Vertical movement of spawning cultured chub mackerel (Scomber japonicus) in a net-cage

• Spawning behavior of net cage-cultured chub mackerel is described in detail
• Differences in the vertical distribution of spawning and non-spawning fish
• The differences could be used to indicate the timing of the spawning period

Vertical behavior of cultured chub mackerel, Scomber japonicus, in a net-cage was investigated using depth data logger and infrared video camera footage during the spawning and non-spawning seasons. Marked differences in the vertical distributions of spawning and non-spawning mackerel were observed. Non-spawning mackerel swam near the bottom of the net-cage during the daytime, coming up to shallow waters during night-time. However, spawning mackerel remained near the bottom of the net at all times except for brief, night-time visits to the surface for gamete release. Spawning events started with the school of fish whirling near the surface, before two or three fish, possibly the female followed by the males, darted upward and away from the school at high speed. Once away from the school, the fish swam in a tight circle two or three times before releasing their eggs and sperm. Estimation of the timing of spawning events based on back-calculations of egg development and then correlating these data with changes in vertical movements revealed that there were 26 events over a three-day period, and that most events occurred between sunset and midnight. Differences in the depth distribution between the spawning season and the non-spawning season could potentially be used as new indicator for the spawning period in chub mackerel reared under artificial conditions. These results could help us to better understand the spawning season in mackerel and allow us to increase the sustainability of the natural reproductive process in captivity.