Case Study: Tilapia

Global spread and invasion of the fish species Tilapia, including the blue and Nile genera, represents a case of good intentions gone awry. The Tilapia has a tolerance for poor water quality and is omnivorous, able to use a wide range of natural food organisms. It also reaches early sexual maturity, which allows populations to grow quickly. These traits make Tilapia second only to carp as the most widely farmed freshwater fish in the world, but also make Tilapia an effective and dangerous invader.

Native to the Middle East and the Nile River region of Africa, Tilapia became a popular choice for fisheries worldwide, given its durability, high survival rates, and culinary value. In efforts to improve food production and bolster the fish-export business in Eastern Africa, the Tilapia was introduced to Lake Victoria, the largest tropical lake in the world, which serves as the main source of sustenance for the people of Kenya, Tanzania, and Uganda. Instead of improving the fishing opportunities in the lake, however, the Tilapia has caused the decimation of several fish populations.

When introduced in the 1950s, Tilapia constituted only a minor percentage of the lake’s fish biomass, while small indigenous cichlids made up 80 percent. A survey taken in 1980 indicated that cichlid numbers had fallen drastically, comprising only 1 percent of fish weight. Tilapia’s aggressive and omnivorous feeding behavior outcompeted native species or utilized native species themselves as a food source. Indigenous cichlids play a major role in reducing detritus, but with severely decreased numbers, their ability to consume this organic material is diminished. As the detritus decays and sinks to the lake floor, it absorbs oxygen. The decimated cichlid population led to unprecedented amounts of detritus, which has depleted oxygen at the lowest levels of the lake and created dead zones. Lake Victoria, considered one of the most species-diverse lakes in the world, has seen its cichlid population drop from 400 to 200 species due to introduction of the Tilapia and other invasive species, such as the Nile Perch.

Related Links:
-- ^{http://fineartforum.org/dept/srac/tilapia.htm}
-- ^{http://www.oneworld.org/patp/pap_victoria.html}