Measurement of the size, shape and structure of Atlantic bluefin tuna schools in the open ocean

Abstract

How do patterns arise spontaneously in the collective motion of fishes? What specific advantages are conferred when individuals organize into distinct school formation structures? Automated methods are increasingly important for the rapid retrieval, conversion and integrated statistical analysis of large sets of data. Here, we apply a semi-automated image analysis to measure the size, shape and structure of schools of Atlantic bluefin tuna ( Thunnus thynnus L.) in the open ocean. We approximate their school shape as ellipsoids with boundary edge refinement. The distance between nearest neighbors follows a bi-modal distribution and varies between 0.3 and 0.9 BL, conferring maximum to 35% energetic savings, respectively, based on hydrodynamical considerations. Smallest schools comprise individuals in a linear to diagonal arrangement in the horizontal plane, with largest schools containing individuals arranged according to a diamond-lattice structure. Intermediate-sized schools show significant variation in positional, relative neighbor bearing and directional orientation in the horizontal and vertical dimension. Individuals maintain a fixed number of neighbors at preferred bearing angles, improving their ability to search and detect prey by schooling. In addition, individuals gain energetic advantages necessary to migrate over large distances, while maintaining high sustained swimming speeds. Estimated school packing density lies between theoretical estimates for aggregative and optimum packing of spheres. The trade-off between the hydrodynamical and visual advantage, while affecting school internal structure, also explains the shape of observed formations as a collective trade-off between school compactness and elongation. Future simultaneous tracking of schools and individuals would advance our understanding of survival advantages conferred to individuals through adjustments in school compactness and elongation as schools migrate and search for patchy-distributed prey.