Catastrophic Change in SpeciesRich
Freshwater Ecosystems
The lessons of Lake Victoria
Les Kaufman
L iving communities change frequently.
Hurricanes, volcanoes,
predator outbreaks, and disease
destroy old patches of forest and reef,
making way for new species. Shifting
continents redistribute the earth's great
floras and faunas. Collisions with extraterrestrial
debris may bring passage
from one era of life to the next.
Humans, too, are a force for ecological
change, perhaps now the dominant
force. Conversion of tropical forest
to cropland or human habitation
rapidly degrades species richness.
In the marine realm, the plight of
the whales, sea turtles, and seabirds is
widely known. Marine fishes and invertebrates
are thought to be buffered
against extinction by high fecundity
and widely dispersed pelagic larvae
(Robins 1991), but when more is
known about marine animals more
may be found to be endangered (Earle
1991, Kaufman and Strysky 1991,
Tudge 1990). As for freshwater fishes
and invertebrates, the evidence is more
telling. Freshwater communities are
exposed to the full brunt of deforestation,
mining, poor land-use practices,
exotic species introductions, and development.
Therefore, it is not surprising
that more than one in five
freshwater fishes may be threatened
or endangered (Kaufman 1989, Williams
and Nowak 1986).
Entire freshwater fish faunas are
disappearing (Avise 1990, Kaufman
1989, Tudge 1990, Williams and
Les Kaufman is chief scientist at the New
England Aquarium's Edgerton Research
Lab, Central Wharf, Boston, MA 02110.
? 1992 American Institute of Biological
Sciences.
There is at least an
even chance of
maintaining a diverse,
productive, and healthy
tropical ecosystem in
Lake Victoria
Nowak 1986, Williams et al. 1989).
The infamous snail darter is but one
representative of the vanishing mollusc
and stream fish fauna of the Mississippi
Basin and Appalachia. The
desert pupfish is one of dozens of
endangered desert fishes and insects
in the US southwest and Mexico (Williams
et al. 1989). Aquarium fishes
bred domestically and available in pet
stores are almost gone from their native
haunts in Madagascar, Sri Lanka,
and Borneo (Andrews and Kaufman
in press). The endemic species flock of
Lake Lanao, in the Philippines, is virtually
extinct (Kornfield and Carpenter
1984). Though usually with fewer
species to lose, high-latitude freshwater
ecosystems are showing similar
signs of stress and disruption (Taylor
et al. 1984, Wells and McLain 1972,
Williams et al. 1989).
The recent history of the fishes of
Lake Victoria, East Africa (Figure 1),
exemplifies a pace and magnitude of
change that is alarming. In 1988, the
World Conservation Union Red Book
of Endangered Species listed the hundreds
of endemic fishes of Lake Victoria
under a single heading: "Endangered."
The most exuberant expression
of vertebrate adaptive radiation
in the world (Table 1) is now in the
midst of the first mass extinction of
vertebrates that scientists have ever
had the opportunity to observe, an
event as exciting as it is depressing.
More than 30 million people who
depend on the lake are feeling the
consequences of roller-coaster changes
in the fauna and lake environment.
The most important freshwater food
fishes in East Africa have already vanished
from the marketplaces and very
nearly from the planet. A fishery that
once drew on hundreds of species,
mostly endemic, now rests on three: a
native pelagic minnow called the
omena (Rastrineobola argentea) or
dagaa in Tanzania; the introduced
Nile perch (Lates niloticus), known as
mbuta; and the introduced Nile tilapia,
Orechromis niloticus (Figure 2).
Scientists, fishermen, and environmentalists
have decried the loss of
Lake Victoria's native species, and
others have praised the introduction
of Nile perch. Some have even referred
to that fish as a savior. Now the
savior threatens to destroy itself, the
lake ecosystem, and a major source of
protein in the midst of the world's
fastest-growing human population. In
Lake Victoria, as elsewhere, human
welfare is intimately linked to concern
for species conservation and ecosystem
integrity.
The African great lakes and
their fishes
To understand Lake Victoria, one must
see both the lake and its fauna in the
BioScience Vol. 42 No. 11846
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context of its giant East African neighbors,
Lake Malawi and Lake Tanganyika
(Table 2). The faunas of all
three lakes exhibit the products of
rapid speciation from very few ancestors.
Best known is the fish family
Cichlidae, of which more than 90%
of the species in each lake are endemic
(Greenwood 1984). Catfishes, mormyrids
(elephant-nosed electric fishes),
carps, gastropod and bivalve mollusks,
insects, and crustaceans also have produced
clusters of endemic species, but,
with the possible exception of the
Lake Tanganyika gastropods, these
radiations are much less diverse and
morphologically varied than those of
the cichlids. Closely related but less
species-rich cichlid flocks also occur
in the nearby, smaller lakes Edward,
George, Kivu, Kyoga, and Nabugabo
(Greenwood 1974, 1981).
The cichlid faunas in the three great
lakes in East Africa are strikingly similar
and often cited as examples of
evolutionary parallelism (Eccles and
Trewavas 1989, Fryer and Iles 1972,
Witte 1984). Three apparent lineages
are present in the lakes: haplochromines,
tilapiines, and lamprologines.
The haplochromines are a speciesrich
and geographically widespread
lineage. The tilapiines are species poor
but also widespread. The lamprologines
are species rich, but in the
great lakes occur only in Lake
Tanganyika.
As a group, the cichlids stand in
contrast to the Nile perch and omena,
both of which invest heavily in fecundity
and little in parental care. The life
history profile of chiclids in general
entails small broods and extended
parental care. Most East African
haplochromine and tilapiine cichlids
brood relatively few (5 to 100) large
eggs and develop their young in the
mouth, but the substratum-spawning
lamp-rologines and tilapiines place
large clutches (hundreds to thousands)
of small eggs in nests on the lake
bottom. All defend their young until
they are self-sufficient. They have limited
dispersal (except for seasonal inshore
movements to spawn or release
young) and strong site attachmentcharacteristics
that should in theory
make them highly vulnerable to extinction
(Jablonski 1986, Gaston and
Lawton 1990).
