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 difficult 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 characteristics during the pre-spawning period over 6 consecutive years by means of hydro-acoustic surveying. 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 herring. 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 reproductive success.