A10 Interference in Shoals

Topic

Interference in fish schools describes the disruption of the physical mechanisms that hold the school together as a functional collective entity: the cohesion, coordination, and synchronization among the constituent individuals. This disruption of school cohesion can originate outside the group—due to violent hydrodynamic disturbances, high-intensity acoustic noise, or anomalous electromagnetic fields—or internally, when a sick or injured individual swims at an anomalous speed or along an unusual trajectory, thereby breaking the flow signals that the rest of the school uses to coordinate its own movement.

Under normal conditions, the school cohesion that interference disrupts relies on three types of signals exchanged between individuals and their neighbors: visual signals, which convey the position and relative speed of neighbors; hydrodynamic signals, detected by each individual's lateral line from flow disturbances generated by nearby fish; and acoustic signals, produced by the individuals' own swimming movements. A violent hydrodynamic disturbance—such as the pressure wave generated by a passing vessel—can temporarily disrupt the velocity field within the school, effectively masking the flow signals each individual's lateral line needs to orient itself relative to its neighbors, resulting in desynchronized alarm responses among the fish in the school.

The degree of school alignment—how well-coordinated the group's movement in a single direction is—can be described using an order parameter ranging from a maximum value, where all individuals swim in perfect alignment, to a value of zero, where the group's movement becomes completely disordered. When additional angular noise affects all individuals in the school simultaneously—such as during a sudden acoustic shock—the order parameter can collapse from its maximum value to a state of total disorder. This collapse occurs over a characteristic time inversely proportional to the intensity of the external noise: the more intense the acoustic shock, the faster the school's order collapses.

Once the external noise ceases, the school does not remain disorganized; it reorganizes following the same dynamics by which a disordered group spontaneously regains its ordered phase. This process continues until the school's order parameter returns to the same maximum alignment value it held prior to the disturbance, thereby completing the cycle of disorganization and reorganization. The time required for this reorganization increases with the number of individuals in the school and decreases with the strength of the coupling between them—that is, the extent to which each individual tends to mimic the swimming direction of its neighbors.

This relationship between school size and reorganization time results in contrasting behaviors depending on the type of disturbance: larger schools are more resilient to localized disturbances, thanks to the greater collective inertia provided by a larger number of individuals; however, these same large schools take longer to reorganize once a global disturbance—such as an acoustic shock affecting the entire group—has caused their cohesion to collapse. Thus, the very number of individuals that initially provides the school with its cohesion, coordination, and synchronization—as described at the beginning of this model—also determines how long the group takes to regain these qualities after experiencing an interference.

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