Curiously, this view is sharply at
odds with certain other of the cichlids'
EMIN PASHA GULF4\ I MWANLM A "
EM,. ^SKOME ISL. MWANZA GULF
100 km
Figure 1. Lake Victoria, showing bathymetry. Note Winam G
of islands in the northwest sector is the Sesse Islands. Adapte
attributes. They are aggressive, behaviorally
and physiologically adaptable,
phenotypically plastic, and prone
to extraordinary evolutionary diversification
(Avise 1990, Fryer and Iles
1972, Liem 1974). Much of this versatility
has been attributed to a fundamental
reorganization of the pharyngeal
jaws that cichlids share with
several closely allied fish families
(Kaufman and Liem 1992).
Many cichlids can individually alter
tooth and skull morphology in
response to a change in diet (Greenwood
1965, Kaufman 1989, Meyer
1990, Sackley 1991, Witte et al. 1990).
One cichlid, the Victorian snail-crusher
Astatoreochromis alluaudi, was introduced
to West Africa to help control
the snail vectors for bilharzia, but
these cichlids fed on insects instead of
snails and stopped producing the massive
dentition and musculature necessary
to crush snail shells (Sloot-weg
1987).1
Cichlids have escaped from tropi-
1Haplochromine Ecology Survey Team, 1992,
University of Leiden, personal communication.
cal fish hatcheries into the canals and
everglades of south Florida, where
they reproduce more successfully than
do native sunfishes (Centrarchidae;
(Courtenay and Robins 1973, Hogg
1976, Taylor et al. 1984). Introduced
all over the world as a ready source of
home-grown protein for developing
nations, tilapiine cichlids may have
affected hundreds of native fish com-
munities.
In short, cichlids have adapted to
an incredibly wide range of conditions.
One might expect such adaptability
and ecological versatility to
offer some measure of protection
against extinction. That it has not
done so in Lake Victoria may be one
indication of the magnitude of change
that has taken place.
Recent changes in the Lake
Victoria ecosystem
The recent history of Lake Victoria is
one of dramatic change in limnological
parameters and native fishery
stocks from the late 1960s to the
present (Ogutu-Ohwayo 1990). OverDecember
1992 847
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a
D
c d
Figure
hundre
extincti
an anch
200 kg
fishing
deleterious land-use practices, and
pollution from various sources all contributed
to the oxygen depletion and
mass extinction of indigenous fishes
now taking place.
The earliest known anthropogenic
changes in the fish assemblages were
the result of overfishing. Gill nets and
other modern fishing methods, introduced
by the British in the early part
of the century, led to the rapid depletion
of important anadromous fishes
that once had spectacular spawning
runs up the rivers that flow into Lake
Victoria, and of the ngege, Oreochromis
esculentus (Figure 2b), which
originally supported the lake's most
important fishery (Graham 1929,
Ogutu-Ohwayo 1990).
Several exotic species of tilapia were
introduced from other lakes and rivers
during the late 1950s to replace the
devastated stocks of native fishes. At
first these introductions were not very
successful, but the Nile tilapia, 0.
niloticus, eventually took hold and
prospered.
High rainfall during the 1960s submerged
and destroyed extensive beds
of littoral vegetation, eliminating important
spawning and nursery grounds
for native species. This natural occurrence
must have contributed to the
decline of at least those native species
dependent on nearshore habitats.
Beginning in 1954, Nile perch (L.
niloticus) were introduced to the lake
(Fryer 1960). Nile perch persisted as a
BioScience Vol. 42 No. 11
*yL
.- ^ U-S. .-"?es
^ .
*i ijj ^11 ii4
~. _ llllllll
848
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minor component of the fauna for
two decades, until the early 1980s.
Then the species underwent a staggered
series of population explosions
in the eastern, northern, and southern
waters of the lake. The trigger for the
Nile perch irruptions is not known; it
is interesting and mysterious that the
fish should have persisted for so long
and at such low densities before the
explosion.
The leading hypothesis for the native
fishes' demise is that they were
consumed by the introduced Nile
perch: the decline in native fishes and
the increase in Nile perch population
are almost perfectly reciprocal (Figure
3, Ogutu-Ohwayo 1990, Witte et
al. 1992). Indeed, Nile perch seem to
have regulated fish diversity in the
past. Those East African lakes naturally
inhabited by Nile perch have few
haplochromine species, whereas those
without the predator host species-rich
communities (Greenwood 1981).
The dramatic disappearance of
native fishes from Lake Victoria took
place within a remarkably short time
period, mostly between 1975 and 1982
(Witte et al. 1992). As recently as
1978, the haplochromine fauna was
intact. Haplochromines contributed
approximately 80% of the biomass
and Nile perch less than 2%, with the
remainder consisting of the introduced
Nile tilapia and native noncichlids.
By 1983 in Kenya (Figure 4) and by
1986 in Tanzania, the native fish community
had been virtually destroyed,
and Nile perch comprised better than
80% of the catch. The remaining 20%
consisted of Nile tilapia, the tiny native
omena, and a small remnant of
other native fishes. A community of
more than 400 fish species collapsed
to just three co-dominants: the Nile
perch, the Nile tilapia, and a single
indigenous species, the omena (Hughes
1986). Meanwhile, as its own supply
of forage fish dwindled, the Nile perch
exhibited an astonishing ecological
shift. In essence, it has now become a
whale, subsisting largely by scooping
up great heaps of a tiny native shrimp,
Cardina nilotica. The next most important
item on the Nile perch menu
is Nile perch; cannibalism is now a
major link in the food chain.
At first, Lake Victoria was cited as
a classic example of what can go
wrong with alien fish introductions in
tropical fresh waters (Barel et al. 1985,
Table 1. Examples of trophic diversification in Lake Victoria. Nomenclature follows
Greenwood 1979, except for undescribed species, which are listed, by convention, as
"Haplochromis" with nickname in quotes to avoid confusion with described taxa.
