Ethiopia’s Lake Tana is losing the fight to water hyacinth

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The aquatic weed water hyacinth is causing major problems in Ethiopia’s Lake Tana.

Solomon Kibret, University of California, Irvine

Lake Tana is the largest lake in Ethiopia. It holds 50% of the country’s fresh water. It is also the source of the Blue Nile, which contributes up to 60% of the Nile’s water. Not only is the lake important as a water source for over 123 million people in the Nile Basin, it is also a source of food in the form of fish. But weeds are threatening this life-giving resource.

The lake has been listed in the top 250 lake regions of Global Importance for Biodiversity. It has 28 species of fish, of which 21 are endemic. Commercially, the lake’s most important fishes include the large African barbs, Nile tilapia and African catfish. The annual commercial value of fish production at Lake Tana is about USD$1.1 million.

The potential fish production of the lake is estimated to be 13 000 tons yearly. But its current fish production is less than 1000 tons a year. Recent studies show a serious decline in fish stocks due to the spread of the aquatic weed water hyacinth around fish spawning grounds.

Water hyacinth, Eichhornia crassipes, is an exotic free-floating invasive plant that is native to South America. People who tend aquariums and gardens are believed to have spread the plant inadvertently across the Atlantic to Africa and Asia.

It restricts water flow, blocks sunlight from reaching native water plants and depletes the oxygen in the water – often choking aquatic animals like fish. It also has an economic impact by interfering with navigation, irrigation, power generation and fishery.

The infestation

The weed forms thick mats that cover the open water. Recent data show that Lake Tana is critically infested with water hyacinth and it’s putting the aquatic biodiversity at extreme risk.

In 2011, the Regional Environmental Bureau named water hyacinth as the most dangerous weed affecting Lake Tana. By then, about 20 000 hectares of the north-eastern shore of the lake was infested. In 2014, researchers from Ethiopia found out that about one-third of the lake’s shoreline, around 128km, was invaded by water hyacinth.

In just two years, the estimated coverage of the weed doubled from 20 000 to 40 000 hectares. The weed is now estimated to cover 50 000 hectares of the lake. To make matters worse, inflowing rivers carry heavy loads of soil and suspended sediment into the lake, which affects the water quality and creates favourable conditions for the spread of the weed.

The release of untreated waste water from industries around the lake adds to the deterioration of the lake ecosystem. As a consequence, the lake has lost 75% of its fish stock in recent years.

Farmers trying to remove water hyacinth from Lake Tana.
CGTN Africa

Control measures

A water hyacinth infestation is hard to get rid of. But there are three ways to do this: removal, chemical spraying (using herbicides) and biological control.

Removing the weed, either manually or using machines, could reduce coverage and slow its spread. But it’s expensive and takes time. Local authorities are mobilising an estimated 162 000 people to remove the weed by hand. This happens only when the lake shores are accessible and when farmers have time.

Lake Victoria, lying in Tanzania, Uganda and Kenya, offers lessons for dealing with the water hyacinth problem. An estimated 60 000 hectares is covered by the weed in Kenya alone. A weed harvesting machine can clear only 10 hectares a day, so it would take 6 000 days (more than 16 years) to remove the weed entirely. Methods like biological control have been shown to be more effective.

Herbicides have been widely used to reduce the spread of the weed, but they may harm the environment. They can kill native plants that are necessary for a healthy functioning of the lake’s ecosystem.

This control method is expensive for developing countries and requires highly skilled people. In Sudan, the costs of chemical treatment for water hyacinth control were estimated to be £1 million each year.

The best approach

Biological control has been widely used. It appears to be the most economical and effective approach to manage water hyacinth in the long term. It uses natural enemies, with little cost and usually no negative environmental impact. Two weevil – or beetle – species, Neochetina eichhorniae and Neochetina bruchi, have been widely used with success. They have shrunk the coverage of the weed and controlled its spread in 33 countries, including the United States, Uganda, Nigeria, Ghana, India and Australia. But this method takes years of work by the insect to clear the weed. For instance, the weevils took two years to control the weed at Lake Victoria in Uganda.

Neochetina weevils eat only water hyacinth. Studies show that these weevils rely on the water hyacinth’s root system for crucial stages of growing. They feed heavily on the plant tissue: larvae eat the inside of the plant and adults eat the outside. Feeding damage by both life stages inhibits the growth of the plant by slowing the flowering process.

