Tracking individual herring within a semi-enclosed coastal marine ecosystem: 3-Dimensional dynamics from pre- To post-spawning

Abstract

Pelagic fish typically swim in shoals, but a full understanding of behaviour, including individual spatiotemporal dynamics and the relationships between individuals and the social unit, can only be achieved through studies of individual fish. We studied horizontal and vertical movement of tagged individual Atlantic herring (Clupea harengus L.) inside a semi-enclosed ecosystem throughout the dynamic spawning season. Twenty-four fish were successfully tagged with acoustic transmitters and followed for up to 62 d (average 28 d) using a network of ultrasonic receivers. Herring samples from gillnets provided information about gonadal maturity. During pre-spawning, all individuals remained within the range of a single receiver, but with marked and predictable diel vertical migrations (DVMs). As maturation progressed, the number of herring on the spawning grounds gradually increased, while swimming depth and DVMs gradually decreased. However, the daily vertical distance moved by single individuals increased markedly as spawning approached, and the individual variability in both horizontal and vertical positioning also increased over time. During the period assumed to include spawning, individuals moved more frequently between the receivers, with dives towards the bottom presumed to represent spawning events. The results indicate a development from low variability in individual positioning and strong school coherence before spawning, to high variability during spawning that could reflect individual states of maturation. The study demonstrates that novel acoustic tagging technology opens up the possibility of investigating the dynamic trade-offs between collective behaviour and individualism in schooling pelagic fish.