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Impact of the baIted hook fishery on the recovering endemic fish species in Lake Victoria

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As gillnet yields of Nile perch stagnated in the middle of the 1990s, the hook and line fishery intensified in Lake Victoria. Frame surveys show that the number of hooks used in this fishery grew tenfold in the Tanzanian waters between 1992 and 2002. The increased use of endemic fish species as bait has gone largely unnoticed, but is thought to impede the recovery of these species in the lake. A survey was conducted in March 2003, covering 51 sites, to assess the impact of this fishery targeting Nile perch on the endemic species. The species used as bait were juvenile Clarias (41%), haplocromines (37%), dagaa (Rastrineobola argentea) (10%), Synodontis (3%), Mormyrus (2%), and Labeo (1%). The total annual weight of the mostly undersized and illegally caught (beach seining, gillnets below 3″ mesh) baitfish was estimated at 7,465 t. The size of the Nile perch caught is bait-dependant, with catches using Clarias bait having a modal length at 61–62 cm TL of which 20% is above the legal upper size limit. Catches with haplochromines and dagaa bait have a mode at 49–50 cm TL, with 62% below the legal lower size limit. Seasonal variations in the fishery and management implications of its legalization are discussed, while the threats to biodiversity conservation and the stability of the ecosystem are addressed.
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Impact of the baited hook fishery on the recovering endemic fish species in
Lake Victoria
Oliva C. Mkumbo a; Enock Mlaponi b
a Senior scientist, Lake Victoria Fisheries Organization, Jinja, Uganda b Tanzania Fisheries Research
Institute, Mwanza, Tanzania
To cite this Article Mkumbo, Oliva C. and Mlaponi, Enock(2007) 'Impact of the baited hook fishery on the recovering
endemic fish species in Lake Victoria', Aquatic Ecosystem Health & Management, 10: 4, 458 — 466
To link to this Article: DOI: 10.1080/14634980701704197
URL: http://dx.doi.org/10.1080/14634980701704197
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Impact of the baited hook fishery on the recovering
endemic fish species in Lake Victoria
Oliva C. Mkumbo1,and Enock Mlaponi2
1Senior scientist, Lake Victoria Fisheries Organization, P.O. Box 1625 Jinja, Uganda
2Tanzania Fisheries Research Institute,P.O. Box 475, Mwanza, Tanzania
Corresponding author: ocmkumbo@lvfo.org
As gillnet yields of Nile perch stagnated in the middle of the 1990s, the hook and line fishery intensified
in Lake Victoria. Frame surveys show that the number of hooks used in this fishery grew tenfold in the
Tanzanian waters between 1992 and 2002. The increased use of endemic fish species as bait has gone largely
unnoticed, but is thought to impede the recovery of these species in the lake. A survey was conducted in
March 2003, covering 51 sites, to assess the impact of this fishery targeting Nile perch on the endemic
species. The species used as bait were juvenile Clarias (41%), haplocromines (37%), dagaa (Rastrineobola
argentea) (10%), Synodontis (3%), Mormyrus (2%), and Labeo (1%).
The total annual weight of the mostly undersized and illegally caught (beach seining, gillnets below 3
mesh) baitfish was estimated at 7,465 t. The size of the Nile perch caught is bait-dependant, with catches
using Clarias bait having a modal length at 61–62 cm TL of which 20% is above the legal upper size limit.
Catches with haplochromines and dagaa bait have a mode at 49–50 cm TL, with 62% below the legal lower
size limit. Seasonal variations in the fishery and management implications of its legalization are discussed,
while the threats to biodiversity conservation and the stability of the ecosystem are addressed.
Keywords: endemic, Nile perch, artisanal, management, baitfish, Clarias
Introduction
The original fish fauna of Lake Victoria com-
prised over 28 genera with more than 350 species.
Cichlids formed the majority of these, with about
300 haplochromine and the two tilapiine species
Oreochromis esculentus and O. variabilis (Graham,
1929; Greenwood, 1974, 1981; Witte and Van Oijen,
1990). In the 1950s, the fishery targeted the tilapiines
and Labeo victorianus (Worthington, 1933; Cadwal-
ladr, 1965; Fryer, 1973). Other local fisheries existed
for Protopterus aethiopicus,Bagrus docmak, Clar-
ias gariepinus and species like Barbus altianalis and
Mormyrus kannume (Garrod, 1961). In 1969/1971,
haplochromines constituted 80% of the estimated
750,000 t fish biomass of Lake Victoria (Kudhon-
gania and Cordone, 1974).