Trophic groups Taxa
Plant eaters
Pelagic phytoplanktivore
Detritivore 1 (soft algae)
Detritivore 2 (diatomaceous ooze)
Rock scraper
Plant scraper
Algal browser
Leaf chopper
Algal gardener
Arthropod eaters
Crab eater
Prawn eater
Generalized insectivore
Rock picker
Fat-lipped sucker
Forceps-toothed picker
Rock-reef, low-foraging zooplanktivore
Rock-reef, high-foraging zooplanktivore
Soft-bottom zooplanktivore
Sand/rock, pit-hunting zooplanktivore
Pelagic zooplanktivore
Sand sifter
Parasite picker
Mollusc eaters
Winkler
Oral snail crusher
Oral clam crusher
Pharyngeal crusher
Fish eaters
Ambush piscivore
Pursuit piscivore
Snout-engulfing paedophage
Egg snatcher
Scale eater
Balon and Bruton 1986). Local fisherman
complained bitterly about Nile
perch. The huge, clumsy fish destroyed
the fishmen's nets and equipment,
which had been designed for smaller
quarry. Nile perch flesh is oily, and
without refrigeration quickly rots.
Sun-drying, a method of preservation
that worked for the cichlids, does not
work for large Nile perch. Nile perch
could be smoked, but this method
requires wood. Trees are in short supply
around most of Lake Victoria, a
decline that some think may have
been accelerated by this new demand.
Such was the story of disaster told
in the foreign press. But during the
late 1980s the situation changed, as
foreign aid and investors helped bring
refrigerator trucks and processing
plants to the lake shore. The fatty,
flaky white flesh of Nile perch became
"Haplochromis" "kribensis"
Enterochromis spp.
Oreochromis esculentus
Neochromis spp.
Haplochromis lividus
"Haplochromis" "kruising"
Xystichromis phytophagus
Neochromis nigricans (?)
"Haplochromis" "smoke"
Psammochromis spp.
Astatotilapia spp.
"Haplochromis" "rockpicker"
Paralabidochromis chilotes
Paralabidochromis victoriae
"Haplochromis" nyereri
"Haplochromis" "double stripe"
Astatotilapia piceata
"Haplochromis" "deep black"
"Haplochromis" "argens"
Psammochromis spp.
"Haplochromis" teunisrasi
Ptyochromis sauvagei
Macropleurodus bicolor
Hoplotilapia retrodens
Labrochromis spp.
Pyxichromis orthostoma
Prognathochromis dentex
Lipochromis maxillaris
Astatotilapia barbarae
Allochromis welcommei
popular on African hotel menus and a
successful export to Europe and Israel.
Scientists deplore the loss of
haplochromine biodiversity (Barel et
al. 1985, Witte et al. 1992), but the
importance of biodiversity is readily
questioned when a commodity of immediate
value appears in its place.
The African view of the Lake
Victoria's Nile perch is complicated,
and largely misunderstood by outsiders
who have caught only bits and
pieces of this ecological soap opera
(e.g., Avise 1990, Dawes 1986, Harrison
et al. 1989, New Scientist 1988,
Tudge 1990). The small-scale subsistence
fishery that focused on the native
cornucopia has taken a severe
beating, with most species simply gone
from the marketplace. My conversations
with villagers in Kenya and
Uganda revealed that, regardless of
December 1992 849
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m
II:
Nile perch
omena
haplochromines
68 e8 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 86 88 87 88 89 80
Year
Figure 3. Percentage of the commercial catch in Kenya made up of haplochromine
cichlids, omena, and Nile perch between 1968 and 1990. The remaining 40% to 70%
of the catch before 1981 was dominated by native lungfish (Protopterus), catfishes
(Synodontis and Clarias), carps (Labeo and Barbus), elephant-nose fishes (Mormyrus),
and both native and introduced tilapiine cichlids (Oreochromis and Tilapia). (From
Getabu 1987, FAO Report #388.)
how they may feel about the Nile
perch, they miss the native fishery
with its great variety in taste and
texture.
The revenues generated by the Nile
perch fishery are much greater than
those ever realized from the lake's
native species, but the relationship
between changes in the lake's fauna
and changes in revenues generated by
the fishery is not a simple one. Landings
data, such as those presented in
Figure 5 for Kenya, suggest that the
usable productivity of the lake increased
by at least half an order of
magnitude over 15 years. Landings
data can be misleading, however. Because
the traditional nearshore fishery
was less mechanized, less efficient,
and less wide-ranging than the
new fisheries aimed at Nile perch and
omena, even if productivity were unchanged,
landings would be expected
to soar. Some Ugandan and Tanzanian
fisherman may be selling their
catches in Kenya, where prices tend to
be higher, thus skewing estimates of
landings by country.
Experimental fishery surveys are
less vulnerable to such bias and tell a
strikingly different story. Experimental
surveys are carried out on a routine
basis by the Kenyan, Ugandan, and
Tanzanian fishery research organizations;
estimates of standing stocks for
introduced and native fishes in Kenya's
Winam Gulf region are presented in
Figure 4. The sharp switchover from
native to introduced species is still
there, but by these data, standing
stocks of introduced species were only
marginally higher in 1990, if at all,
than were the stocks of native species
20 years earlier.
Although more tonnage is being
landed, and more valuable tonnage at
that, the total amount of fish biomass
produced annually by the lake may
not have changed as much as first
thought. From a marketing standpoint,
the new product may be easier to
handle because it lacks the native species'
bewildering variety. This change
is analogous to the common practice
of clearcutting rainforest and replacing
it with one or a few fast-growing,
easily processed, exotic timbers.
The distribution of wealth resulting
from the Nile perch fishery is also
different from that of the original,
artisanal fishery. Some local fisherman
may actually be worse off despite
the apparent plenty. Large-scale operations
that exploit the introduced
species for foreign currency are doing
well. The small-scale fisherman and
fishmongers, who never went hungry
and who relied for their livelihoods on
the traditional tastes and interesting
diversity of the native species, are a
vanishing breed. The specter of protein
malnutrition in the lake basin has
been raised by socioeconomists, an
incredible irony in a place exporting
nearly 200,000 tons of fish protein
annually.
Mysterious die-offs of Nile perch,
and evidence that the stocks are being
overfished, have raised additional concerns.
