JPH0751028B2 - Ectoparasite control method for seawater-cultured fish - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for exterminating ectoparasites parasitic on seawater-cultured fish.

(B) Conventional technology When parasites such as yellowtail and yellowtail in seawater farms are parasitized, their growth is hindered and they eventually die, or even if they do not die, the fish body discolors and the commercial value decreases. . Two main species of such parasites are Benedenia seriolae, which is a body surface parasite, and Heteraxine heterocerca, which is a fish-gill leaf parasite.

Further, recently, in a farm of yellowtail and striped jack, damage caused by the infestation of Caligus spinosus, which is a kind of white lice, has become a problem.

It is said that these parasites have a large tendency to undergo fish engraftment in aquaculture farms that employ the subdivision system in waters where the influence of ocean water is relatively strong.

An organic tin-based fishnet antifouling agent was developed to effectively prevent the adherence of organisms on the net in the small-scale splitting method, in which a certain sea surface is divided and cultivated according to the type of fish to be cultured or the degree of growth of the fish. In parallel with its spread, the damage caused by the parasites also decreased, but as is well known, from the pollution point of view, it is again seen at the farm where the organic tin-based fishing net antifouling agent is no longer used. Damage caused by parasites has become a problem.

It is said that the control of parasites in the farm should be carried out periodically in consideration of the difference in the life history of the parasites including hatching of the parasite eggs and the control effect on the fish / eggs. However, the freshwater bathing method and the concentrated saltwater bathing method have long been known as countermeasures (Shuzo Egusa, Koseisha Koseikaku, published on May 10, 1984, "Infectious Diseases of Fish" P-468, P -472), and the medicated bath method is being studied as a method of using a pesticide (Shizuoka Fisheries Research Station, 1965, Research Report on Fish Disease Control (196
6) Toshikazu Hoshina "Study on Benedenia seriolae";
"Fish Disease Research" 1 (2) 1967.1 Shogoro Kasahara "Extermination of parasitic flukes of Hamachi by sodium peroxyphosphate";"Fish Disease Research" 4 (2) 1970.3 Yasuyuki Oiwa, Atsushi Minamizawa "Exterior of Hamachi by Sodium Percarbonate" Extermination of the parasite Benedenia-I ”;“ Fish Disease Research ”5 (2) 1971.3 Shogoro Kasahara“ Examination of Disodium Peroxide Phosphate Medication Bath Method for Ectoparasite Fluke of Hamachi ”; Fish Disease Research 2 (2) 1968.1 Yataro Fujita "Extermination of calixes parasitic on farmed yellowtail").

(C) Problems to be Solved by the Invention The well-known fresh water bath method and concentrated salt water bath method are parasites by scooping fish into fresh water or concentrated salt water prepared in a ship tank or the like and immersing the fish for 3 to 6 minutes. The insects lose their vitality due to the osmotic pressure of fresh water or concentrated salt water, causing them to drop out of the fish body and die, but this method affects the ecology of fish because the salt concentration of the treated water is different from that of seawater. There is an inconvenience of having to transport the water to the farm where it is to be implemented, and especially when processing a large amount of fish, there are serious difficulties in implementation. On the other hand, the medicinal bath method is, as described in the above-mentioned literature, sodium pyrophosphate, sodium percarbonate, disodium peroxide, 0,0-dimethyl-2,2,2-trichloro- 1-Hydroxyethylphosphonic acid and other, although not necessarily recommended in terms of efficacy, try to exterminate parasites from fish by using the action of agents such as formalin and glacial acetic acid, and a corresponding effect was confirmed. In addition, the disadvantages of the fresh water bath and the concentrated salt bath method can be partially solved.

However, a drug which has been studied in the conventional medicinal bath method and has been found to have a medicinal effect has a difficulty in use based on its chemical and physical properties.

That is, in the medicinal bath method as well as in the fresh water bath and the concentrated salt water bath method, the fish are housed in a certain compartment and treated, but these chemical agents described above are alkaline in seawater to change the properties of seawater,
And / or it is sparingly soluble in seawater and may form inorganic precipitates in seawater and adhere to the gills of fish, causing respiratory distress.
Or, it is dangerous, and it is necessary to pay sufficient attention to how to use it, considering its effect on the ecology of fish.

