Three-dimensional analysis of spawning behaviour of skipjack tuna Katsuwonus pelamis using stereo-image measurements.

Although many pelagic fish species exhibit group spawning, the mechanisms underlying this mating system remain unclear; more fundamentally, detailed descriptions of group-spawning behaviour itself are lacking. Here, the spawning behaviour of skipjack tuna, Katsuwonus pelamis, characterised by 'group spawning', was recorded in an aquarium tank using stereo-image measurements to estimate the size of reproductive participants and quantify swimming kinematics during reproduction. Events were observed in which an individual (referred to as the 'leader') was chased by multiple conspecifics (referred to as the 'follower') and subsequently engaged in spawning. However, leaders (presumed female) were rarely chased by multiple followers (presumed male), a scenario more conducive to visible sperm release. In contrast, single followers chased leaders of varying body sizes, including smaller and likely immature individuals. Furthermore, leaders chased by multiple followers darted and turned at higher speeds. Reproduction may begin with one male pursuing one female; when the male targets a female with a high likelihood of spawning, the female accelerates, potentially attracting additional males to join the chase. By linking locomotor performance to reproductive outcomes, this study advances understanding of group-spawning mechanisms in pelagic fish species and contributes to broader theories of sexual selection and mating system evolution.