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Africa Invaded: Louisiana Crayfish

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The Louisiana crayfish Procambarus clarkii, also known as the red swamp crayfish, supports a lucrative aquaculture industry in its native range, and is a popular component of the region’s Cajun cuisine. Over the last 50 years it has been introduced to Africa, Europe and Asia, in most cases with negative consequences.


A food source, a biocontrol agent and even a popular family pet

The Louisiana crayfish is indigenous to the southern parts of the United States and northern Mexico. It has been introduced to other regions primarily to diversify local
fisheries or for aquaculture purposes; although in a few cases in Africa it was released as a biological control agent against the snail hosts of bilharzia (schistosomiasis). In the United States it was stocked outside its natural range as a food source for gamefish such as largemouth bass and bluegill, and spread by anglers using it as bait. In Japan it became a popular family pet, and was also traded by aquarium and garden pond hobbyists in parts of Europe.

Highly adaptable

Once introduced, the species quickly becomes established in the wild through escaped or deliberately released animals. Louisiana crayfish can survive in a variety of natural and manmade habitats, such as rivers, wetlands, dams and irrigation canals, where they burrow into soil banks along the shoreline. They are able to tolerate a wide range of salinities as well as oxygen-poor conditions, high pollution levels and fluctuating water levels, and adults can travel long distances over land to colonise new areas. More importantly, the Louisiana crayfish is a prolific breeder and a generalist feeder, able to exploit most available food sources.

Apart from plants, the Louisiana crayfish also eats insects, worms, snails, amphibians, crustaceans and small fish, as well as their eggs and fry. Its huge appetite has been blamed for the disappearance of some species of snails in African wetlands, and for the decline of certain amphibians in parts of the United States. In addition, the crayfish is aggressively territorial, so it frequently out-competes and excludes indigenous predators, further reducing local biodiversity.

A number of other impacts are associated with invasion by Louisiana crayfish. Their burrowing weakens dam walls, creates leaks in levees and aquaculture ponds, and increases erosion along watercourses. Although they are often farmed in combination with rice crops, they inevitably raise the cost of rice culture by burrowing into
dykes and eating the rice plants.

The Louisiana crayfish is a vector of the ‘crayfish plague’ Aphanomyces astaci, which caused a collapse of the crayfish industry in Europe after it was introduced with the American red signal crayfish in the 1860s. In some parts of the world it is also a vector for harmful human parasites, including the lung fluke Paragonimus westermani and the rat lungworm Angiostrongylus cantonensis, which are passed on to humans who eat undercooked crayfish.

Control

Once established in an area, Louisiana crayfish are extremely difficult to eradicate. Limited success has been achieved with traps baited with fresh fish or meat and left overnight. Research is now being conducted using sex hormones – or pheromones – as bait, in the hope that crayfish looking for a mate will be more readily lured into traps. For small ponds and dams, drainage and physical removal of crayfish has sometimes been effective, but the animals can escape capture by burrowing deeper into the mud or moving over land to nearby pools. 

Natural enemies have kept crayfish numbers in check in some areas. For example, where there are large heron colonies in wetlands in southern Europe, the birds exact a
heavy toll on the crayfish population. In the United States, the species is not normally a problem in sport-fishing dams stocked with trout, bass, catfish and bluegills, but introducing these predatory fish to control crayfish in other areas has generally not been successful and causes secondary impacts.

Chemical control is not recommended for crayfish. Toxic pesticides are likely to kill non-target species, threaten water quality and contaminate water supplies. Recently, research has been conducted on the potential of a bio-degradable surfactant – Genapol OXD-080, a fatty alcohol polyglycol ether – to control crayfish in rice paddies. However, the non-selective action of the product means that it is a threat to biodiversity. Trials showed that it risked contaminating irrigation canals, and killed mosquitofish at concentrations well below those needed to achieve control of crayfish.

From Kenya to Europe’s fine food outlets

In 1970, the Louisiana crayfish was introduced to Lake Naivasha – situated in the Rift Valley in Kenya – to diversify the lake’s fisheries. The crayfish thrived, and a decade later the population had grown to a density of four individuals per square metre of shoreline. A flourishing fishery exported millions of crayfish to France, Holland and Belgium, where they were served in top city restaurants. Unfortunately, the crayfish population was consuming vast quantities of aquatic vegetation, and in 1982 the lake’s indigenous floating and submerged plants completely disappeared. The following year the fishery collapsed, and in 1987 crayfish density was estimated at only 0.25 individuals per square metre. The
aquatic vegetation slowly recovered, but crashed again in 1996, coinciding with a peak in crayfish numbers. It appears that the Louisiana crayfish is a keystone species in the lake ecosystem, resulting in a cyclical ‘boom or bust’ scenario for plant and crayfish populations.

 


 

Reference: Matthews S. & Brandt K.   Africa Invaded: The growing danger of invasive alien species  Global Invasive Species Programme 2004