The Research Of Phototaxis In Different Growth Stages Of Three Populations Of Tilapia(Oreochromis)

The light plays an important role in the fish life activities. The phototactic characteristic of fish refers to the characteristics of stimulated directional light movement, backlit movement is called negative phototactic characteristics.The light exciting reaction with fish brings about many kinds of hypothesis,including of curiousness, appropriate intensity of illumination, the bait cluster, reflex, forced movement, fascination and instinct, etc. In this study, the photogradient method was used to study the phototactic behaviors of three populations of tilapia which were Egypt O. niloticus,GIFT and O. aureus during five different growth stages:Ⅰ.30.0～45.0mm;Ⅱ.50.0～65.0mm; Ⅲ.70.0-85.0mm; Ⅳ.90.0～105.0mm; Ⅴ.110.0～125.0mm, researching the reaction of Egypt Nile tilapia which were exposed to different light colours such as red,yellow,blue and green light to explore the preferred light intensity and suitable photographic light color meanwhile.The results were as follows:1. The results declared that three populations of tilapia during different growth stages in different light intensities all behaved positive phototaxis:I group of fish which mostly in the area15cm from light source were quickly swimming to light source and showed strong phototactic characteristic; II group in light of the fish in activities were more active, but less frequent than I group; III group in light of the fish in activities were more stable, and the phototaxis standard was lower than I and II group;From Ⅳ group, the fish tended to stay in lower intensity of illumination area, and the phototaxis standard obviously decreased as well; The number which the distribution of V group fish in each light source area was similar, and the fish were insensitive to light stimulation, so the phototaxis standard decreased to minimum. The phototaxis in three populations of tilapia is closely related to age and size,and juvenile fish performance strong phototactic characteristic.Physiological conditions and feeding ways of tilapia change gradually along with the growth and development of fish, and phototaxis also can produce corresponding change. It is said that this change of phototaxis may be inferred in relation to visual structure and function of tilapia. 2. The results showed that the everage phototactic rate offour different group in different populations of tilapia increased with the boosted light intensity which the range was2501ux～1000ux, and the everage phototactic rate reached the maximum when the light intensity was1000lux, which the maximum of everage phototactic rate is I group such as73.3%、73.3%and76.7%. There was no significant difference in everage phototactic rate between2501ux and500lux(P>0.05), but it showed obvious difference between2501ux and10001ux(P<0.01).When the intensity was above1000lux, the everage phototactic rate decreased with the increase of light intensity. No significant difference in everage phototactic rate between1000lux and2000lux(P>0.05), however, there was significant difference between20001ux and40001ux(P<0.01).3. No significant difference in effects of phototactic reaction to different color lights during different growth stages of Egypt Nile tilapia(P>0.05), but it showed very significant difference during the same growth stages to different color lights (P<0.01).The differences of everage phototactic rate in reaction to red and green light were unconspicuous (P>0.05), but there were very significant differences in yellow and blue light (P<0.01). It emphasised the fact that the yellow and blue light had great influences on phototaxis behaviors of Egypt Nile tilapia, and the sensitivitie to red and green light was low. The differences which all sorts of color lights were absorbed in the water were great, therefore,the phototactic reaction of Egypt Nile tilapia in same light intensity to different color of light was unlikeness. There was obviously different in effects of phototactic reaction to different color lights during different development stages of Egypt Nile tilapia, as well as light intensity.4. During the experiment in each light color of Egypt Nile tilapia in same group,the average phototactic rate could be reached to a maximum value, and the corresponding light area was the suitable illumination area of Egypt Nile tilapia in this light color.The results revealed that the appropriate illumination intensity of I group was the highest level, and it showed obvious phototaxis in the strong light intensity. The phototactic reaction of Egypt Nile tilapia in different groups to yellow light was the most sensitive, and the second was blue light, and the minimum was green. Well, from I group to V group, the suitable illumination intensity of Egypt Nile tilapia decreased gradually in yellow light condition, but there was not so extensive of reduce. It showed a large decrease extent in blue, red and green light condition during different development stages of Egypt Nile tilapia. The most obvious was green, and it also showed that the Egypt Nile tilapia had a certain selectivity from different colors of light. From the study, it could be draw a conclusion that different developmental periods of Egypt Nile tilapia always tended to choose the bright or dark area, which were both ends of the aquarium,but selodm stayed at the middle area, so it is important that the intensity of illumination is relative value but not absolute value.The study was based on the perspective of behavioral science to study the phototaxis of three populations tilapia which were Egypt O. niloticus, GIFT and O. aureus duringfour different growth stages. It could have a certain practical significance for improving the feeding rate of tilapia, providing certain reference for light in the application of fresh water in fishing at the same time.