Tuesday, January 10, 2012

Alien Crayfish a threat to Africa's unique endemic aqauatic animals species

By Ochieng’ Ogodo
A ‘weird’ alien fish that eats fish eggs and fingerlings is beginning to cause worry across Africa. It is feared that the Red Claw Crayfish, also known as the  Louisiana Crayfish with  much less edible meat-native to the United States of America and is one of the species from the northern hemisphere-is becoming a threat to unique endemic species of aquatic animals in  lakes and other water bodies.
It is not known exactly when the first Louisiana Crayfish were imported to Africa but it is thought in the 1970s both into Kenya and South Africa.
Geoffrey Howard, Global Coordinator for Invasive Species, Invasive Species Initiative, IUCN Species Programme, said the Red Claw Crayfish was imported to South Africa as an aquaculture species for specialty food supply. It was brought to Lake Naivasha in Kenya to breed up and send the crayfish to Scandinavia where native freshwater crayfish had been wiped out by a disease brought to Europe by the same Louisiana Crayfish much earlier.
In Kenya, they were also brought to the dams around Nairobi, Kiambu and Limuru to rid them of the freshwater snails that are the vectors for bilharzia.
But the Red Claw Crayfish, which is bigger and more aggressive and so more dangerous, can kill and eat the native freshwater crabs, other crustaceans and many forms of aquatic life according to Howard.
It can reduce fishable fish population
“By eating fish eggs and fingerlings, said Howard, “they can reduce the populations of fishable fish and so affect fisheries.  By removing animals and plants from wetlands they can upset the balance of ecosystems and reduce valuable ecosystem functions.”
But of greatest importance to conservationists and those interested in biodiversity is their threat to unique endemic species of aquatic animals in three eastern Africa Great lakes of Malawi, Tanganyika and Victoria where there are hundreds, probably thousands, of species found nowhere else. 
 If the crayfish gets into the lakes and become invasive, they could remove many of the unique species forever.
They are not native to Africa and have no natural enemies or competitors to keep their populations in check.  They are omnivorous and can eat small fish, fish eggs, other crustacea, molluscs and water plants.  They also burrow into the edges of dams, rivers and lakes to make nests and can destroy the dam infrastructure or the banks of rivers and lakes. The burrowing behaviour can cause water canals to leak and earth dams to collapse.
Unlike most aquatic animals, said Howard, the freshwater crayfish can stay out of water for many hours at a time, especially at night and on wet days.  Thus they are not restricted to the catchments of any particular water bodies and can walk across dry land for many kilometers. 
“Walking” fish
“They can also walk and swim upstream in rivers that feed lakes and can even more easily move downstream in rivers and streams.  In addition, they have been moved by people using them as fishing bait and possibly for food and those collecting specimens for aquariums,” he said. 
In these ways the crayfish have become widespread although it is not possible to be precise how far because there has never been any funding in Africa to detect and map their spread. 
In Kenya, for instance, they are now quite widely distributed in the Lake Victoria Basin, in the Naivasha Basin and its feeder rivers; in the Ewaso Ngiro (north) Basin and in many water bodies and wetlands around the wetter areas of the country. 
They are also in Rwanda, Uganda, along the Nile and especially common in Egypt; and in the Zambezi basin.  Seychelles and Mauritius are no exception and are also wild in some parts of South Africa.
Said Howard, “No statistics because they are rarely seen or recognized as a threat.  But they have certainly affected the fishery in Naivasha.”
Whereas it is possible to control (eradicate) crayfish from very small water bodies by trapping and possibly poisoning, research is developing hormonal and pheromonal methods but these are not yet available to us.  Mechanical barriers can be used to stop their spread – but only if we know their precise distribution. That is the problem at present.
Arne Witt, invasive-species coordinator for CABI said, with no host specific natural enemies (predators, parasites and/or diseases) it to proliferate - as a result it can build up populations rapidly to the detriment of indigenous plant and animal species.
“ It has the ability to feed on green plants, animals including aquatic invertebrates, especially insect larvae, amphibians, plant detritus, plankton, periphyton and benthos – it has been held responsible for the disappearance of many aquatic plant species from specific systems especially floating-leaves and submerged native species,” he said
Louisiana Crayfish has the ability to change or switch diets based on the availability of food items and as such has an impact on the ecological functioning of any water body. It can also change water quality affecting habitats for native aquatic fauna and flora.
Whereas some people have benefited from catching and eating or selling the crayfish, it is a bit of a boom and bust venture as crayfish can build up numbers quickly and then the populations can collapse once they have eaten themselves out of house and home and no industry can sustain itself on an irregular supply of a natural resource according to Arne.
Need for control
It obviously reduces the amount of prey items available for other organisms such as fish and birds. “It destroys the whole ecological balance in systems – all life on this planet is dependent on diversity – if we were to become dependent on one resource we could put ourselves at immense risk.” 
Arne said there is not much that can be done in water bodies where it currently exists but in long term a host specific disease may be found to reduce abundance. In the short term we may consider enhancing or facilitating the capture of this crayfish on a regular basis in order to reduce numbers. Removing water hyacinth and other aquatic weeds from water bodies such as Lake Naivasha may also expose them to higher levels of predation by birds. We also need to ensure that people stop moving them around – this should be made illegal and punishable by law.
It poses a significant threat to the ecological integrity of our water bodies.

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