Study on the relationship of fish's vision angle with nearest neighbor distance under laboratory conditions

Data showed a rule that the nearest neighbor distance(NND) decreases as the vision angle of own fish increases in Pseudorasbora parva school.To explore whether other fish schools follow the same rule,30 individuals of Hemigrammus rhodostomus have been selected as a comparative object,which was smaller in body length and their average body length was(20.3±0.2)mm.The size of the aquarium length,width and height were 78.0 cm,52.0 cm and 65.0 cm respectively.The water depth was 5 cm.A video camera was set up to record the position of fish from overhead of aquarium in lab.Two-dimensional location data of each individual can be selected from the records in time sequence.Two parameters,NND and vision angle,were selected for characterizing the structure and to explore the relationship in different fish schools.Results show that:1) the vision angles of P.parva and H.rhodostomus all remained within the 80°,that is,the vision angle of own fish observing the neighbors is not greater than 80°.2) the vision angle range that neighbor fish occupies the own fish view doesn't increase as the body length increases in different species fish schools.3) If NND remains stationary,the maximum vision angle(neighbor fish occupies the own fish view) and NND have the relationship as yx=A,in which y is for vision angle(unit:degree), x is for NND(unit:BL),A is a constant and related with body length of individuals.The rule is applicable to P.parva and H.rhodostomus schools.4) When the NND is minimum between the own fish and neighbor fish,schooling is accomplished and maintained by vision angle.In other words,the vision angle is the main parameter to control the distance between each other and plays a dominant role.Lateral line and other sense organs may have their functions,but they might be less important.