Kenya – The Invaders Are Coming

Cover image: Masai giraffe (Giraffa tippelskirchii) at Lake Naivasha, Kenya. © Jonathon Roberts, 2017.

The Great Rift Valley carves its way across Africa and the Middle East like a scar, running 6,000 km from Lebanon to Mozambique. Its African stretch is strongly volcanic, and the arid environment is characterised by two types of water bodies: the freshwater Great Lakes like Victoria, Tanganyika and Malawi, and the generally smaller soda lakes such as Baringo, Bogoria (famed for its breeding flamingos), Nakuru and Magadi. Lake Naivasha in Kenya is an exception to this: rather small, with only a 139 km2 surface area, and with no outflowing rivers, Naivasha is kept fresh by an underground seepage beneath its south shore. This prevents dissolved salts from accumulating as the water evaporates, the process which has made the other small lakes in the area so strongly alkaline. Papyrus swamps and fever tree groves fringe the lake’s shore, alive with the chattering of weaverbirds and swallows and the creaking duet of tropical boubous; further out, jacanas and ibises wade through floating mats of vegetation, cormorants and kingfishers make dives after fish, and fish-eagles soar overhead. Colobus and vervet monkeys chase through the trees, and at night herds of hippos emerge to graze on the lakeshore.

A confiding vervet monkey shows its best angle
An inquisitive vervet monkey (Chlorocebus pygerythrus) at Hell’s Gate National Park, near Naivasha. © Jonathon Roberts, 2017.

Naivasha is Kenya’s largest freshwater resource after Lake Victoria, of critical importance for the people who live on its shores. Agriculture and horticulture are big business, but the lake is also an important fishery, for export and subsistence. Due to the arid, low-rainfall environment of western Kenya, Naivasha is prone to dramatic fluctuations in water level, sometimes drying up entirely – most recently during the early 1800s. This imposed severe constraints on any species wishing to make Naivasha its home: birds could fly away, and cladoceran eggs and algal spores could lie dormant in the lakebed for decades until water returned, but fish were more susceptible to the droughts. Naivasha was, until recently, home to only a single species of fish (a small, undescribed Aplocheilichthys lampeye probably endemic to the lake), but this is now extinct. What went wrong?

 

Naivasha provided early European colonists with easy access to fresh water and fertile volcanic soils, and a moister, cooler climate than much of the rest of Kenya, so was quickly settled. Ranching predominated, displacing the nomadic Maasai peoples and their herds west onto the Mau Escarpment. By the early 1900s, the lakeshore population had grown significantly, and the native fish made for poor eating, so in 1925 the Athi River tilapia (Oreochromis spilurus niger) was introduced from the eponymous river system to the east. This was the beginning of a string of introductions and invasions that turned Naivasha’s ecosystem on its head.

Allow me to introduce…

Several other fish species were introduced over the following four decades to bolster Naivasha’s fisheries. Although the Athi River tilapia has gone extinct in the lake, numerous others remain common today, including straightfin barb (Barbus paludinosus), blue spotted tilapia (Oreochromis leucostictus), and redbelly tilapia (Tilapia zillii) from elsewhere in Africa; and rainbow trout (Oncorhynchus mykiss), largemouth bass (Micropterus salmoides), and millionfish (Poecilia reticulata) from the Americas. Later introductions were less successful, probably as the established fishes had already assembled a food chain with few unexploited niches – Nile tilapia (Oreochromis niloticus) and Nile perch (Lates niloticus) barely made it onto the menu before they vanished, and eastern mosquitofish (Gambusia holbrooki), introduced from North America to control mosquito populations, met a similar fate. Red swamp crayfish (Procambarus clarkii) from North America was much more successful, occupying the spot of bottom-feeder and coming to dominate fish catches after its introduction in 1970. Some of these organisms were herbivores and fed on the water-lilies that once carpeted Lake Naivasha in a heaving carpet of scented white flowers; others, like the bass, were predators. Somewhere in this tumult, probably due to predation and competition, the Naivasha lampeye went extinct, vanishing from the world unnoticed between 1960 and 1980.

Fish weren’t the only organisms introduced to Naivasha. Giant salvinia (Salvinia molesta) from Brazil and water cabbage (Pistia stratiotes) from Lake Victoria reached the lake in 1962 and 1965 respectively, joined by water hyacinth (Eichhornia crassipes) from the Amazon basin in 1988, likely escaping from ornamental lakeshore ponds during Naivasha’s floods. All of these plants float on the surface of the water, by means of gas-filled bladders or water-repellent hairs; without competition from native water-lilies they spread rapidly across Naivasha, at times carpeting the entire lake edge and preventing fishing boats from reaching open water. They starve the water of light and oxygen, but provide shelter for the crayfish, allowing their populations to boom – which then devour all the plants and suffer from increased predation, locking the species in population cycles. In an attempt to deal with the floating plants, four herbivorous weevil species from their native ranges (Cyrtobagous salviniae, Neochetina bruchi and N. eichhorniae, and Neohydronomus affinis) were introduced for biocontrol in 1995-9. These beetles seem to have no effect on the native flora, but their impact on the introduced plants is patchy, largely dictated by lake levels depriving the plants of water.

