群体大小对青幼鱼群体特征的影响

The effect of group size on school structure in juvenile black carp

为考察群体大小对青幼鱼(Mylopharyngodon piceus)群体特征的影响,在(25±0.5)℃条件下对1、2、4和8尾青幼鱼组成的群体(N=12)进行视频拍摄和轨迹分析,得到游泳速度和空间分布特征数据,实验同时测定了不同群体大小对恐吓刺激的反应差异。结果显示:青幼鱼自发游泳速度多处于0—10 cm/s,不受群体大小的影响,但不同个体速度同步性随群体大小增加而显著下降;青幼鱼最近邻距离多数位于0—10 cm,且随着群体大小增加,最近邻距离和群体排列的极性均随群体数量上升而显著下降;青幼鱼群体中位置分布具有个体差异,且随着群体数量上升差异加剧;青幼鱼对恐吓刺激的反应率不受群体大小的影响,但群体内部不同个体对刺激的反应的一致性随群体大小上升有所下降。结果表明:青幼鱼具备集群性,在2—8个体范围内随群体大小上升凝聚力上升,协调性下降;群体特征的改变对恐吓刺激反应率没有显著影响,但协调性的下降导致反应一致性降低;青幼鱼群体中不同个体在集群活动中可能存在社会分工,且这种分工的作用在大群体大小上升表现更为明显。

Group living is a commonly observed phenomenon in fish species in nature, and about half of all fish species shoal for part or all their lives. Fish derive many benefits from collective living including defense against predators and enhanced foraging success. Thus, how the behavior of individuals and the interaction between them produce group-level behavior is fundamental to collective behavior research. In the present study, using fine-scale movement data of juvenile black carp （Mylopharyngodon piceus） （body mass：（2.43±0.04） g, （5.34±0.03） cm, N=180）, the effect of group sizes on motion and shoal structure were investigated at （25±0.5）℃. To achieve our goals, twelve repetitions of black carp juveniles with different group sizes （1, 2, 4, and 8 individuals per group） were videoed for 15 min （30 frame/s）, from which 27000 coordinates of each individual were acquired and the swimming speed during individual spontaneous movements and the shoaling distribution structure of fish groups with different sizes were analyzed. The response ratio and conformity to artificial fright stimulus of fish with different group sizes were also measured. Juvenile black carp mostly swam at a speed between 0 and 10 cm/s （84%-97%）. Although the proportion of fish that swam at high speed decreased with group sizes, the mean swimming speed （（4.90±0.79）, （3.80±0.57）, （4.15±0.84） and （2.93±0.82） cm/s in 1, 2, 4 and 8 individuals group, respectively） showed no significant difference among different group sizes. The synchronization of swimming speed among fish individuals within each group were （64.22±0.84）, （30.76±4.06）, （38.86±5.07）% in 2, 4 and 8 individuals group, respectively, which decreased with group size. The nearest neighbor distance （NND） mostly occurred between 0 and 10 cm （91%-98%）, which significantly decreased with group size. Juvenile black carp showed high levels of polarity of orientation within a group （84% to 95% individuals moving in the same direction）, which significantly decreased with group size. The leadership of individual fish as suggested by moving in the front of a shoal showed high inter-individual difference and the variation was aggravated with an increase of group size. The response ratio showed no difference among fish of different group sizes possibly due to the combined effect of group size on information communication efficiency across group numbers and on boldness and vigilance of individual fish. However, the conformity of response to the fright stimulus within a group decreased, possibly due to increased inter-individual differences in large-size groups. This suggested that juvenile black carp showed typical schooling behavior. Group cohesion as suggested by NND （small NND indicates high group cohesion） increased while coordination as suggest by synchronization of speed, polarity and conformity of response to the fright stimulus decreased with group size increasing from 1 to 8 individuals. The disparity of different individuals in school distribution suggested that a different social role existed among different group members. Further investigations should attempt to identify the effect of group size on movement dynamics and distribution structure comparatively across cyprinids, in terms of evolution and phylogenetics.