There is serious worry about
the future of Nile perch, a concern
much stronger than the lingering nostalgia
for native food fishes and the
biodiversity of haplochromines. But
most people do not realize that there
is now an even more serious problem:
the entire lake has been placed in
jeopardy by profound changes in the
structure and dynamics of the ecosys-
tem.
Along with irruptions of Nile perch,
the early 1980s brought another surprise:
the regular appearance of dense
algal blooms and associated low oxygen
levels in the lake's shallow waters
(Ochumba and Kibara 1989). Anoxia
brought stinking masses of dead Nile
perch to the surface. Fish kills had
probably occurred on Lake Victoria
before,2 but not with such frequency
or intensity. In March of 1987, the US
National Undersea Research Program
sent a remote-operated vehicle and
team of investigators to Lake Victoria
to meet with local scientists as an
overture to a planned Large Lakes of
the World Program.
During a series of demonstration
dives on the lake, the group discovered
that below 30 meters there was
no fish life, and the bottom was littered
with dead and dying organisms.
This observation was the first hint
that the anoxia described by Ochumba
(Ochumba and Kibara 1989, based on
data collected in the early to mid-
1980s) might in fact be extensive in
the lake's deeper waters and thus
worthy of grave concern.
The Lake Victoria Research Team
was formed to spur multinational research
in the northern waters of the
2R.L.Welcomme, 1992, personal communication.
FAO, Fisheries Research and Environment
Division.
BioScience Vol. 42 No. 11
100
80
60
40
c-
0
a)
C)
0
Q)
0~
20
0
850
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lake, as part of a larger Lake Victoria
Research and Conservation Program
coordinated through the New England
Aquarium in Boston. The National
Undersea Research Program provided
seed support for hydrological and remote-operated-vehicle
survey work,
and the National Science Foundation
funded preliminary biotic survey activities.
The US Agency for International
Development, the Pew Foundation,
the Canadian International
Development Research Council, and
the Conservation Food and Health
Foundation also assisted.
The results of the first phase of
work by Lake Victoria Research Team
members are alarming. It appears that
coincident with the Nile perch explosion
was a violent change in the physical
environment of the lake. Before
1978, aerobic life penetrated into the
lake's deepest waters. The lake was
well-mixed, although oxygen was
lower in the deeper waters (more than
60 meters) for brief periods during the
long (approximately February through
April) and short (December) rainy
seasons (Talling 1966). Fish biomass
was high even in deep water, though
the rains may have initiated inshore
migration by fishes requiring higher
oxygen levels (Hoogerhoud 1983).
In contrast, now Lake Victoria appears
to be stratified for the entire
year. Seasonality is still apparent, but
mixing is restricted to periods during
June and July when the oxycline (the
depth at which oxygen falls precipitously),
though still persistent, sinks
to approximately 50 meters3'4 (Figure
6a). The region between 50 and 25 m
is subjected year-round to frequent,
severe deoxygenation. We have observed
local upwellings of hypoxic
water, along with extensive fish kills,
at the surface near rocky islets. Thus,
most of the lake's volume is incapable
of supporting any but the most rugged
species. Adult Nile perch themselves
are probably not tolerant of prolonged
hypoxia.
One interesting sidelight was the
discovery that in the deepest portions
of Rusinga Channel at the mouth of
Kenya's Winam Gulf oxygen levels
are consistently higher than at equivalent
depths in the main lake (Figure
3L. Kaufman, 1992, manuscript in preparation.
4R. Hecky, 1992, manuscript in preparation.
Freshwater Institute, Canada.
Table 2. Physical characteristics and biodiversity profiles for the three largest lakes of
East Africa. Lakes Victoria and Malawi excel in species and trophic diversity. Lake
Tanganyikan cichlids exhibit the greatest reproductive, form, and phyletic diversity.
Lake
Physical characteristics Victoria Malawi Tanganyika
Surface area (sq km) 62,000 28,000 32,600
Maximal depth (m) 100 700 1470
Effective depth (m) 100 (before 1986) 200 150
(depth at which 40 (after 1987)
oxygen levels fall
below 1 ppm)
Biological characteristics
Species diversity Medium (350+) High (500+) Low (170+)
(Cichlidae)
Trophic diversity High High Medium
Reproductive diversity Low Low High
Form diversity Low Medium High
Phyletic diversity Low Medium High
Age of radiation 14-225 <1500 3000+
(thousands of years)
6b), from which the gulf is partially
isolated by a shallow sill. This protected
region is inhabited by severa
tiny, undescribed deepwater haplochromines
(e.g., Figure 2a).
Perhaps the greatest surprise from
the work using the remote-operated
vehicle was the observation that the
lake's small, native detritivorous
shrimp, C. nilotica, was abundant and
active in the midst of massive kills of
omena at and below the point where
oxygen levels declined toward zero.
This observation is important because
it suggests that the shrimp might have
a refuge from Nile perch. The predator,
which as an adult is believed to
require a level of at least 5 ppm oxygen
in its water, would probably be
unable to destroy this food base the
same way that it wiped out the old
one.
Myriad other aspects of the lake's
ecology also appear to have changed:
productivity and turbidity have both
increased, papyrus swamps seem to
be on the decline, snails seem greatly
increased in abundance, locals say
lake fly (mostly Chaeoborus swarms)
are larger and more prevalent than
before, and water hyacinth, a noxious
floating plant introduced from South
America, has invaded Lake Victoria
from Rwanda via the Kagera River;
now it is spreading and increasing in
the lake. It will take a while to assess
the validity, the timing, and the relative
importance of such changes.
The collapse of the species-rich
Lake Victoria ecosystem and its replacement
by a highly simplified,
largely exotic community raises of
plethora of varied and important questions.
The answers must address the
dynamics of speciation and extinction,
the wisdom of alien introductions,
and the practices of fisheries
management in the tropics.
What happened to
Lake Victoria?