In particular, since it is sparingly soluble in seawater, it is necessary to prepare a solution dissolved in fresh water in advance, and in the case of large-scale use, the damage to the labor is serious and the fishermen can adopt it. Not a method of implementation.

The present invention is intended to provide a method for exterminating parasites parasitizing cultured fish by efficiently performing an easy operation even in a large-scale treatment without any disadvantages and difficulties as in the conventional method. .

(D) Means for Solving the Problem The present invention is to store hydrogen peroxide in a swimming compartment in a cage that has been reduced in size by a partition wall that can block the passage of water to cultured fish in a seawater farm. Added, the concentration is 200 ~ 3000ppm
Provided is a method for exterminating ectoparasites parasitic on seawater-cultured fish, which comprises removing the partition walls after washing the fish body under appropriate conditions within a treatment time range of 1 to 20 minutes.

The present invention is carried out by directly administering an aqueous hydrogen peroxide solution to a swimming compartment of a fish in which aquaculture cages such as yellowtail, yellowtail, amberjack, striped horse mackerel, and sea bream are reduced. Since the size of each maritime farms pens are not necessarily constant is usually water depth of about 7～10M, planar areal 100 m ² position, administering an effective amount of hydrogen peroxide relative to the large amount of seawater Is not economical at all. Therefore, in the present invention, a suitable group of fish are densely packed and the amount of seawater is limited.

Hydrogen peroxide used as a pesticide in the present invention is extremely easily dissolved, diffused and diluted in seawater, and therefore, in order to maintain a concentration that ensures a reliable parasite control effect, for example, plastic sheet A thin cloth that blocks water flow is used to surround a part of the cage, and an aqueous hydrogen peroxide solution is sprinkled or added at one time with, for example, a pump or a bucket. The reduction range of the swimming area (processing area) is not particularly limited, but it is decided according to the size and scale of the school of fish desired to be processed. For example, as shown in Fig. 1, a processing tank is formed in seawater with a plastic sheet. Or, as shown in Fig. 2, put a plastic sheet to circulate the fish net of the swimming cage to encircle the whole circumference as much as possible to reach the sea floor and surround it. As shown in the figure, the swimming cage of the shrunk school of fish is surrounded by a plastic sheet as if it circumscribes the fish net to form a pool on the entire circumference and bottom. Thus, for the treatment tank shown in FIG. 1, the fish in the cultured cage are scooped up and transferred to the treatment tank, and after treatment, the tank is dismantled and the fish are discharged into the original cage. In the case of Fig. 2, after surrounding the cage with a plastic sheet as described above, the bottom of the fish net is pulled up as much as possible to condense the fish school here, and then the fish net is restored and the plastic sheet is sprinkled. To do. In addition, in the case of FIG. 3, after pulling up the bottom of the fishing net of the aquaculture cage in advance, as described above, after concentrating the fish school in the pool surrounded by the plastic sheet on the entire circumference and bottom surface,
Remove the plastic sheet and restore the fishing net.

In any of these methods, it is optional to appropriately reduce the cage plane. Thus, by adopting the implementation method of treating in these reduced treatment compartments, it is possible to surely maintain the desired hydrogen peroxide concentration and carry out the extermination treatment.

According to the study of the inventors, hydrogen peroxide parasitizes fish bodies under the following treatment conditions: B. seriolae and H. hetero.
It has been determined to act effectively against cerca and Caligus sp. and to have suitable properties for use according to the procedure of this invention.

That is, the concentration of the aqueous hydrogen peroxide solution added to the cage is not particularly limited, but in consideration of the fact that it directly sprinkles on the fish body, it is usually diluted with seawater at a concentration of 35% or less. It is preferable to use. Treatment of fish bodies with hydrogen peroxide in the cage resulted in a concentration of 20% in seawater.
It is performed by maintaining 0 to 3000 ppm and maintaining the treatment time in the range of 1 to 20 minutes, but the maintenance concentration and the treatment time have a mutual dependence on the parasitic effect and the degree of influence on the ecology of the fish body. , Below the concentration and below the treatment time, the effect of exterminating parasitism is not sufficient, and above the concentration and above the treatment time, there is a danger of affecting the fish body, so combine the conditions appropriately within the above concentration and time range. It is particularly preferable to carry out the treatment with a hydrogen peroxide aqueous solution concentration of 400 to 1000 ppm and a treatment time of 3 to 10 minutes. In this way, the school of fish is introduced into a previously reduced swimming compartment, an aqueous hydrogen peroxide solution is added to this to wash the fish body, and then the thin cloth for the partition wall is wiped off to finish. As a method of adding an aqueous solution of hydrogen peroxide and uniformly dispersing it to the above concentration, various known methods can be applied, but in practice, it is preferable to spray the aqueous solution of hydrogen peroxide from the upper part of the compartment.

