dc.description.abstract | 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. | en |