The impact of a strong natural hypoxic event on the toadfish Aphos porosus in Coliumo Bay, south-central Chile

The occurrence of natural hypoxic events and coastal marine areas affected by anthropogenic hypoxia has increased over the past decades. In Coliumo Bay, a small shallow bay of the eastern South Pacific, an intense event of natural hypoxia took place in January 2008, causing the mass mortality of fish and other components of the benthic and pelagic communities. In the short term (3 months) recovery was observed in species richness as well as in total fish density. Nevertheless, in the medium term (2 years), this event produced negative effects on population size. In particular, the toadfish Aphos porosus, a dominant fish species in Coliumo Bay, showed a progressive decrease in population size since the occurrence of the hypoxic event, associated with a decrease in recruitment. We hypothesized that this reduction in population size lowered the probability of encounter of reproductive adults, decreasing the number of eggs, larvae and ultimately recruits, thus driving the population towards a low density threshold, and eventually, favoring an Allee effect condition. These results provide evidence on the causal relationship between coastal natural hypoxic events, mass mortality, life history and population dynamics. The slow recovery of the A. porosus population size suggests a process in which the population remains vulnerable to new perturbations. The observed population response is an example of the trajectory that some species may follow when faced with increasing occurrences of natural hypoxic events as predicted, for instance, by global warming scenarios.