Tilapia as an invasive species

“Invasive species are organisms (usually transported by humans) which successfully establish themselves in, and then overcome, otherwise intact, pre-existing native ecosystems. Biologists are still trying to characterise this capability to invade in the hope that incipient invasions can be predicted and stopped. Factors may include: an organism has been relieved of the pressures of predators or parasites of its native country; being biologically “hardy”, for example, has short generations and a generalist diet; arriving in an ecosystem already disturbed by humans or some other factor. But whatever the causes, the consequences of such invasions – including alteration of habitat and disruption of natural ecosystem processes – are often catastrophic for native species.”

IUCN/SSC Invasive Species Specialist Group (http://www.issg.org)

Since tilapia is such a popular and easily cultivated food fish it has been spread by man to many different corners of our planet. Sometimes the introductions have been deliberate with tilapia being intentionally set free outside its native African and Middle Eastern range to serve as a food fish or to combat aquatic weeds and diseases carrying mosquitoes. In other cases, the introductions have been involuntary – many feral populations of tilapia hail from specimens escaped from aquacultures. If you keep tilapia in your farm or as a pet, it is very important never to give it a chance to escape into the wild since invasive species can cause a lot of problems for native ecosystems; either on their own or by introducing dangerous pathogens to the environment. A pet fish that you can no longer care for and fails to find a new home for should be euthanized, not released into the wild.

When introduced to a region outside its native range, tilapia can disrupt the ecosystem in several ways. Tilapias are fond of digging and can turn clear waters turbid, thus limiting the amount of available light for animals and plants. Tilapia can also compete with other species for food, space and spawning sites, and since they are flexible omnivores they will eat a long row of different organisms.

The sturdiness and adaptability of tilapias does not only make them ideal for aquacultures; it also makes them very prone to cause problems in ecosystems to which they are introduced. They can usually find food in all sorts of environments and will happily eat anything from detritus (decomposing organic matter) to the fry and larva of native species. They also eat adult fish and invertebrates, plankton and aquatic plants. Although primarily considered freshwater fishes, quite a few tilapias can adapt to brackish conditions and some species will even survive in sea water. To make things even worse, tilapia tend to grow fast and reproduce rapidly, thus outcompeting many slower, native species unfamiliar with this ferocious competitor.

The main weakness which restricts the spread of tilapia into new environments is their need for fairly high temperatures year round. Tilapia is a tropical species and low temperatures will kill it or weaken its immune system, rendering it open for disease.

Examples of tilapia as an invasive species

– Unfortunately, Tilapia has been introduced to the remote Galapagos Islands located far off the coast of Ecuador. Due to their isolation from the mainland, the ecosystems on these islands are as unique as they are delicate with an amazingly high degree of endemic species. Any foreign species risk disrupting this sensitive balance and the sturdy tilapia has become a serious threat to local wildlife.

– In the United States you can find numerous tilapia populations in the wild, and a group of Oreochromis mossambicus tilapia has for instance managed to adapt to life in the Salton Sea, an inland saline lake in Southern California.

– In Singapore, introduced O. mossambicus tilapia have been causing problems since they were introduced by the Japanese during World War II, but this problem might be on its way of solving itself due to the introduction of yet another tilapia variant. Ever since its introduction, O. mossambicus tilapia has been a common sight in both freshwater and brackish environments in Singapore, and a thriving population even managed to establish itself in the sea off the northern coast. Since the late 1980s, O. mossambicus tilapia has however become an increasingly rare sight throughout Singapore and this decline has been attributed not to overfishing or pollution but to the introduction of other tilapias, most likely the hybrid O. mossambicus x O. niloticus and possibly also O. urolepis hornorum and O. aureus. When O. mossambicus breeds with certain tilapia species and hybrids, the resulting offspring exhibit a much skewed sex ratio. A very high degree of the specimens become male, which naturally hampers future reproduction due to a lack of available females. It is also common for hybrids to produce fewer fry per spawning than the true species and this also contributes to the decline of the tilapias in Singapore waters. Offspring containing genetic material from O. niloticus also risk losing the high tolerance towards saltwater exhibited by pure O. mossambicus specimens.