Impact of introduced carp (Cyprinus carpio)in subtropical shallow ponds in Central Mexico

In Mexico, as in many other subtropical and tropical countries, there has been a recent trend towards stocking non-native carp (Cyprinus carpio) in lakes and ponds as a source of food in rural areas. However, the results of a study in a series of small(1–8 ha.), shallow (<2 m), semi-natural ponds in Acambay, a high altitude valley in the basin of the Lerma river in the volcanic belt in central west Mexico, illustrate that the stocking of carp over a threshold value may have a detrimental ecological impact at several trophic levels. Ponds with carp tended to be turbid with high levels of suspended solids, and with few rooted macrophytes and epibenthic invertebrates. In contrast, ponds without carp had clear water and abundant rooted macrophytes and associated invertebrates, particularly gastropod molluscs. The direct uprooting of macrophytes by benthic foraging carp appeared to be the most important mechanism in switching the ponds from a clear macrophyte-dominated to a turbid state. The subtropical study ponds thus appear to confirm the alternative stable-state hypothesis developed intemperate lakes, although the importance of benthic rather than pelagic interactions was emphasised. The implications of stocking carp for native fauna of high intrinsic conservation value and as a food supply for local people are outlined. This revised version was published online in July 2006 with corrections to the Cover Date.