In population dynamics, depensation is the effect on a population (such as a fish stock[1]) whereby, due to certain causes, a decrease in the breeding population (mature individuals) leads to reduced production and survival of eggs or offspring.[2] The causes may include predation levels rising per offspring (given the same level of overall predator pressure) and the Allee effect, particularly the reduced likelihood of finding a mate.
When the level of depensation is high enough that the population is no longer able to sustain itself, it is said to be a critical depensation. This occurs when the population size has a tendency to decline when the population drops below a certain level (known as the "Critical depensation level").[2] Ultimately this may lead to the population or fishery's collapse (resource depletion), or even local extinction.[3]
The phenomenon of critical depensation may be modelled or defined by a negative second order derivative of population growth rate with respect of population biomass, which describes a situation where a decline in population biomass is not compensated by a corresponding increase in marginal growth per unit of biomass.