The recent changes in Lake Victoria
are astonishing, but they are not without
precedent: this lake has a tumultuous
past. The Victorian Basin as a
whole was formed approximately
750,000 years ago when crustal uplifting
and doming reversed the westward
flow of the Kagera and Katonga
rivers into the new lake basin (Kendall
1969). The basin's lakes have varied
in depth, shape, and number several
times since its formation (Greenwood
1974, 1981, Scholz et al. 1990). On
reflection, there must already have
been several cycles of explosive speciation
and mass extinction, with the
recent faunas of lakes Victoria, Kyoga,
Edward, George, Nabugabo, Kanyaboli,
and Kivu being only the latest
bursts in a series of massive speciation
events (Greenwood 1981). Lake Victoria
itself may have completely dried
up only 14,000 years ago (Scholz et al.
1990). The speciation rate that this
history would imply (more than 350
species arising in 14,000 years) is over-
whelming.
The current crisis probably began
with the eutrophication of the lake
basin. Algal productivity in Lake
December 1992 851
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I _ _ 0 . Introduced
0 U _ Native
70 75 77 83 90
Year
Figure 4. Demise of the native fishes of Lake Victoria, as illustrated by fisheries
surveys in Kenyan water by the Kenyan Marine and Fisheries Research Institute.
Standing stock estimates derived from the following numbers of hauls: 1969-1970
= 19; 1975 = 69; 1977 = 167; 1982-1983 = 54; 1989-1990 = 41. (Data courtesy P.
Ochumba, A. Asila, and J. Ogari of KMFRI.)
I I I I I I I I I I I I I I I I I I I I I I I
UI
I
Nile perch
omena
haplochromines
88 89 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 88 87 88 89 90
Year
Figure 5. Total landings for haplochromines, omena, and Nile perch in the Kenyan
waters of Lake Victoria, 1968-1990. (From Getabu 1987, FAO Report #388.)
Victoria has increased fourfold over
several decades, with a concomitant
drop in water transparency (OgutuOhwayo
1990). This change could be
due to an increase in anthropogenic
nutrient loads and/or changes in nutrient
dynamics within the lake. Even
before the Nile perch irruptions, the
Lake Victoria ecosystem probably had
already been altered by eutrophication.
The huge stocks of detritivorous
haplochromines observed before the
collapse of the 1980s might have been
at least partly a reflection of the increase
in primary productivity.
It is remarkable that the Lake
Victoria ecosystem was able to cope
with increased productivity over several
decades without an earlier collapse.
This resilience may have been
related to the high proportion of
haplochromine cichlids, which eat
virtually everything-especially each
other, with more than half the haplochromine
species once preying on other
haplochromines (Greenwood 1981,
Witte et al. 1992). In addition to the
tight internal recycling that they provided,
the haplochromines helped to
move biomass and nutrients in the
course of their vertical and horizontal
migrations (e.g., Goldschmidt 1989).
This biological mixing could have
been as important in lake dynamics as
is the physical mixing of the water
column. First, it could have retarded
the accumulation of rotting biomass
in the deeper portions of the lake. The
importance of this effect should not
be underestimated: 80% of the haplochromine
biomass, or approximately
60% of the lake's total biomass, consisted
of cichlids that fed in the lake's
thick layer of detritus. The second
effect of biological mixing was to
expose much of the lake's biomass to
potential export through predation
on fishes by birds (Wannink 1990),
snakes, crocodiles, otters, and other
amphibious organisms, including, of
course, the human population.
In sum, Nile perch may have
decoupled the recycling capacity in
Lake Victoria by converting the recycling
machinery itself, (i.e., the haplochromines)
into Nile perch flesh. A
primary question is whether the
present, still-presumptive links from
detritus to shrimp to Nile perch are
indeed less efficient than those of the
original food web. If so, then the
impact of this cessation of recycling
and biological mixing, combined with
a long-term increase in anthropogenic
nutrient loading on the lake and resulting
increase in algal growth, could
have caused a build-up of biomass in
the lake's deeper waters. Biological
oxygen demand would surge, and the
deeper water column would be rapidly
depleted of oxygen during periods
of thermal stratification.
Native fishes, seeking reliable refugia
in their seasonal moves between
shallow and deep water habitats, may
have faced a choice of death by asphyxiation
in deeper water or death
by Nile perch predation in the more
BioScience Vol. 42 No. 11
100
90
80
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60
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b
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x
0
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1 2 3 4 5 6 7 8 9 10 11 12
Month
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Figure 6. Mean monthly oxygen concentrations at the surface and bottom (depth 40-60 m) of Lake Victoria (measurements were
made with a Hydrolab multiprobe). a. Open lake, based on six stations in the open waters of northeastern Lake Victoria (Kenya
and Uganda). b. Rusinga Channel, Kenya, based on two stations located near the mouth of Winam Gulf. Note higher oxygen levels
in Winam Gulf during those months for which comparative data are available. (From Lake Victoria Research Team; 22 missions
conducted during 1989-1991.)
oxygen-rich shallows. Each forcedeoxygenation
and predation-selected
for a mutually exclusive refugium.
It is most likely this combination
of complementary impacts, rather than
any one factor by itself, that brought
about the rapid disintegration of the
hap-lochromine assemblage.
Alternative hypotheses can be constructed,
but their assessment is hindered
by a lack of comprehensive
data. For example, little is known
about the potential for the new food
web to functionally replace the old
one. The shrimp Caridina is both a
detritivore and, and as seen from the
remote operated vehicle, a facultative
anaerobe, so it could transport organic
material from the benthos into
the lake's upper waters. We do not
know, however, if the shrimps actually
extend over much of the lake
bottom, or if a significant portion of
the population is available for consumption
by Nile perch.
It is clear from temperate lakes,
however, that changes in shrimp abundance
can have profound impacts on
aquatic food webs (Spencer et al.
1991). Lake fly larvae (Chaeoborus)
such as those in Lake Victoria are
well-known for their vertical migrations
and tolerance of low oxygen
levels, and when they metamorphose
into winged adults, they could export
benthic nutrients to the birds that feed
on them. We do not know, however,
if there are trophic links that could
funnel benthic-derived nutrients into
the fly larvae. One of the most interesting
questions concerns the extent
to which Nile perch juveniles can
replace haplochromines as a manifold
for nutrients to higher trophic levels
(mostly adult Nile perch). Small Nile
perch differ from large ones only in
scaling, whereas 60% of the haplochromine
species were piscivores
that varied considerably in form and
behavior as well as size.
