Emerging school structures and collective dynamics in spawning herring: A simulation study

Schooling fish are known to display various collective behaviours depending on ecological context and life history situation. In Norwegian spring spawning herring (NSS-herring) (Clupea harengus) different trade-offs during the seasons of feeding, overwintering, migration and spawning are likely to influence school morphology and behaviour. In the field, hydro acoustics are able to record collective patterns, but hardly the mechanisms of how individual decisions and interactions lead to the observed formations. Individual based models (IBMs) on the other hand, are promising simulation tools for investigating how low-level individual behaviour influences large-scale behaviour. We have used this approach with a rule based school model in order to gain understanding of how certain school patterns can emerge during the spawning of NSS-herring. Response to predation and motivation towards spawning are added to the response to nearby fish. Simply by varying population size and synchronisation of spawning motivation we find different system responses in terms of school morphology and dynamics. With high motivational synchronisation, the system is mainly represented by one integrated school, whereas low degree of synchronisation presents a system with frequent split-offs of small schools. An intermediate degree of synchronisation leads to a more complex situation with schools or layers in a dynamic vertical contact and formation of vertical 'hourglasses' or cylindrical shaped schools. This suggests that the degree of motivational synchronisation between individuals in a school will determine whether or to what degree a school splits into different components or remains integrated. We also find that with increasing population size there are new system behaviours emerging, not present with lower population size. Larger populations lead to horizontal extension of the pre-spawning components resulting in a double layer system where vertical bridges connecting the two layers are established. The cylindrical bridges are truly emergent properties of the system, formed and maintained by ovulating and spent herring moving across these structures. Similar school formations with vertical connections have been observed acoustically in spawning herring schools.