By such an addition treatment, a uniform concentration can be adjusted within a short time within the compartment in combination with the stirring action by the swimming of the fish within the compartment. In addition to the extermination effect in the parasite in the treatment with such a chemical agent, the most important thing to be noted is the abnormal condition of the fish's ecology and the presence or absence of mortality after the treatment. That is, since the fish are treated in an abnormal environment in a narrowed compartment with a high concentration of fish shadows, it is necessary to shorten the time during which the fish come into contact with hydrogen peroxide as much as possible at a given concentration. Hydrogen oxide is extremely easily dissolved in seawater at any concentration, and diffuses uniformly and quickly as the school of fish swims.Therefore, adjust the concentration of hydrogen peroxide and the treatment time so that there is no hindrance to the growth of fish bodies. Is possible.

Experimental Example 1 A seawater farm in Mie Prefecture, December 1987, is bred by a split system with a length of 25 to 30 cm and a weight of approximately 800 g, both of which are B. seriolae.
Three year-old fish, which had been parasitized with A. japonicus, were selected as test fish, and the extermination effect in the parasitism by an aqueous hydrogen peroxide solution was tested in a laboratory. That is, seawater at 18 ° C
Was added, and hydrogen peroxide was added to each concentration so as to aerate while adhering Hamachi which was adhering to the parasite. Then, after treating for a certain period of time, it was returned to clear sea water and the effect on the fish body after 96 hours was observed and compared with the control group.

The method for obtaining the removal rate of parasites was as follows. First, the treated seawater was filtered through a plankton net to count the number of parasites that had fallen off. The number of parasites remaining in the fish that had been bred for 96 hours after the treatment was counted, and the removal rate was calculated by the following formula.

The experimental results are shown in Table 1.

Experimental Example 2 The properties when hydrogen peroxide and other peroxides were added to seawater were tested. Each compound was prepared with pure water so as to have a concentration of 5 w / v%, and each treating agent was added to 1 filtered seawater so as to have a predetermined concentration. At that time, except for hydrogen peroxide, it became cloudy white.
The properties of seawater were investigated. The results are shown in Table 2.

Example 1 Parasites were exterminated at a certain yellowtail farm in Mie prefecture. The cage (2) formed by the fishing net (1) shown in Fig. 1.
Polyethylene sheet (3) in another cage (2 ') next to
Create a 1m × 2m × 1.5m seawater pool (4) surrounded by, fill it with 2m ³ of seawater, and prepare a 3w / v% hydrogen peroxide solution prepared in advance in the pool (4) of 400ppm in seawater. Was added with a pump. About 1 kg of yellowtail was scooped from the original cage (2) and about 300 fish were transferred to the pool (4).

An average of 30 to 60 B. seriolae lived on this yellowtail. The yellowtails were left in a dense state for 5 minutes, then the pool (4) was disassembled and the yellowtails were discharged into the cage (2 ').
At this time, the hydrogen peroxide concentration in pool (4) was 150 ppm.

After 3 hours, 10 scoops of hamachi were randomly picked up, and B.seriol
As a result of examining the engraftment status of ae, it was not observed at all.

For several days, we observed the intake of dead fish and food in Hamachi, but no abnormalities were observed.

Example 2 In the above-mentioned farm, one side 10m and a depth 10m shown in FIG.
The Hamachi cage (2) was draped in a curtain shape so that the vinyl sheet (3) was circumscribed to the fishing net (1) forming the cage (2) up to a water depth of 6 m, and the cage (2) was surrounded.

In this cage (2), about 700 g of yellowtail grew about 10,000, and 20 to 50 B. seriolae lived on the yellowtail on average.

