Alien fish and plants harm native species in Kenyan lakes

In 2019 researchers made a list of the fish and plants that humans introduced into Lake Victoria and Lake Naivasha in Kenya. They say the effect on the lakes’ natural species and the surrounding environment has been more negative than positive, with some natural species struggling to survive. 

People may introduce new species of fish into lakes for sport fishing, to boost commercial fishing stocks or even to control weeds. Because of the nutrient build-up in the lakes, water plants have started to blossom. These new fish and plants harm the native species, which are at risk for becoming extinct. 

In this study, researchers looked at all the non-native plant and fish species that humans introduced into Lake Victoria and Lake Naivasha in Kenya. They wanted to know which species were the most successful, and what the costs and benefits were for native species and the lake ecosystems. 

Researchers noticed that more than half of the species in Lake Victoria were non-native Nile perch and Nile tilapia. They said these fish species outcompete the native fish for food, shelter and predation, which is when one animal kills and eats the other animal. 

But, surrounding communities relied on the Nile tilapia food since it was abundant and cheap. 

Researchers said that in Lake Naivasha, the Largemouth bass, Tilapiines and Louisana red swamp crayfish have been intentionally introduced to improve commercial fisheries. 

The Common carp played a role in fisheries too, but they were accidentally introduced into the lake. They said that generally these introductions improved the Kenyan fish markets, but they negatively impacted native species because of predation and competition for food and shelter.

They also found that non-native plants such as the Water hyacinth and the Nile cabbage blocked landing sites in the lake, which prevented fishermen from catching fish. Some of these plant species were accidentally introduced, and they blocked navigation and prevented other plants at the bottom of the lake from photosynthesising (using sunlight to produce food). However, some fish species used these plants beneficially for shelter.

Researchers say authorities should ensure that in future non-native species are not introduced into lakes without exploring their potential negative impacts on endemic African species.  

The authors of this study were from Kenya. 

Abstract

Fish and other aquatic organisms have been introduced into new environments for various reasons. Provision of sports fishery, supplemental to stocks, to fill empty niches, control weeds, and disease vectors and even create a commercial fishery are some of the reasons why fish species are introduced into new ecosystems. Lake Victoria and Lake Naivasha in Kenya have seen several introductions as discussed in this paper. In Lake Victoria for example, some of the species introduced include Nile perch, Lates niloticus, Nile tilapia, Oreochromis niloticus, Tilapia zilii, and Oreochromis esclentus among others. The most successful introductions have been that of the Nile perch, Nile tilapia and water hyacinth. Lake Naivasha too has witnessed a number of introductions into its fisheries too. Some of the successful introductions into the lake are those of the Common carp, Cyprinus carpio, the largemouth bass, Micropterus salmoides, the Nile tilapia, O. niloticus and Louisiana red swamp crayfish, Procambarus clarkia. Aside from the fish species mentioned, some invasive plant species have also been introduced into these two lake ecosystems. Of notable success and impacts have been the water hyacinth, Eichhornia crassipes and Nile cabbage, Pistia stratiotes. These introductions have had both negative and positive impacts on the fisheries and the ecology of the two lakes. Some of the impacts have been predation on the native species by the introduced species like in the case of Nile perch and largemouth bass in Lake Victoria and Naivasha respectively. Competition for resources and hybridization with the native species has also been witnessed in the case of the tilapiines in the two lakes with undesirable results. Blockage of navigation routes caused by the invasive plants has been a major problem in the two lakes sometimes leading to massive postharvest losses by the fishermen. On the other hand, some introduced species have led to an increase in fish landings as well as utilization of the niches that were initially vacant within these ecosystems. It can therefore be concluded that introduced species often have more negative effects on the native species and therefore such introductions should be based on sound scientific research in order to minimize their effects within the new environments.

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