断続光に対するマアジの行動反応に関する研究

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With regard to the behavioral response of fish to lights, studies have been conducted, mainly, onphototaxis-aided alluring, and the results are being applied widely to attracting fish. A constant light was used in most of these studies, however, the influence of the inconstant light on a fish school's behavior has hardly been examined, in spite of great concern about the effect of such stimulation. The aim of this study was to clarify, from an ethological point of view, that an inconstant lightingcondition, in which light goes on and off with a short cyclical time as an intermittent or flickering light,would give a different stimulation from that of a constant light to fish. The observations of the fish school's behavior in response to intermittent lights which have different blinking frequencies and contrast ratios between bright and dark light intensities and different wavelengths of lights were conducted.Then the difference between the fish school's behavior in response to the intermittent light under a stateof dark adaptation and under light adaptation was examined by changing the background irradiances underthese projected lights. In order to apply the obtained results to controlling the swimming behavior of afish school, a prototype multiple beam projector was constructed and used to examine the barrier effectof a leader net made of projected beams, upon the behavior of fish. I Response of a School of Japanese Horse Mackerel to Intermittent Light1) MethodAn outdoor concrete round tank 7 m in diameter and 1.5 m in depth of Banda Marine Laboratory ofTokyo University of Fisheries was used for the experiments. The tank was facilitated with a set ofpartitions extending outward from its center leaving a circular free passage 1.3 m wide along the wall ofthe tank. The inside surface of the wall was coated with black paint. A light projector equipped with alight source, rotating shutter device and color filters was installed at a position 50 cm above the waterlevel near the center of the tank. The free passage was illuminated diagonally from the light projector.The tank was covered with a lightproof roof and equipped with 8 miniature bulbs to regulate thebackground irradiances homogeneously. One-year-old Japanese horse mackerels, Trachurus japonicus, which had a striking phototaxis, wereused for the experiments. Specimens were between l7 to 20 cm in total length. Vivid and healthyhorse mackerels were selected at the site of a stationary trap net, transported to the laboratory and usedfor the experiments within 24 - 36 hours, in order to use them as much as possible under their naturalconditions. Care was taken based on confirmation by a preliminary experiment that, when the fish werereleased into the circular free passage, they had a good command of cruising for about two days afterbeing caught, after which their behavior as a school fell into disorder. In the experiments, the projector was operated to give a constant light for 20 minutes and anintermittent light for 20 minutes, alternatively. Under such lighting conditions, the movements of theschool were traced. Changes in schooling behavior were observed and characterized by the school'sappearance times in the irradiated area. The irradiated area was located in the range of the fish's visualfield in which a direct beam flux projected from the light source could be seen (the circular free passagewas divided into 16 equal sections). The difference ratio (R) in appearance times was calculated fromthe data using the following equation: R=(1- D/S)X100 (%) where "S" stands for the number ofappearance times in the irradiated area under a constant light and "D" for those under an intermittent light. The experiments were conducted only at night,in order to avoid artificial noises and lights around the tank. The same kind of experiments were repeatedly carried on, through different blinking frequencies and contrast ratios between bright and dark light intensities of intermittent light,and changing of wavelengths of projected lights and background irradiances. The experiments were carried on for a period from August to November each year,between 1983 and 1987.The total numbers of experiments and fish used for the study were 299 and 3,900, respectively.The water temperature in the tank was maintained in a range of 20 - 25℃.2) Typical mode of response There was a wide difference between the number of times the school appeared in the irradiated areaunder an intermittent light and under a constant light. That is, in the case of the constant light, the fishschool approached the irradiated area, but the school avoide the intermittent light (the school did notapproach the area at all when the intermittent light was projected). These results show that anintermittent light induces a high degree of aversion by the fish to lieht.3) Response to different blinking frequencies of intermittent light The number of times the school appeared in the irradiated area under intermittent lights varied withblinking frequencies. A minimum number of appearance times was seen when blinking