Could it happen again?
The story of Lake Victoria, told and
retold at scientific meetings, has become
a kind of ballad, warning of the
dangers of meddling with nature. Are
other tropical lakes equally vulnerable
to this kind of disturbance? Strong
superficial similarities among the cichlid
faunas of Lake Victoria, Tanganyika,
and Malawi suggests that
they are (Witte 1984). On the other
hand, comparison of different facets
of biodiversity in the East African
lakes reveal properties unique to each
fauna (Table 2). These differences are
worth exploring in terms of their implications
for conservation biology.
Lake Malawi has the highest species
diversity of the three great lakes.
Lake Tanganyika's cichlids exhibit
the greatest variation in body form, as
well as the greatest diversity in reproductive
behavior. Lake Tanganyika
also has the highest phyletic diversity-the
greatest number of distinct,
ancient lineages, each of which gave
rise to independent radiations within
the lake basin (Nishida 1991). Lake
Tanganyika's great age and complex
history probably explain the high phyletic
diversity, which in turn provided
the broad morphological substratum
from which the lake's high form and
reproductive diversity are derived. In
contrast, the haplochromine flock in
Lake Victoria exhibits four characteristics
that clearly set it apart from the
cichlid faunas of Lake Malawi and
Lake Tanganyika:
* Species diversity out of proportion
to age. The cichlid fauna of Lake
Tanganyika could be anywhere from
3 to 100 times older than that of the
Lake Victoria, yet Lake Victoria has
(or had) two to five times as many
haplochromine species as Lake Tanganyika.
The implied rates of speciation
in Lake Victoria are phenomenal
regardless of whether one estimates
age on the basis of tectonic, sedimentary,
or molecular clues.
* High functional diversity. The Victorian
haplochromines exhibit behavioral
and dietary specializations
comparable to those of the Malawian
and Tanganyikan cichlids. In fact,
Lake Victoria shares guilds with Lake
Malawi that are absent in Lake
Tanganyika. The best example is th
functional group of piscivores know
December 1992
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I I I I I I I I l
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Resolutions of the Workshop on People, Fisheries,
Biodiversity, and the Future of Lake Victoria
After careful consideration of available evidence, based on research results, we conclude that the environment of
Lake Victoria is changing rapidly and the fishery is unsustainable at its present composition and yield. Due to
overfishing and the development of an export market, the amount of fish protein now available for local
consumption is inadequate. The changing condition of the fishery will affect the social and economic welfare of
millions of people in the Lake Victoria basin. Prospects for the future are cause for grave concern and reason for
immediate action designed to resolve key unknowns and establish research and management policies that can be
developed and applied expeditiously. The resolutions presented below outline the urgent need to understand
current changes in limnology and the environment, to understand how fish biology affects the sustainability and
management of the fishery, and to understand and mitigate the social and economic impacts of anticipated changes
and varied management options.
General recommendation
It is necessary to form a Lake Victoria Fisheries Commission to harmonize research and management strategies
for the Lake Victoria Basin. An important scientific function of the commission should be to foster close
international cooperation, standardization of research and management methods, cross-calibration of scientific
instruments, and continuity of monitoring.
Limnology and environment
* There is need to develop a general ecosystems model of Lake Victoria that includes the physical, chemical,
biological, and human factors required to understand and predict lake productivity.
* Due to increased oxygen depletion, loss of fish habitat and fish kills are extensive. There is urgent need to
understand the controls on oxygen distribution and levels in Lake Victoria, including the influence of low levels
on fish stocks.
* Tremendous alterations in the food web have occurred in the last 30 years. Understanding the effects of these
changes on water quality and lake productivity requires determination of the flows of nitrogen, phosphorus, sulfur,
silicon, oxygen, and carbon into the lake ecosystem.
* There is need to determine the energy flow through the two major trophic pathways (grazing and detritus) that
couple fish productivity with primary production.
* A research and management program for the wetland and forest habitat in the riparian zone of the lake and its
waterways should be undertaken, with special attention to the water hyacinth.
Fish biology
* We must quantify interactions among food-web structures, water quality, and life-history characteristics of
fishes in order to understand the relationship between fish production and the causes of eutrophication and anoxia
in Lake Victoria.
as baby suckers, or paedophages
(Wilhelm 1980). Paedophages are
predators with large mouths, thick
rubbery lips, and odd, stout, outward-pointed
anterior teeth. Paeodophages
take the head end of brooding
females into the mouth, suck out
the eggs or young, and then eject the
spent female to eventually produce
another meal.
* Low phyletic diversity. The entire
Victorian haplochromine flock is descended
from a single ancestral stock,
no more than 225,000 years ago
(Meyer et al. 1990). Lake Malawi may
be monophyletic, but it has had time
to produce several distinct descendent
lineages. Lake Tanganyika's cichlids
are definitely derived from a diverse
ancestry (Nishida 1991).
* Low form diversity. The high species
and functional diversity of Victorian
haplochromines is out of proportion
to their limited variation in body
form. The low form diversity greatly
complicates taxonomic work with
these organisms, though data on both
behavior and morphology support
their validity as biological species
(Hoogerhoud et al. 1983).
These attributes make the Victorian
radiation especially important to
evolutionary biologists, but they also
raise a red flag. Species that share so
much in genetics and outward appearance
may also share vulnerabili-
ties.
No matter how adaptable they may
be individually, all of the Victorian
haplochromines are likely to share
many of the same weaknesses. Just as
genetic diversity among individuals
may be critical to the long-term survival
of a species, the genotypic and
phenotypic diversity among species
could provide a community with some
hedge against mass extinction and
ecological collapse. If so, Lake Victoria
may have been unusually vulnerable
to the dramatic events that have
overtaken it.
BioScience Vol. 42 No. 11854
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* There is continuing need for stock assessment such as that now underway with support from the European
Community. Such assessments are necessary to guide ecosystems modelers, policy makers, fisheries investors,
and fisheries managers.
* Refuge areas (fish parks) should be established to protect the diversity of the native fish species and maintain
spawning stocks of commercially important species.
* The possible contribution of aquaculture and management of alternative fisheries to the fishery industry should
be fully explored, including their potential for restoring and rehabilitating indigenous stocks.
