Effect of changing visible light regimes on aggregation of Antarctic krill (Euphausia superba)

Antarctic krill (Euphausia superba) is the keystone species in the Antarctic marine ecosystem, and is also the marine protein source that left for human beings. E. superba is creatures that inherently form schools and display intricate social interactions. It is also known to engage in both vertical and horizontal movements in its natural habitat. The social dynamics within these schools are crucial to understanding various biological aspects of E. superba. Consequently, researches into its behavior are essential for comprehensive grasp of krill biology. The effect of visible light color on the aggregation behavior of E. superba was investigated through in-situ aquaria observation on the sea. Understanding the characteristics of the distance between individuals and their swimming angles within schools will help understand the size of schools in the field and improve the accuracy of the target strength (TS) model in acoustic estimate of this species. The krill school images were synchronously acquired by an optical stereo camera in this study. The high-precision automatic measurement of the geometric information of the key feature points of the krill individuals was completed with the help of photogrammetry technology, then obtained the individual distances and swimming angle in the schools under different visible light colors from blue (short-wave) to red (long-wave). Research had found that blue light had a better positive phototaxis to krill compared with long-wavelength red light. The absolute individual spacing of krill schools was (14.51±9.71) cm, and the relative individual spacing was (2.84±1.51) BL. The lighting conditions affected the distance between individuals in the krill schools, and there were significant differences in distance and swimming angle between individuals in the krill schools between short-wavelength blue light and long-wavelength red light. The swimming angle of E. superba under different light colors was (46.08±35.50) ° and concentrated around 60°. There was no significant difference in swimming angle between different light colors. This study helps to understand the characteristics of krill schools in autumn and winter seasons, and provides scientific references for the input parameters of the TS model for krill resource evaluation, in order to improve the evaluation accuracy of the model.