Silver carp (Hypophthalmichthys molitrix) swimming in a river in the USA, where it is an invasive species. Four species of carp, including this one, were introduced to the US in the 1970s to control parasites and plant growth in fish farms and canals. These carp escaped into the Mississippi river during flooding and rapidly established breeding populations. Silver carp have since spread to rivers in at least 21 states. They can outcompete local species, reproduce rapidly and harm native ecosystems.