The bottom (1 ') of the fishing net (1) of the cage was tucked up to a water depth of about 2 m to make the hamachi densely packed in a reduced cage of about 200 m ³ . Immediately, a 2 w / v% aqueous hydrogen peroxide solution prepared in advance was added in a shower form in an amount of 500 ppm in 200 m ³ of seawater for 1 minute with a pump.

After leaving for 15 minutes, the vinyl sheet surrounding the cage (3)
, And returned the hamachi to the original cage (2). The hydrogen peroxide concentration in the cage before removal was 100 ppm. The intake of dead fish and food of yellowtail was observed and investigated for 24 days after the number of parasites had taken up. As a result, no dead or weakened yellowtail was observed, and 10 randomly picked yellowtails did not show any parasite engraftment.

Example 3 In the above hamachi farm, about 10,000 B. seriolae grew on an aquaculture cage (2) having a side of 10 m and a depth of 10 m shown in FIG. Was.

The bottom (1 ') of the fishing net (1) of the cage (2) was tucked up to a water depth of about 2 m, and the hamachi were densely packed in a reduced area of about 200 m ³ . Immediately hold the vinyl sheet (3) to form a wrapping pool (4) so as to circumscribe the reduced fishing net (1), and add a hydrogen peroxide solution to the pool (4) at a concentration of 400 ppm by a pump. It was added like a shower for 30 seconds. After left for 5 minutes, the vinyl sheet (3) was spun off, and the cage (2) was returned to its original state.

Parasitism was not observed in the hamachi, which was randomly picked up after 24 hours, and no deceased fish had been observed until several days later, and food intake was good.

Experimental Example 3 In a seawater farm in Shizuoka Prefecture, October 1988, a three-year-old fish striped with a length of about 30 cm and a body weight of about 1 kg, both of which are infested with Caligus sp. As a test fish,
The control effect of the parasite by the aqueous hydrogen peroxide solution was tested in the laboratory.

Seawater 50 at 15 ° C. was placed in a 75-volume plastic container, hydrogen peroxide was added to each concentration, and three acacia to which parasites were attached were placed while aerating. Then, after treating for a certain period of time, it was returned to clear sea water and the effect on the fish body after 96 hours was observed and compared with the control group.

The method for obtaining the removal rate of parasites was as follows. First, the treated seawater was filtered through a plankton net to count the number of parasites that had fallen off. The number of parasites remaining in the fish raised for 96 hours after the treatment was counted, and the removal rate was calculated by the following formula.

The experimental results are shown in Table 3.

Experimental Example 4 Fish species, test methods, and evaluation methods were the same as in Experimental Example 3.

The drug used was Depterex (emulsion: 50% active ingredient) described in the literature.

The experimental results are shown in Table 4.

When using Depterex, the concentration at which parasites can be completely removed requires 100 ppm of contact for 1 minute or more. However, if the contact time with the drug is 3 minutes or more, all fish will die, so it is extremely dangerous as a parasite treatment agent and cannot be used in practice.

In addition, Depterex is a pesticide and is not a treatment agent that should be used due to the danger to the human body.

Example 4 Parasites were exterminated at a certain Japanese horse mackerel farm in Shizuoka prefecture. Create a 1m x 2m x 1.5m seawater pool (4) surrounded by a polyethylene sheet (3) in another cage (2 ') next to the cage (2) formed by the fishing net (1) shown in Fig. 1. , Fill it with 2 m ³ of seawater, and add 5 w / v% hydrogen peroxide solution prepared in advance to the pool (4) with a seawater concentration of 300 ppm.
Was added with a pump. 1kg from the original cage (2)
Scoop up the striped horse mackerel and pool about 400 fish (4)
Moved to.

An average of 40 to 70 Caligus sp. Lived on this Japanese horse mackerel. The striped horse mackerel was left in a dense state for 3 minutes, then the pool (4) was disassembled, and the striped horse mackerel was discharged into the cage (2 ′). The concentration of hydrogen peroxide in pool (4) at this time is 100 ppm.
Met.

After 3 hours, pick up 5 strips of horse mackerel randomly, Caligu
As a result of examining the engraftment status of s sp., it was not observed at all.

For several days, we observed the consumption of dead fish and foods of Japanese horse mackerel, but no abnormalities were observed.

Example 5 In the above-described striped horse mackerel farm, a side of 10 m shown in FIG.
Approximately 2000 striped horse mackerels growing in a cultured cage (2) having a depth of 10 m had an average of 30 to 50 Caligus sp.

