'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 expla ined by changes in the relative importance of reproduction, predation and f eeding during spawning. Horizontal area, vertical extension, school shape a nd relative density were quantified using sonar and echosounder, and relate d to gonad maturation stage and stomach fullness recorded from gillnet samp les. The majority of the herring completed spawning within 3 d. During this period, the herring were feeding and predatory fish were present in the ar ea. An extended cylindrical school shape prior to spawning indicated that i ndividual herring within the school had different depth preferences, with r ipe individuals descending towards the spawning substrate on the bottom whi le immature individuals preferred the pelagic environment for safety and fe eding. As the majority of the fish matured, the school segregated verticall y into a pelagic component that contracted to a tight ball and a demersal c omponent that spread out in a flat layer on the bottom. Post-spawners seeme d to return to the pelagic school. After spawning, the 2 components rejoine d each other and formed a loose flake (i.e. loose, uneven layer) at the sur face. Schooling fish have traditionally been considered to make 1 of 3 beha vioural decisions: to stay, join or leave a school. Waiting without losing contact with the rest of the school, 'await', is suggested as a fourth opti on.