Fertilisers and sewage discharges entering the Nile delta have boosted fish stocks in Mediterranean coastal waters nearby, a study suggests.
A team of researchers found that the dramatic increase in fish populations coincided with a sharp rise in the amount of fertilisers used by farmers.
At least 60% of the area’s fishery production is supported by the run-off entering the Nile’s water, they added.
The findings appear in the Proceedings of the National Academy of Sciences.
“More than 95% of Egypt’s population and all of its agriculture are concentrated in less than 5% of Egypt’s land, along the banks of the Nile and throughout the 25,000 sq km Nile delta,” the researchers wrote in their paper.
“For more than 5,000 years, Egyptians depended on the annual flooding of the Nile, which irrigated and fertilised the floodplain and eventually discharged to the Mediterranean Sea.”
As early as the 19th Century, the nation’s population began to exceed its resources, which led to discussions on whether to dam the Nile in order to control the flow of the great river.
When the Aswan High Dam opened in the mid-1960s, the annual floods (caused by summer rains in East Africa) were reduced by about 90%.
As well as being used to generate hydroelectricity, the dammed water was also used to irrigate three crops a year instead of just one.
But interrupting the natural flow of the Nile was not without problems.
Reduced flooding meant the arable land was not having its nutrients replenished, yet it was producing an extra two harvests each year.
It also led to a smaller volume of fertile floodwater entering the Mediterranean Sea, which in turn produced a sharp fall in the number of fish being landed by Egypt’s fishermen.
“But in the late 1980s, the coastal fishery began to exhibit a surprising recovery,” the researchers observed.
“Today, landings are more than three times the pre-dam level.”
Although improvements in technology and a greater number of fishing boats could account for some improvement, the scientists said it could not account the all of the increases.
“A recent assessment of potential anthropogenic nutrient sources in Egypt suggested that these sources may have more than replaced the fertility carried by the historical floodwaters.”
They added that since the completion of the Aswan High Dam, Egypt’s population had doubled, calorie intake and meat consumption and risen by more than a third, and the use of fertilisers had increased four-fold.
“This is really a story about how people unintentionally impact ecosystems,” explained co-author Autumn Oczkowski from the University of Rhode Island Graduate School of Oceanography.
“The Egyptians were fertilising the land, and then fertilising the sea with the run-off,” she said.
“It also corresponded with a population boom and the expansion of the public water and sewer systems.”
The team of scientists collected more than 600 fish during 2006 and 2007 from four regions affected by the run-off and two regions that were not.
Results showed the fish had consumed algae and plankton that in turn had flourished in waters rich in anthropogenic sources of nitrogen.
This led to the researchers concluding that there was a correlation between an increase in fish stocks and the increase in nutrients from human activity entering the Nile delta.
Ms Oczkowski acknowledged that the findings differed from the prevailing view that excess sewage or fertilisers entering bodies of water was detrimental to marine ecosystems.
“We’re programmed in the West to think of nutrient enrichment of coastal systems as bad,” she said.