Biological control using weevils has been successful in Lake Victoria. A recent study on the adaptability and efficacy of weevils for water hyacinth control in the Ethiopian Rift Valley showed promising results. Potential negative effects, however, should to be studied before realising the weevils to new environment. Once the weevils are released, there is no operational cost as they naturally reproduce and continue feeding until all the weed is cleared.

The ConversationResearchers are also looking at the potential of using weevils for water hyacinth control around Lake Tana. At least 2-3 million people living around the lake will be relying on the success of all these efforts.

Solomon Kibret, Postdoctoral researcher, University of California, Irvine

This article was originally published on The Conversation. Read the original article.


Citizen scientist scuba divers shed light on the impact of warming oceans on marine life

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A volunteer diver surveys marine life at Lord Howe Island.
Rick Stuart-Smith/Reef Life Survey, Author provided

Madeleine De Gabriele, The Conversation

Rising ocean temperatures may result in worldwide change for shallow reef ecosystems, according to research published yesterday in Science Advances.

The study, based on thousands of surveys carried out by volunteer scuba divers, gives new insights into the relationship of fish numbers to water temperatures – suggesting that warmer oceans may drive fish to significantly expand their habitat, displacing other sea creatures.

Citizen science

The study draws from Reef Life Survey, a 10-year citizen science project that trains volunteer scuba divers to survey marine plants and animals. Over the past ten years, more than 200 divers have surveyed 2,406 ocean sites in 44 countries, creating a uniquely comprehensive data set on ocean life.

Reef Life Survey takes volunteers on surveying expeditions at hard-to-reach coral reefs around the world.
Rick Stuart-Smith/Reef Life Survey, Author provided

Lead author Professor Graham Edgar, who founded Reef Life Survey, said the unprecedented scope of their survey allowed them to investigate global patterns in marine life. The abundance of life in warm regions (such as tropical rainforests and coral reefs) has long intrigued naturalists. At least 30 theories have been put forward, but most studies have been based on relatively limited surveys restricted to a single continent or group of species.

By tapping into the recreational scuba diving community, Reef Life Survey has vastly increased the amount of information researchers have to work with. Professor Edgar and his colleagues provide one-on-one training to volunteers, teaching them how to carry out comprehensive scans of plants and animals in specific areas.

Dr Adriana Vergés, a researcher at the University of New South Wales specialising in the impact of climate change on ocean ecosystems, said that the Reef Life Survey has already substantially improved our understanding of the marine environment.

“For example, Reef Life Survey data has greatly contributed to our understanding of the factors that determine the effectiveness of effectiveness of marine-protected areas worldwide. The team have made all their data publicly available and more and more research is increasingly making use of it to answer research questions,” she said.

Some of the divers have been working with Reef Life Survey for a decade, although others participate when they can. One volunteer, according to Professor Edgar, was so inspired by the project that he began a doctorate in marine biology (he graduated this year).

There’s a strong link between fish numbers and water warmth, which means warming oceans are likely to change global fish distribution.
Rick Stuart-Smith/Reef Life Survey, Author provided

Warming oceans means fish on the move

One of the important insights delivered by the Reef Life Survey datatbase is the relationship between water temperature and the ratio of fish to invertebrates in an ecosystem. Essentially, the warmer the water, the more fish. Conversely, colder waters contain more invertebrates like lobster, crabs and shrimp.

Professor Stewart Frusher, director of the Centre for Marine Socioecology at the University of Tasmania (and a former colleague of Professor Edgar) told The Conversation that he believes we will see wide-scale changes in fish distribution as climate change warms the oceans.

“Species are moving into either deeper water or towards the poles. We also know that not all species are moving at the same rate, and thus new mixtures of ecosystems will occur, with the fast-moving species of one ecosystem mixing with the slower moving of another,” he said.

As species migrate or expand into newly warmed waters, according to Professor Frusher, they will compete with and prey on the species already living in that area. And while it’s uncertain exactly how disruptive this will be, we do know that small ecosystem changes can rapidly lead to larger-scale impacts.

In order to predict and manage these global changes, scientists need reliable and detailed world-wide data. Professor Frusher said that, with research funding declining, scientists do not have the resources to monitor at the scales required.

The Conversation“Well-developed citizen science programs fill an important niche for improving our understanding of how the earth is responding to change,” he said.

Madeleine De Gabriele, Deputy Editor: Energy + Environment, The Conversation

This article was originally published on The Conversation. Read the original article.