For the first time, tuna spawning in a transport cage being towed from the western Mediterranean spawning ground to a fattening facility off the coast of northeastern Spain was examined during the 2008 fishing season. Daylight and night surface plankton samples were collected using bongo nets located in front of and behind the transport cage. The results for the different time intervals revealed clear and massive nocturnal spawning from 03:00 to 05:00, when the rear bongo was completely jammed with eggs (up to 250 000–300 000 eggs per 1000 m3). Egg size and morphology were consistent with the features of Thunnus thynnus eggs, and identification was confirmed by genetic analysis. Microscopic examination showed the eggs to be in the very early developmental stages. Spawning took place every night over the entire journey. The study showed that neither captivity nor handling/environmental changes along the route inhibited T. thynnus spawning to a very precise biological clock. The study also revealed the diel temporal concurrence of T. thynnus spawning and jellyfish larvae at the sea surface.