentration of the rearing water, the difference in seawater conditions from the previous acclimation step, i.e., the ratio of seawater to the total volume of the rearing water, is preferably within 25%, more preferably within 20%. In addition, the rearing water in the last acclimation step before moving to the return rearing step is preferably 55% seawater condition or less, more preferably 50% seawater condition or less.

[0072] Furthermore, the seawater used to prepare the breeding water is not limited to artificial seawater, and may be ordinary seawater with a salinity of 28 to 38 psu that has been pumped from the ocean and filtered to remove impurities.

[0073] In addition, in the above examples, an embodiment has been described in which individuals weighing 0.3 to 0.5 g are used as the minimum individual stage of salmonid fish in order to acclimate them to seawater in a large number of fish at low cost in a minimum space. However, the present invention is not limited to this embodiment, and there is no restriction on the weight of the juvenile salmonid fish to which the present method is applied, preferably as long as the individuals have completed absorption of the yolk sac, which improves their ability to adapt to seawater.

Claims

1. an acclimation step of acclimating the salmonid fish, which has been reared in freshwater rearing water for a predetermined period of time or more, to rearing water having a predetermined salinity for a predetermined period of time or more; a return breeding process in which the fish is returned to freshwater breeding water after the acclimation process; A method for cultivating salmonid fish, comprising:

2. The method for cultivating salmonid fish as described in claim 1, characterized in that the acclimatization step comprises acclimating salmonid fish that have been raised in freshwater breeding water for a predetermined period of time or more to breeding water with a predetermined salinity for a predetermined period of time or more, and then acclimating them to breeding water with a predetermined salinity higher than the predetermined salinity for a predetermined period of time or more.

3. 3. The method for cultivating salmonid fish according to claim 1, wherein the acclimation step is carried out for a period of one week or more in breeding water having a predetermined salinity.

4. 3. The method for cultivating salmonid fish according to claim 1, wherein the acclimation step is carried out for a period of one week in breeding water having a predetermined salinity.

5. 3. The method for cultivating salmonid fish according to claim 2, wherein the rearing water used in the acclimation process is a mixture of seawater and freshwater, and the rearing water to which the fish are initially transferred from the freshwater rearing water contains up to 50% seawater by volume.

6. 3. The method for cultivating salmonid fish according to claim 2, wherein the rearing water used in the acclimation process is a mixture of seawater and freshwater, and the rearing water immediately before being transferred to the return rearing process contains up to 70% seawater by volume.

7. 7. The method for cultivating salmonid fish according to claim 2 or 6, wherein the difference in the proportion of seawater to the total volume of the rearing water in which the salinity is gradually increased in the acclimation step is within 25%.

8. 7. The method for cultivating salmonid fish according to claim 5 or 6, wherein the seawater is artificial seawater.

9. 3. The method for cultivating salmonid fish according to claim 1 or 2, wherein the acclimation step is carried out on fry weighing 0.3 g to 0.5 g after completion of yolk sac absorption.

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