Allee effects in marine systems

The fitness or population growth rate of populations with Allee effects increases with in- creasing population size or density up to a certain threshold. Allee effects are possible in marine pop- ulations, as they are less open than has been assumed and may have a metapopulation structure. We modelled the population consequences of Allee effects and show that increases in mortality interact with critical Allee thresholds, such that an Allee effect with no population consequences at low mor- tality can drive a population to extinction when mortality is increased. In heavily fished species, popu- lations with strong Allee effects go extinct at lower levels of fishing mortality, or conversely as fishing mortality increases, weaker Allee effects can drive population to extinction. We found little empirical evidence in the literature for widespread Allee effects in marine populations, although we found some suggestive observations, particularly for broadcast spawners and in exploited populations This might be due to methodological problems or long time lags. Many marine species have components of their life history or ecology which could in theory generate Allee effects; however the population level con- sequences of these potential mechanisms remains virtually unexplored. We suggest that including Allee effects in models of vulnerable populations may be critical for the precautionary management of exploited and threatened marine species.