frequencies werein the range of 0.62 - 1.36c/s, with the difference ratio being 92 - 98%. When blinking frequencieswere more or less than the aforementioned range, the number of times they appeared increased further. Thus, it was concluded that the school of fish showed the highest degree of aversion to theintermittent light when the blinking frequency range was within the range of 0.62 - 1.36c/s. Thesefrequencies correspond to l/37 - l/l7 of the critical fusion frequency of the horse mackerel's eye,obtained by means of electroretinography.4) Response to different contrast ratios of intermittent light The number of times the school appeared in the irradiated area under intermittent ligltts increased incomparison with a decreasing contrast ratio. In the case of the intermittent light having a contrast ratioof 60:1or more when the background irradiance was kept at less than 0.01lx, the difference ratiosshowed more than 75%; and in case of 2:1, it indicated 2%. Consequently,it was found that the scoolof fish showed a considerably high degree of aversion to the intermittent light when the contrast ratio was60:1 or more, but the ratio at which the school did not show any aversion to light at all was 2:1. Thevalue of the differential threshold of brightness and darkness of fish's eye has been considered to be in therange of 0.02-0.2. The fish's aversion to the intermittent light was seen when the contrast ratio was atleast 300 times as much as the threshold value, but disappeared when it decreased to 10 times that value.5) Response to different wavelengths of lights The number of times the school appeared in the irradiated area under intermittent lights varied withthe wavetengths of the projected lights. The difference ratio was obtained by projecting blue and redlights through certain color filters in addition to the white light (the entire spectrum). 'Ihe differenceratio of the blue light was almost equal to that of the white light, and about twice as much as that for thered light, The underwater energy of the blue light was only 1/6 of that of the white light, and was about2/5 of that of the red liglt. The results showed a good correlatian with the quantity of stimulation given to the eye, when thequantity was considered as the integrated value of energy which reached the eye multiplied by visualsensitivity.6) Response to different background irradiances When the fish's dwelling environment became brighter, the state of the retina in the fish's eyechanged from dark to light adaptation, and the number of appearance times into the irradiated area underintermittent lights began to increase. The difference ratios were over 94% with background irradiances less than 0.01lx, 56 - 28% with0.01 - 0.02 lx, and 20% wrth 0.2 lx. The most prominent degree of aversion to the light was seen whenbackground irradiance was less than 0.01 lx, and it decreased with increasing background irradiance but acertain degree of aversion still remained up to an irradiance of 0.02lx and then, it disappeared whenirradiance had reached 0.2lx. A physiological study discloses that the retina of horse mackerelcommences its retinomotor response even if environmental irradiance is less than 0.01lx, and the stateof the retina changes into light adaptation or fairly good approximation of light adaptation, underirradiances of 0.01 to 0.02 lx. Such a decrease of fish's aversion to light under irradiances of 0.01 to0.02 lx was conceivable to be caused by the fish's eye changing from dark to light adaptation. The relation between fish's aversion to the intermittent light and the contrast ratio of theintermittent light under a state of dark adaptation and under light adaptation was compared. The limitsof the contrast ratio, within which the school showed a high degree of aversion to light, were more than60:1 under the state of dark adaptation, and 5,000:1under that of light adaptation. Furthermore, thelimits of the contrast in which the school did not show any aversion to light at all were in the ratio 2: 1under the state of dark adaptation, and 500:1 under that of light adaptation. It was concluded, that thelimits, within which the school showed aversion to the intermittent light under the circumstances of darkadaptation were different from such limits under light adaptation by <special>102</special>. A physiological study disclosesthat when each eye of fish undergoes light adaptation, sensitivity to light by each retina is decreased by<special>l0-2</special> to <special>10-5</special> times. These physiological outcomes reasonably supported the results, which wereinduced from the present behavioral studies. The outcomes introduced above, showed that an inconstant lighting condition in which a light goes onand off with a short cyclic time gave a considerable high degree of fish's aversion to light (avoidancebehavior), quite contrary to an attracting influence (gathering behavior). Moreover, it was found thatdegrees of fish's aversion to light vary not only with blinking frequencies and contrast ratios ofintermittent light and wavelengths of light, but also with the state of dark/light adaptation of fish's eye. In addition, the