The bottom (1 ') of the fishing net (1) of the cage (2) was tucked up to a water depth of about 2 m, and the striped horse mackerel was concentrated in a reduced area of about 200 m ³ . Immediately hold the vinyl sheet (3) and wrap it around the reduced fishing net (1) to enclose the pool (4)
To form a pool of hydrogen peroxide solution (4)
The amount in which the internal concentration became 300 ppm was added in a shower form for 30 seconds by a pump. After standing for 3 minutes, the vinyl sheet (3) was spun off, and the cage (2) was returned to its original state.

No evidence of engraftment during parasitism was observed in the Japanese horse mackerel picked up randomly after 24 hours. As a result of observation up to several days later, no fish died and food intake was good.

Example 6 When a hydrogen peroxide-generating compound other than hydrogen peroxide was directly dissolved in seawater, it became cloudy, and it was unknown whether or not it was dissolved. When dissolved in fresh water, there is no cloudiness of the solution, but the concentration is still markedly reduced, and there is a time constraint until the solution dissolves into a chemical bath, and it is difficult to obtain a large amount of fresh water at the seaside. However, it requires a great deal of labor to dissolve and then ship it by sea. The stability test results of the prepared solutions are shown in Tables 5 and 6
Shown in the table.

Example 7 (Heteraxine heter)
Effect on ocerca)) At a certain yellowtail farm in Mie prefecture, a test was conducted on a yellowtail (weighing about 250 g / tail) infested with aphids (which also had worms attached) in a small-scale splitting method. As a fish, an extermination test with hydrogen peroxide was performed on site.

That is, put 50 of seawater (water temperature 21 ° C) in a 75 volume plastic container,
Hydrogen peroxide was added to each concentration to add 3 aphids-attached yellowtail, which was treated for 3 minutes, and then returned to clear sea water and aerated for 96 hours while performing aeration, and the effect on fish bodies was observed.

The method for determining the parasite removal rate was as follows. First, each treated seawater was filtered through a plankton net, and the number of parasites (aphid and podworm) that had fallen off was counted. Then, the removal rate was calculated by counting the number of insects remaining on the gills and body surface of the fish that had been bred for 96 hours after the treatment. This experimental result is No. 7
Shown in the table.

Example 8 (Effect of Red sea bream'Elamy '“Bivagina tai” disease) In a red sea bream farm in Mie prefecture, red sea breams (body weight about 350 g / tail) are infested with red sea bream by a split method. Using the fish as a test fish, an extermination test with hydrogen peroxide was performed on site.

That is, put 50 of seawater (water temperature 21 ° C) in a 75 volume plastic container,
Hydrogen peroxide was added to each concentration to add three red sea breams to which aphids were attached. And after processing for a certain time,
After returning to clear seawater and performing aeration, the animals were bred for 96 hours and the effects on the fish bodies were observed.

The method for determining the parasite removal rate was as follows. First, each treated seawater was filtered through a plankton net, and the number of parasites that had fallen off was calculated. Then, the number of parasites remaining on the gills of the fish bodies raised for 96 hours after the treatment was counted to obtain the removal rate. The experimental results are shown in Table 8.

Example 9 Aphid parasites of red sea bream were exterminated at a certain red sea bream farm in Mie Prefecture.

Place a new IKES in contact with the IKES of the fish to be treated, and put a canvas sheet treatment tank (2.5m
X 2.5 m, depth 1.5 m, internal capacity of about 9 tons).

About 1/2 of seawater was put in the treatment tank, and hydrogen peroxide was added to about 500 ppm. And red sea bream than Ikesu in culture
4,000 fish were driven into the treatment tank together with seawater, and after swimming in the treatment tank for about 10 minutes, one end of the treatment tank was released and the red sea bream was discharged into a new cage.

The hydrogen peroxide concentration in the treatment tank in this case was 250 ppm. A part of the treated water was filtered with a plankton net to confirm the fallen aphids.

After 96 hours, no dead fish were observed and normal feeding behavior was observed.

Example 10 At a certain amberjack farm in Mie Prefecture, the aphid parasite of the amberjack was exterminated.

