Posted by: genghisprawn | August 22, 2008

A Nilotic Plague of Crayfish

As CNN reports, Procambarus clarkii (the Louisiana or red swamp crayfish) is earning a name for itself as “the cockroach of the Nile.” The article draws effective attention to the perils and, perhaps, unintended windfalls of this alien species’ introduction, but as a piece of scientific reporting it’s pretty shabby. I’ve highlighted some particularly pause-worthy segments below.

A statement from aquatic ecologist Magdy Khalil of Ain Shams University:

“In the 1980’s somebody came to me and said that there was a new creature in the river Nile,” says Khalil. “After two days of examination, we determined it was the fresh water crayfish. It has no natural predator in the Nile.”

Granted, crayfish of any sort are newcomers to the Nile, but I wouldn’t expect its Potamonautid crabs and larger Palaemonid prawns to go wholly unmolested. For what it’s worth, captive Fahaka or Nile puffers (Tetraodon lineatus) take readily to Cajun cuisine:

With no natural predator, scientists say, the crayfish was free to roam from where the Nile meets Egypt’s Mediterranean coast, down toward the arid nation’s border with Sudan.

Whoa there, Nelly! There’s no indication as to whether P. clarkii has already expanded its range to this extent, or whether this is just a projection. Irrespective, Wadi Halfa or bust probably won’t pan out without a follow-up introduction into Lake Nasser to get this species past Aswan High Dam.

Aswan High Dam (Wikipedia)

Aswan High Dam (Wikipedia)

The Nile’s warm waters and abundant food supply helped the crayfish evolve, Khalil says, to reproduce at twice the rate of other species.

Which other species, exactly? Since the Nile is bereft of native Astacidea, is Khalil comparing the reproductive rates of Nile P. clarkii to those of native river decapods? Alternatively, is he contrasting Nile P. clarkii with conspecifics back home in Louisiana? With a presumed date of introduction in the 1980s, I think favorable environmental conditions are more likely to underpin P. clarkii’s proliferation than rapid selection for massive bounds in fecundity.

Theories on just how the American crayfish found its way to the Middle East have reached urban legend status.

But it is widely accepted by scientists studying the crayfish invasion that this all started when an Egyptian businessman attempted to expand his fish farm industry by investing in shrimp.

He bought what he thought were [shrimp] eggs to hatch in his fish farm,” Khalil says with a wry smile.

“When they hatched into crayfish, they ate all the fish, then burrowed through mud partitions into neighboring fish containments and ate those fish too.”

As the story goes, the businessman was so enraged that he took the crayfish and dumped them in the Nile.

Crayfish eggs adhere to the pleopods or swimmerets of the “berried” female, and juveniles are retained there for some time after they hatch.

Berried Procambarus clarkii

Berried Procambarus clarkii (Smart Center)

Although aquaculture- or conservation-minded researchers have investigated artificial incubation, where viable crayfish eggs are stripped from the swimmerets and stored in an aerated container (see here and here), I’m not aware of any crayfish-farming operation that grows their stock from loose “berries.” More to the point, the preeminent shrimp in the freshwater aquaculture industry is Macrobrachium rosenbergii, which similarly maintains eggs amongst the pleopods until they hatch into zoeae. In aquaculture, this species is usually stocked as post-larvae; starting from eggs alone would be wholly unprecedented (and for good reason — heavy mortality due to mechanical damage or insufficient oxygen during shipment seem pretty much guaranteed, and additional air pumps would be required for what the fluttering of the female shrimp’s swimmerets would do for free).

Berried Macrobrachium rosenbergii

Berried Macrobrachium rosenbergii (FAO)

M. rosenbergii eggs differ dramatically from those of P. clarkii in both size and overall appearance, and moreover yield tiny floating larvae which require brackish water for proper development.

The verdict is clear: this explanation, too, is an urban legend. Nonetheless, introduction through aquaculture does seem like the most likely explanation.

If some enterprising Egyptian crayfish-rancher didn’t intentionally seed the river, P. clarkii’s ability to crawl around over land — and hop the inundated sides of aquaculture ponds during heavy rainfall — make the situation self-explanatory.

