State-dependent spatial and intra-school dynamics in pre-spawning herring Clupea harengus in a semi-enclosed ecosystem

Distribution and aggregation patterns of pelagic fish in marine ecosystems are diffi- cult to predict and understand, particularly since focussed studies are typically limited to single field efforts. Our study targeted coastal pre-spawning herring Clupea harengus in a small semi- enclosed coastal marine ecosystem, investigating their spatial and intra-school dynamic charac- teristics during the pre-spawning period over 6 consecutive years by means of hydro-acoustic sur- veying. Prior to spawning, the herring were consistently located at a specific site characterised by a deep trench that led to the deepest basin in the system and provided a potential escape route from predators. As the herring approached spawning, they moved in the direction of the spawning grounds, mimicking the long-distance spawning migration of Norwegian spring-spawning her- ring. There were consistent trends across years in the school characteristics demonstrating state- dependent schooling dynamics. Schools were found closer to the bottom and more densely packed during late than early pre-spawning. Furthermore, a higher diversity in maturity stage be - tween individual herring, assumed to be associated with stronger motivational conflicts, resulted in increased heterogeneity in school packing density. Our repeated investigations from a small site pinpoint some key behavioural traits in herring, trading off survival, energy saving and repro- ductive success.