Place a new cage in contact with the fish cage to be treated and put a canvas sheet treatment tank (2m x
2m, depth 1.5m, capacity 6t).

About 1/2 of seawater was put in the treatment tank, and hydrogen peroxide was added to about 500 ppm. Then, 500 amberjacks were driven into the treatment tank together with seawater from the cages during cultivation, and after swimming for about 5 minutes in the treatment tanks, one end of the treatment tanks was released, and the amberjacks were discharged to new cages. This operation was repeated to treat 5000 amberjacks. At this time, the hydrogen peroxide concentration in the treatment tank was 250 ppm, and the seawater temperature was 21 ° C.

When five amberjacks after 96 hours of treatment were randomly examined for the presence of aphids, they were not observed, and it is considered that they were all removed by the treatment with hydrogen peroxide.

The effect of hydrogen peroxide was confirmed by counting the number of dead fish 25 days before the treatment with hydrogen peroxide and 30 days after the treatment. The results are shown in Table 9.

Example 11 (Confirmation of influence on fish growth) At a certain yellowtail farm in Mie Prefecture, a medicinal bath test with hydrogen peroxide, sodium percarbonate, and fresh water was conducted by using a yellowtail parasitized yellowtail as a test fish. The efficacy was compared.

1000 hamas with an average weight of 550 g were used on September 1 and September 2
After treatment with each drug once, the growth and survival rate of yellowtail were examined. The results are shown in Table 10.

Thus, the method of the present invention using hydrogen peroxide not only does not inhibit the growth of fish at all, but rather has the effect of promoting the growth of fish, and is extremely convenient as a method of exterminating ectoparasitic flukes of seawater-cultured fish. I know there is.

(E) Effect of the Invention According to the present invention, B. se that hydrogen peroxide water is parasitic on cultured fish
It has been confirmed that it has an effective action for exterminating riolae, H.heterocerca, Caligus sp., etc., and by clarifying the technical conditions for its application, the following effects are obtained.

1. A large amount of cultured fish in cages can be stored in a reduced area, which enables efficient parasite control directly in seawater, and is small-scale and labor-intensive as in the freshwater bath and concentrated salt bath methods. As in practice and when using other peroxide compounds, the inconvenience of having to be used by dissolving in fresh water beforehand has been eliminated.

This is because hydrogen peroxide easily dissolves in seawater at any concentration,
This is the effect of being able to utilize the property of diluting.

2. It is possible to adopt a method of use that exerts an exterminating effect without affecting the ecology of cultured fish. The effects of osmotic pressure on fish in freshwater baths and concentrated saltwater baths and other peroxide compounds are used. As in the case, there is no fear of changing the properties of seawater or causing respiratory distress due to precipitates formed. This is an effect obtained based on the above-mentioned properties of hydrogen peroxide and by being able to control the concentration and the treatment time in a prudent manner. Also, there is no need to worry about the danger to fish as when using other chemicals (pesticides, etc.),
Can be processed safely.

3. Based on the above effects, an effective in-service method that fishers can implement was provided.

[Brief description of drawings]

FIGS. 1, 2 and 3 are perspective views of an apparatus showing an embodiment of the present invention. FIG. 1 shows the apparatus used in Examples 1 and 4, and FIG. 2 shows Example 2. The apparatus used and FIG. 3 show the apparatus used in Examples 3 and 5. 1 ... Fishing nets that form cages, 1 '... Bottom fishing nets, 2 ... Aquaculture cages, 3 ... Synthetic resin sheets, 4 ... Seawater pool.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunio Nishimura 2-10-15 Higashi-Awaji, Higashiyodogawa-ku, Osaka City, Osaka Prefecture Katayama Chemical Industry Co., Ltd.

Claims (2)

[Claims] 1. A seawater-cultured fish is housed in a swimming compartment in a cage that has been reduced in size by a partition that can block the passage of water, and hydrogen peroxide is added to the compartment to a concentration of 200 to 3000 pp.
A method for controlling ectoparasites of seawater-cultured fish, which comprises removing the partition walls after washing the fish body by appropriately selecting conditions within a treatment time of 1 to 20 minutes. 2. The ectoparasite control of seawater-cultured fish according to claim 1, wherein the reduced swimming compartment of the cultured fish is surrounded by a thin cloth capable of blocking water passage in the cage or forms a pool. Method.