Extinction Risk from Human Impacts on Small Populations of Marine Mammals

ABSTRACT Population models used to set limits for whaling, fisheries bycatch and other human-caused mortality (HCM) usually focus on relatively large populations and do not include Allee effects (declines in population growth rate at small population sizes). These models are not suitable for managing small and endangered populations of marine mammals. We use a stochastic age-structured population model to investigate the effect of HCM on extinction risk. Compared to environmental variability and catastrophes, Allee effects had a strong influence on risk. Depending on the scenario, HCM (1) delayed the rate of population recovery (with no increased risk), (2) increased extinction risk because populations lingered at low levels, (3) increased extinction risk because the population was pushed below an Allee threshold, or (4) increased extinction risk over 100 years because the rate of extinction for a doomed population was accelerated. Population dynamics in small populations are poorly known for most marine mammals. Therefore, we recommend that managers consider the range of potential population dynamics for the species under consideration and make precautionary decisions on allowable levels of HCM. For critically depleted populations (e.g., small populations, well below historic levels) even low levels of HCM have the potential to substantially increase extinction risk.