* Proposals for the introduction of exotic flora and fauna to the Lake Victoria ecosystem should be submitted
to the Lake Victoria Commission for evaluation. Such proposals must be treated with great caution and
rigorously evaluated with respect to biological, economic, and social impacts. Introductions must meet with the
approval of all three riparian nations. Management of species already introduced is essential to the welfare of
existing fisheries.
Policy, management, and socioeconomics
* The quality of runoff water from the land is deteriorating and may affect fish habitat. Fishery scientists should
define water quality requirements of the fishery and work closely with limnologists in drafting overall guidelines
for the water quality of Lake Victoria.
* Proposals for fisheries planning and development should be submitted to the Lake Victoria Commission for
detailed social and environmental impact assessment before implementation. All proposals must be cognizant
of the changing patterns of ownership and the control of harvesting and processing facilities. They should also
analyze the differential incomes received by the various participants in the fishery industry.
* The fisheries planning process needs to be integrated with the development of information systems
encompassing regional, social, economic, and biological data. Programs should be developed to assist those
groups most likely to suffer the effects of predicted declines in the current fishery.
* Research should identify the factors that facilitate or impede local participation in fisheries management, and
fisheries officials should be familiarized with programs in other countries that involve local fishers and fisheries
management.
* The supply of fish within the lake basin available to local populations is insufficient to meet their needs. Studies
should be conducted to determine if the expansion of the Nile Perch fishery and export marketing have
contributed to reduced dietary protein and malnutrition in local people.
* An economic assessment of the value of the native fauna is needed, taking into account their value for food,
aquarium trade, medicine, and tourism.
* The feasibility of levying a tax on fish exported from the basin to support the research, management, and
monitoring of the fisheries should be determined. Proceeds should be directed initially toward the effective
implementation of regulations on fishing gear, methods, and allowable catch.
Workshop was held in Jinja, Uganda, 17-20 August 1992. It included scientists from Uganda, Tanzania, Europe, and North America. The
meeting was sponsored by the US National Science Foundation.
What can be done?
Talk of large-scale manipulation to
improve or "rescue" Lake Victoria
never fails to fuel heated debate, because
not everyone regards management
of severely damaged ecosystems
as a practical enterprise. At the rate
that tropical ecosystems are deteriorating,
however, it would behoove us
to begin to think seriously about man-
agement.
There is some room for optimism.
Huge Lake Erie, declared "dead" only
30 years ago, is once again an important
recreational and fishery resource.
The success in Lake Erie is empowering,
so long as it is understood that
Lake Erie was rehabilitated, not restored;
the present biotic assemblage
is dominated by introduced species. In
addition, the story of Lake Erie is not
finished. The introduction and spread
of the exotic zebra mussel (Roberts
1990) is a new challenge.
To do nothing at all in Lake Victoria
would clearly be foolish, because
there are genuine threats facing the
people of the lake basin. Most immediate
is the threat to the fishery posed
by the lake's hypoxic bottom waters
and physical instability. There have
been many local fish kills caused by
mixing events that have trapped fish
populations in oxygen-depleted water.
Exploration with remote-operated
vehicles has shown that the fish
kills seen on the surface are even more
extensive on the lake bottom at 30 m
and deeper. Thus, a large-scale fishery
collapse from a major mixing event is
a real possibility. Another threat may
come from sharp reductions in the
abundance of snail-eating fishes,
which will probably affect the prevalence
of bilharzia. This effect can be
complex, either raising or lowering
the incidence of disease according to
the age distribution of the snails.
Wholesale species extinctions have
made it impossible to restore Lake
Victoria, but that observation does
not rule out an attempt to rehabilitate
it. Cautious management of the waDecember
1992 855
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tershed, the lake ecosystem, and the
fishery could have positive effects.
Much more must be known about the
lake ecosystem before any sophisticated
manipulations can be attempted,
but meanwhile there are numerous
actions that can bring definite benefits
to the people of the region.
The identification and reduction of
anthropogenic nutrient inputs is one
potential action. This challenge could
be combined with afforestation
projects with indigenous species to
rebuild some of the native forests and
their biodiversity, reduce materials
and contaminants in runoff, and help
restore the dwindling supply of firewood.
Careful management of the
Nile perch population could aid both
the Nile perch fishery and the survival
of native species. The establishment
of separate management areas for
native and introduced species could
be a useful approach, because there do
appear to be distinct refugia, or at
least geographically circumscribed
remnants of the native fauna. Native
food fishes, if kept from extinction,
can certainly be restored to the marketplace
through aquaculture.
Many of the native species are of
great value, and their remnant populations
are potentially significant economic
assets. For example, the anadromous
ningu (Labeo victorianus) lends
itself well to hatchery-style aquaculture
and in situ restoration efforts,
much like those employed for North
American trout and salmon species.
The ngege, though smaller and slower
growing than the introduced Nile tilapia,
is reputedly the most delicious
tilapia species in the world. Its genome
is a prize for the tilapia aquaculture
industry, whose products are
derived from selective breeding and
hybridization of wild gene pools. The
ngege is also one of the most specialized
phytoplanktivores among the East
African lake fishes; it could someday
assist in cropping algal blooms in Lake
Victoria.
Similar arguments can be made for
the lake's haplochromines. They can
be eaten (though never among the
most desirable speciest, they were locally
important both for food and
medicinal use), help control diseasecarrying
snails and insects, serve as
forage for larger edible fishes, and
offer intriguing possibilities for
polyculture because of their functional
diversity.
Perhaps the greatest opportunities
for positive change come from development
projects now slated for the
lake basin. Though traditionally
thought of as harbingers of further
devastation, plans to expand industry
or tourism can, if properly planned
and executed, bring great benefit to
the local people, to the investors, and
to the long-term security of biodiversity
and the environment.