The story takes an interesting turn when Khalil mentions an ongoing collaboration with American researchers to see whether the Nile’s crayfish could become a natural defense against bilharzia (schistosomiasis), a parasitic disease transmitted by trematodes which use freshwater snails as intermediary hosts.

As Egyptian Bilharzia Institute researcher Karem El Homossamey walks along the Nile’s west bank outside of Cairo, he points a few Egyptian women calf-deep in the water, washing carpets.

Prime candidates, he says, for catching the bilharzia infection.

“We must give more attention to public awareness — and the importance of the crayfish to make the people of the Nile like it and put it everywhere,” El Homossamey says.

El Homossamey says it’s because the bilharzia snail is the crayfish’s favorite treat. Ain Shams University ecologist research seems to back that claim.

“We put some fish, lamb, plants, dead chicken — we found the first thing [the crayfish] selected was the snail, because the shell is very thin,” Khalil says.

He argues areas of the Nile where there are more crayfish, human bilharzia infection rates are low.

This leaves me very uneasy. In terms of snail-eating ability, have P. clarkii been compared to native decapods and molluscivorous fish? Have the links between water impoundment, pollution, schistosome snail densities, and native snail-predator populations been adequately analyzed? I fear not. P. clarkii, with its enormous tolerance for heavily-modified habitats, might indeed offer a quick fix. Yet once it’s introduced to a river system, it’s not likely that we’ll ever get it out.

“Voracious omnivores,” as this report on P. clarkii in schistosomiasis-afflicted Kenya calls them, these crayfish might have drastic effects on finfish and macroinvertebrates, aquatic macrophytes, and irrigation-dependent crops. As mentioned in the article, P. clarkii’s burrowing activity is already causing the collapse of water canal networks in the Nile Delta, and its two-clawed depredations on fish-laden nets is raising the ire of fishermen throughout the river. The Kenya paper is inclined to dismiss the possibility, but P. clarkii is also known to serve as the intermediate host of lung flukes in the genus Paragonimus.

Khalil is telling crayfish awareness workshop participants that the crayfish is the answer to spiraling food prices.

High in protein and cheap to buy, crayfish by the kilogram is making its way to Cairo’s street markets. Khalil encourages fisherman to set crayfish traps in their fishing areas, to keep the crayfish out of their nets.

Sponsored by a grant from the United Nations Development Agency, Khalil’s workshops include a buffet tasting of crayfish cuisine, including a crayfish boil that could put New Orleans to shame.

According to the Louisiana Crayfish Promotion and Research Board, the crayfish meat industry is responsible for a $120million/year impact on Louisiana’s economy.

Khalil says if Egypt can develop a similar industry, the profits could be considerable.

“You see in Louisiana they are eating it, cooking it — there are many festivals for the crayfish. We need to do the same here,” Khalil says.

Harvesting crayfish from the Nile probably won’t hurt, but is this really comparable to the Louisiana industry Khalil wants Egypt to emulate? The Agricultural Marketing Research Center gives us the answer:

In the 2006-07 season, the crawfish harvest in the state [Louisiana] totaled 109.2 million pounds of pond-raised crawfish, which were sold for $84.6 million, making pond-raised crawfish the state’s most valuable aquaculture crop. About 1.5 million pounds of wild crawfish were harvested, with a value of $1.3 million.

Crayfish ponds in Louisiana

Crayfish ponds in Louisiana (Aquaculture Network Information Center)

In other words, the monetary value of the Bayou State’s crayfish makes it expedient to ratchet up output as high as possible — to actively farm the creatures. If one wants to rein in on an invasive species, is commodifying it the best approach to take?


Loker, E. S., et al. “Procambarus Clarkii In Kenya: Does It Have A Role To Play In The Control Of Schistosomiasis?” Humanity Development Library 2.0. 22 Aug. 2008

Lutz, C. Greg, Pramod Sambidi, and R. Wes Harrison. “Crawfish Profile.”
Agricultural Marketing Resource Center. 22 Aug. 2008

Shetterly, Robert. “Crawfish (farmed, U.S.).” Guide to Ocean Friendly Seafood: Species Score Card. Blue Ocean Institute. 22 Aug. 2008

Van Marsh, Alphonso. “Egypt’s crayfish invasion.” CNN 17 Aug. 2008. 22 Aug. 2008


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