'Await' in the pelagic: dynamic trade-off between reproduction and survival within a herring school splitting vertically during spawning

Spatio-temporal aspects of spawning behaviour were investigated in a single school of Norwegian spring-spawning herring Clupea harengus, and are explained by changes in the relative importance of reproduction, predation and feeding during spawning. Horizontal area, vertical extension, school shape and relative density were quantified using sonar and echosounder, and related to gonad maturation stage and stomach fullness recorded from gillnet samples. The majority of the herring completed spawning within 3 d. During this period, the herring were feeding and predatory fish were present in the area. An extended cylindrical school shape prior to spawning indicated that individual herring within the school had different depth preferences, with ripe individuals descending towards the spawning substrate on the bottom while immature individuals preferred the pelagic environment for safety and feeding. As the majority of the fish matured, the school segregated vertically into a pelagic component that contracted to a tight ball and a demersal component that spread out in a flat layer on the bottom. Post-spawners seemed to return to the pelagic school. After spawning, the 2 components rejoined each other and formed a loose flake (i.e. loose, uneven layer) at the surface. Schooling fish have traditionally been considered to make 1 of 3 behavioural decisions: to stay, join or leave a school. Waiting without losing contact with the rest of the school, 'await', is suggested as a fourth option.