An excellent example of this potential,
as well as the potential for disaster,
is a project planned for Uganda's
Sesse Islands. More than 80 small
islands and islets located in the lake's
northwest corner (Figure 1), the Sesse
Islands, are still largely forested and
are an important conservation area
for flora and fauna, both terrestrial
and aquatic. They would be an ideal
location for experiments in sustainable
development, habitat conservation,
and species restoration. Whether
the investors can muster the wisdom,
the foresight, and the morals to conduct
this project in an exemplary
manner remains to be seen. If they
fail, Uganda will lose one of the last
shreds of the Lake Victoria Basin still
in a reasonably intact state.
There is at least an even chance of
maintaining a diverse, productive, and
healthy tropical ecosystem in Lake
Victoria, drawing on both native and
introduced species as building blocks.
The details of the new system depend
on the ingenuity of restoration biologists,
multinational cooperation, and
socioeconomic vision. Most important,
those involved must be prepared
to accept the fact that some aspects of
the system will be highly nonlinear or
totally unpredictable and therefore
beyond control.
There is no lack of dissenting views.
Colleagues and proposal reviewers
have stated that Lake Victoria should
be written off so that scarce resources
can be focused on conservation in
Lakes Malawi and Tanganyika. This
idea is not popular around Lake
Victoria. It is considered in the same
vein as the view that AIDS is beneficial
because it will solve the problem
of overpopulation in the region. The
continued presence of Nile perch in
the lake raises obvious doubts about
reintroduction, but data from Tanzania,
Kenya, and Uganda indicate that
at least 10% of the original fauna in
the well-sampled nearshore region is
persistent and sympatric with high
Nile perch densities. It would pay to
learn which taxa are resistant to extinction
in the lake as it is today, why
they are resistant, and what other
native species might with some assistance
also have a chance to survive.
To the scientist, the haplochromines
of Lake Victoria have a great deal to
teach about both speciation and extinction.
To the conservationist, the
remnants of the native fauna are the
building blocks for attempting to reconstruct
the Lake Victoria ecosystem.
We echo Aldo Leopold's advice
to intelligent tinkerers: do not throw
away the pieces. These pieces are the
most critical of all resources for the
future, provided that a safe, productive
context for them can be created
in a future ecosystem.
In 1987, the New England Aquarium
proposed programs to save the
pieces of three disintegrating fish faunas
through captive breeding: Lake
Victoria, North American desert
fishes, and Appalachian stream fishes
(Kaufman 1987). Lake Victoria was
chosen as the first trial project, and
today the program engages the efforts
of more than 30 institutions, led by
the New England Aquarium and the
Columbus Zoo aquarium.
Captive breeding programs only
make sense as part of in situ conservation
efforts (Kaufman 1987, 1989,
Reid 1990). Ironically, their greatest
contribution may be in acting as a
wedge to build awareness and garner
institutional and grant support for
programmatic work, such as that of
the Lake Victoria Research and Conservation
Program. Certainly the fauna
rescue and captive breeding program
for Lake Victoria has had this effect,
even though it had its humble beginnings
in an attempt to maintain an
array of Victorian haplochromines as
a living archive for research purposes,
an effort initiated in the mid-1980s by
researchers at the University of Leiden
and the British Museum. The Lake
Victoria Research and Conservation
program picked up where the earlier
research left off.
Now it is possible to contemplate
using rescued haplochromines in
polyculture and reintroduction. Furthermore,
the experiences with the
Lake Victoria breeding program can
inform efforts to conserve freshwater
BioScience Vol. 42 No. 11856
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fish species around the world, the
focus of a workshop held in Columbus,
Ohio, in October 1992. Hopes,
are running high for coordinated ecosystems,
fisheries, and resource management
planning for Lake Victoria
as a whole.
The Lake Victoria Research and
Conservation Program has focused
on three objectives: to understand
biological processes in the lake and
its watershed well enough to offer
management alternatives to decision
makers in the three countries; to track
and where possible prevent the extinction
of native species; and to restore
endangered food fishes to the
marketplace through aquaculture. In
August 1992, scientists, policy makers,
and government administrators
met on the Ugandan shoreline of Lake
Victoria to discuss the condition of
the lake and the further steps that
might be taken (see box page 854).
Preservation of virgin natural ecosystems,
the world's "last great places"
in the words of The Nature Conservancy's
celebrated program, is a
major preoccupation of conservationists,
and deservedly so. But perhaps
this approach is too limited to the
situation of the moment. Lake Victoria
may inspire some attention to salvaging
orphaned pieces of ecosystems
that can never again be precisely as
they were.
Conservationists scan the scars of
civilization for those places where the
odds and the payoff for biological
salvage and reconstruction are the
greatest. On land, such a place may be
Madagascar, Hawaii, or the dry forest
of Costa Rica (Janzen 1986). On
water, it is without question Lake
Victoria. The value of conservation
efforts, even in badly damaged ecosystems,
should not be doubted. Anything
is better than waiting 20 millennia
for a new fauna, or a new source
of food, to evolve.
Acknowledgments
Seed funds for the Lake Victoria Research
and Conservation Program were
provided by the New England Aquarium
for coordination and public education,
the National Undersea Research
Program for hydrology and
visual exploration by remote vehicles,
the National Science Foundation for
preliminary faunal survey and research
planning, the Environmental Protection
Agency for rescue and propagation
of endangered native food fishes,
the Institute for Museum Services for
the haplochromine captive breeding
program, US-AID and IDRC of
Canada for limnological studies, and
the Conservation Food and Health
Foundation for work by African colleagues.
The author is grateful to the
Pew Scholars Program for Conservation
and the Environment for core
support. The data and insights presented
are the result of a team effort,
led in particular by P. Ochumba, J.
Ogari, and E. Okemwa (Kenya) and
F. Bugenyi, R. Ogutu-Ohwayo, P.
Basasibwaki, and B. Kudhongonia
(Uganda), with major contributions
by W. Cooper, M. Gophen and R.
Hecky, P. Kilham, and R. Robinson.
The author thanks P. H. Greenwood,
F. Witte, R. Lowe-McConnell, R.
Welcomme, and five reviewers for
their stimulating comments. We applaud
the efforts of our other colleagues
and friends of the fisheries
research institutes of Kenya, Uganda,
and Tanzania, who have worked for
so long, with so little, to understand
Lake Victoria and brings its benefits
to their people and the world. This
article is dedicated to the memory of
Peter Kilham.
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