Here’s How We’re Ruining One of the World’s Largest Lakes

The freshwater fishing industry in Tanzania is nine times larger than the ocean fishing industry, and it’s a vital source of income for more than 2 million people, according to the United Nations.  Half of the freshwater haul comes from Lake Victoria.

Nile perch makes up the majority of the catch.  An invasive species that has dominated the lake for half a century, it’s driven many of the native fish to extinction, earning it a reputation as an ecological disaster.  For fishermen, though, it has become a cornerstone of the economy.

But over the last several years, locals here say, fish yields have begun to drop.  The culprit:  a worrisome combination of overfishing and climate change.

Hard statistics are notoriously difficult to come by, as the resource-strapped federal fisheries agency struggles to keep tabs on an industry composed almost entirely of small-scale, informal operators.  But a 2013 government audit painted a disturbing picture. Between 2009 and 2011, according to the audit, yields of Nile perch on Lake Victoria fell about 5 percent.

Further evidence of a population in decline came from the reported size of fish caught. When a fish population is being snatched up faster than it can reproduce, the average length of caught fish tends to decrease, because fewer of them are able to survive into maturity.  The audit found that between 2008 and 2010, the number of fish meeting the minimum size criteria at major processing facility near the lake dropped by more than half.

The main problem, according to the audit, is overfishing.  It blamed the federal fisheries agency for failing to set and enforce catch quotas, and it found that reported yields of Nile perch greatly exceeded the limit at which the population can be sustainably harvested.

But some scientists also point a finger at climate change.  East Africa has warmed by about 2 degrees Fahrenheit over the past century, a trend that is expected to continue. The rising temperatures are gradually altering conditions within Lake Victoria and the region’s other “Great Lakes” — Lake Tanganyika and Lake Malawi.

“The warming of the water bodies in the lakes so far has actually shown some serious impact on the productivity of the lakes,” says Pius Yanda, a climatologist at the University of Dar es Salaam and a contributing author to the 2007 report issued by the U.N.’s Intergovernmental Panel on Climate Change.

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