  Animals form groups for many reasons but there are costs and benefit
associated with group formation. One of the benefits is collective memory. In
groups on the move, social interactions play a crucial role in the cohesion and
the ability to make consensus decisions. When migrating from spawning to
feeding areas fish schools need to retain a collective memory of the
destination site over thousand of kilometers and changes in group formation or
individual preference can produce sudden changes in migration pathways. We
propose a modelling framework, based on stochastic adaptive networks, that can
reproduce this collective behaviour. We assume that three factors control group
formation and school migration behaviour: the intensity of social interaction,
the relative number of informed individuals and the preference that each
individual has for the particular migration area. We treat these factors
independently and relate the individuals' preferences to the experience and
memory for certain migration sites. We demonstrate that removal of knowledgable
individuals or alteration of individual preference can produce rapid changes in
group formation and collective behavior. For example, intensive fishing
targeting the migratory species and also their preferred prey can reduce both
terms to a point at which migration to the destination sites is suddenly
stopped. The conceptual approaches represented by our modelling framework may
therefore be able to explain large-scale changes in fish migration and spatial
distribution.
