Impact of the Allee effect and prey refugia on marine stock collapse and recovery via agent-based modeling

Abstract

The collapse of marine stocks, such as the Atlantic cod, often fails to reverse despite severe fishing restrictions, suggesting factors other than fishing mortality regulate recovery. This study compares two hypothesized drivers of this phenomenon: the Allee effect and the loss of prey refuge. Using a pattern-oriented Agent-Based Model (ABM), the mechanistic roles of these factors were evaluated against a control scenario by assessing their ability to reproduce four signature predator-prey patterns: mutually dependent population cycling, constant period length, chaotic amplitude, and cycle asymmetry. Results demonstrate that prey refuge functions as a stabilizer, dampening oscillation amplitudes and preventing extinction by securing a minimum prey population. Conversely, the Allee effect operates as a destabilizing force, introducing positive density dependence that significantly lengthens cycle periods and widens low-density troughs. This delay in recovery creates a critical window of vulnerability where predation pressure can drive populations to collapse. These findings provide a mechanistic explanation for the failure of depleted stocks to rebound, highlighting that effective management must simultaneously preserve critical habitats to stabilize dynamics and maintain density thresholds of both the target fish stock and their main food source to prevent depensation.