Other organisms have made it into the lake at some point in the past century. The tadpole snail (Physella acuta) arrived in 1950 from Europe; a tubificid worm (Branchiura sowerbyi) and a water flea (Daphnia pulex) also appeared, though when and how is unknown. A small population of coypu (Myocastor coypus), a large American rodent, appeared on the lakeshore in 1965, probably escapees from a nearby fur farm. They survive to this day, though in dwindling numbers, and are rarely seen.

Much better-documented is the arrival of another fish species, European carp (Cyprinus carpio), and its subsequent path of destruction. Freed from an upstream fish farm by floods in 1999, the fish washed down into Naivasha, where they found their bottom-feeding omnivorous niche largely unoccupied. Carp populations boomed, breeding up to five times yearly and growing quickly in the sunlit waters, and within five years they dominated fish catches. Other fishes suffered – rainbow trout is now very rarely caught, and even previously-common species like largemouth bass are only occasional catches.

African fish-eagles
A pair of African fish-eagles (Haliaeetus vocifer) waiting in a fever tree for their next catch. © Jonathon Roberts, 2017.

Carp also have important impacts on non-fish: younger, smaller carp feed on the floating plants, reducing the cover available to the crayfish, which large carp will happily feed on. And though most of Naivasha’s bird life has suffered from the introduction of floating plants, and the loss of the lake’s papyrus fringe as water levels dropped due to abstraction, the carp support a large population of African fish-eagles – up to 130 pairs at any one time, compared to the single pair present in the late-1800s.

Making Waves

Naivasha’s future seems uncertain. A fishery dominated by a single species is prone to collapse, but carp are such competitive generalists that few other species can co-exist in any numbers. The only fish seemingly able to hold its own against the carp is the recently-introduced African sharptooth catfish (Clarias gariepinus), which can survive the lake’s often-low oxygen levels, influxes of pollutants, and grow to be the size of a man – too large for all but the most determined of fish-eagles to tackle. Whether this will ultimately prove boon or bane for Naivasha’s fisherfolk remains to be seen.

Despite attack from below and above, any of the floating plants can re-establish from a segment the size of your thumb, so the water-lilies are unlikely to return to dominance unless the lake dries out completely. Unfortunately, this is more likely than it may seem. Naivasha itself is under pressure from agriculture (both irrigation of arable land and watering of livestock), horticulture for the European cut-flower market, and abstraction for industry, washing and drinking. The Malewa and Gilgil rivers which flow into the lake suffer from the same pressures, and the rains of the Aberdare Mountains which feed these rivers could be lost if the mountain forests are cut down – rivers no longer flow off the Mau Escarpment into Naivasha, following the scarp’s deforestation. And the whole area, already prone to drought, is likely to become increasingly arid under the effects of climate change. Needless to say, this would be a disaster for the people who depend on Naivasha – half a million already live on the lakeshore, likely to surpass two million before 2100. Unless development is closely regulated, runaway abstraction and lakeshore disturbance is likely to destroy the last vestiges of papyrus swamp and the fever tree belt, impoverishing Naivasha’s world-renowned avifauna and imperilling its status as a Ramsar wetland of international importance.

 

So I finally managed to make a blog about my visit to Kenya! It’s only been… eight months? Until next time, when I’ll be talking about Nakuru National Park’s buffalo woes,

Jon

Big skies on Lake Naivasha, Kenya
Big skies on Lake Naivasha, Kenya. © Jonathon Roberts, 2017.

Sources:

  1. Gherardi et al., 2011. A review of allodiversity in Lake Naivasha, Kenya. Biological Conservation 144, pp. 2585-2596. https://ac.els-cdn.com/S0006320711002837/1-s2.0-S0006320711002837-main.pdf?_tid=4bef55ca-e716-42ec-88cf-78b057b0081a&acdnat=1528300876_dcb454c221d65747a8f16769be1ab84d
  2. Keyombe et al., 2015. Length-weight relationship and condition factor of Clarias gariepinus in Lake Naivasha, Kenya. International Journal of Fisheries and Aquatic Studies 2(6), pp. 382-385. http://www.fisheriesjournal.com/vol2issue6/Pdf/2-6-64.1.pdf

 

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