limit within which the school showed aversion to the intermittent light were differentfrom the limits obtained from the physiological studies, such as critical fusion frequency and thedifferental threshold of brightness and darkness. Acclimation to light is another subject to be checked, when behavior of fish school in response tolights is obseved. The experiment was undertaken to clarify the acclimational influence on the fish'saversion to intermittent light. There was almost no change in fish's appearance times, even aftercontinuous projection of intermittent light for 4 hours. That is, an acclimation phenomenon was notdetected. On the contrary, it is known that the alluring effect of a fish attracting lamp on a school of horsemackerel, mackerel, and sardine starts to decrease 2 - 3 hours after the operation. It was concludedthat the occurrence of the acclimation phenomenon in the fish's aversion to intermittent light was far lessthan for the case of the alluring effect by continuous lights.　Behavioral activation and inactivation can also be regarded as the general efｆects of light on thebehavior of a fish school. Moving speeds of the fish school exposed to constant and intermittent lightswere compared. The average moving speed under the constant light was about 20 cm/s, ｗhile thespeed under the intermittent light was 14 cm/s. This means that the fish's moving speeds under theintermittent light was reduced by 30% of its speed under the constant light. It seems that the slowing down of the fish's moving speeds can be considered as an indication offish's aversion to the intermittent light. II Control of a fish school's behavior with a leader net made of a projected beam In order to apply the above-mentioned results to the control of a fish school's behavior, a multiplebeam projector was constructed.1) Multiple beam projeetor The multiple beam projector was designed to irradiate the 8 beams having an angle of 2 degrees tothe optical axis, a diameter of 26 mm, and a maximum projecting irradiance of 2,000 lx. Both theblinking frequency and the contrast ratio could be changed. A light from a halogen lamp was convertedinto intermittent lights by a rotating shutter disk (optical chopper). The wavelength of light longer than750 nm was cut out with infared absorbing filter. The light was distributed to 8 optical fibers, andprojected as 8 beams from the terminal lenses of each fiber. For the experiments, 8 beams having a blinking frequency of 1.36 c/s were projected orthogonallyand vertically to the circular free passage with 12 cm intervals, that is, by creating a leader net of light. Under such lightings, the movements of the school were traced. The same kind of experiment was repeatedly carried on, by irradiances of the beams being changedinto 8 grades from 1,235 to 0.025lx and background irradiances of 0.1lx and 0.003 lx.2) Response to the leader net made of light The leader net of light having irradiances of the beam over 0.25lx for dark adaptation (backgroundirradiance, 0.003 lx), and over 506lx for light adaptation (background irradiance, 0.1lx) were confirmedto exert a prominent barrier effect on the swimming behavior of the school of fish. In other words, theobstruction effect of the net against the passage of a fish school occurred under a dark adaptation with thecontrast ratio of the beam being more than 83: 1 and under a light adaptation with the ratio being over5,060:1. In order to apply such an effect to practical fisheries, an estimation was made on the effective rangeof the leader net of light which could obstruct the passage of a fish school, using the coastal waters ofSagami Bay as an example. Results of the estimation indicated that a fish could find the leader net oflight at a horizontal distance of 30 m, with the effective range of about 16 m, Factors were set asfollows: underwater irradiance, less than 0.01|x; differential threshold of the fish's eye, 0.05; and beamattenuation coefficient, <special>0.5m-1</special>. Vertical downward projection of beams with 2,000 lx irradiance wasused as the condition for the construction of the leader net. Consequently, satisfactory passageobstruction effect by the ieader net of light can be expected in practicai fisheries. Since it has beenreported that underwater irradiance in the Sagami Bay was reduced by <special>10-3</special> even at a depth of 20 mfrom the surface, applcatbn of natural lghts to the leader net of light could be considered a support, byintroducing the surface lights into the sea through optical fibers in the daytime. In addition, thebehavior-controlling effect could be improved further by changing the direction and phase of the beam. This study made it clear through a series of experiments on Japanese horse mackerel, Trachurusjaponicus, that an inconstant lighting conditoon can exert a remarkable influence on fish schoolingbehavior. Further development of the achievement will result in its broad applications to fisheries andculture fisheries in the future, such as in the alluring and controlling of the swimming behavior of a fishschool.

断続光に対するマアジの行動反応に関する研究

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