A12 Shoal Desynchronization

Topic

School desynchronization caused by anthropogenic noise refers to the disruption of the acoustic and hydrodynamic communication system that maintains the school's cohesion and synchronization when exposed to human-generated noise sources, such as maritime traffic, military sonar, seismic exploration, port construction, and aquaculture. Anthropogenic noise affects this communication system in two distinct ways: it can mask the biological signals fish use to coordinate collective behavior—such as communicative vocalizations and sounds produced during swimming—or it can act directly as a physical disturbance to the flow and pressure field perceived by each fish's lateral line.

The masking of the school's biological signals is quantified using "masking excess," a metric that compares the background noise level (in decibels) to the fish's auditory detection threshold at the frequency of interest; when the background noise level exceeds this threshold, the school's acoustic communication is compromised. Furthermore, low-frequency sonar and seismic exploration generate extremely high-intensity pressure pulses—exceeding 200 decibels—that can cause direct mechanical damage to the fish's auditory system (known as auditory barotrauma). This damage is more severe in fish with swim bladders located near the ear, as the swim bladder amplifies the pulse pressure before it reaches the auditory organ.

Beyond auditory damage, the physical disturbance anthropogenic noise introduces to the school's flow and pressure field triggers an alarm response that propagates like a wave of information: individuals closest to the noise source react first, and the alarm is then transmitted outward to those further away. This wave of alarm propagates faster the shorter the reaction time of each individual; it also spreads more rapidly the more densely packed the school is, as neighboring individuals are closer together, allowing the alarm signal to pass from one to another more quickly.

When anthropogenic noise shifts from an isolated pulse to a continuous sound, the school’s desynchronization becomes persistent, and the school itself raises its alarm threshold—a habituation phenomenon where the group ceases to react with the same intensity to stimuli that would have previously triggered an alarm response. Consequently, this habituation diminishes the school's response to genuine threats, such as an approaching predator, precisely because the alarm system has become less sensitive following prolonged exposure to background noise.

Schools chronically exposed to anthropogenic noise ultimately exhibit reduced spatial cohesion—characterized by greater spacing between individuals, higher trajectory variability, and lower foraging efficiency—because the acoustic and hydrodynamic communication system that initially underpinned the school's cohesion and synchronization no longer functions with the same fidelity. These consequences are ultimately reflected in the individuals' body condition and in recruitment—that is, the number of individuals that survive and successfully join the adult population.

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