In order to compensate for the dwindling abun-
dance of the endemic tilapia species, Tilapia
melanopleura,Tilapia zilii, Oreochromis leucostic-
tus and Oreochromis niloticus were introduced into
the lake (Welcomme, 1968). Nile perch was also
introduced in the late 1950s and the early 1960s
(Hamblyn, 1961; Arunga, 1981; Welcomme, 1984).
Catch rates and total yields continued to decrease for
the next twenty years.Haplochromines were the only
under-fished stocks, due to their small size and bony
texture (Scully, 1975). The decrease in the catch
rates in the late 1960s led to the introduction of a
number of additional gears such as small-meshed
458
Aquatic Ecosystem Health & Management, 10(4):458–466, 2007. Copyright C
2007 AEHMS. ISSN: 1463-4988 print / 1539-4077 online
DOI: 10.1080/14634980701704197
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Mkumbo and Mlaponi /Aquatic Ecosystem Health and Management 10 (2007) 458–466 459
gillnets (1.5–2 inches) which caught smaller fish
such as Synodontis spp., Schilbe intermedius,
Barbus profundus,Brycinus spp. and the hap-
lochromines (Marten, 1979; Scully, 1975). Beach
seines were increasingly used in the early 1970s
to catch the haplochromines, but also caught large
numbers of spawning tilapiines and their juveniles
(Marten, 1979), escalating the effects of overfish-
ing. Simultaneously, light fishing for dagaa (Rastri-
neobola argentea) developed in Tanzania and Kenya
(Marten, 1979; Okedi, 1981). This fishery uses very
small mesh sizes (8–13 mm), detrimental to juve-
niles of both haplochromines and tilapiines. The use
of baited hooks also increased, but received little at-
tention. By 1987, Nile perch contributed 60% to the
total yield in Tanzanian waters and haplochromines
less than 10% (Ligtvoet et al., 1988; Bwathondi,
1990; Witte et al., 1995). Although Nile perch
preyed predominantly on haplochromines, all other
species were also recorded in its diet (Gee, 1964,
1969; Hamblyn, 1966; Okedi, 1970; Ogari, 1988;
Hughes, 1986; Ogari and Dadzie, 1988; Ogutu-
Ohwayo, 1990; Mkumbo and Ligtvoet, 1992).
From 1995, heavy fishing for Nile perch was
beginning to take its toll as water quality param-
eters showed some improvement (Lipiatou et al.,
1996; Mkumbo, 2002). As a result, a number of en-
demic species were recovering (Bayona et al., 1998;
Mkumbo and Ezekiel, 1999; Katunzi et al.,2003).
Currently, these endemic species are in high de-
mand as baitfish for the long-line fisheries, and it is
feared that their recovery may be short-lived. This
practice may disrupt the trophic interactions (Hart,
1995) and efficiency of the ecosystem as they are
feeding on the lower levels of the food web (Wanink,
1998). The mixed trophic impact routine (ECO-
PATH II, Moreau, 1995) showed that a decrease of
the biomass of these recovering species may result in
an increase in zooplankton and phytoplankton and
worsen the eutrophication problem in the lake. At
the present time, there is a considerable commercial
fishery for haplochromines during the full moon us-
ing dagaa seines (Mkumbo, LVFO, Jinja, Uganda,
personal observation in Speke Gulf). This study at-
tempts to assess the impact of the bait fishery on the
native stocks and recommends management strate-
gies and/or alternative sources of bait (Mkumbo and
Mlaponi, 2003).
Study area and data collected
This study was conducted at 48 sites, 45 of which
are landing beaches on Lake Victoria; three ad-
Figure 1. Map showing landing sites visited and locations along
river tributaries where bait fish was collected.
ditional sites were located along river tributaries,
where bait was collected (Figure 1).
To assess the extent of the hook fishery, the total
number of fishing canoes and the number of canoes
targeting the resurging species, the type and source
of bait and the means used to catch the bait were
recorded as well as the hook size. Beach Manage-
ment Unit (BMU) leaders and long-line fishermen
were interviewed. Their general perception of the
sustainability of the bait and long-line fishery was
also assessed.
To determine the population structure of the catch
and to assess the performance of the fishery, the
length of individual fish was measured in cm TL
and total weight was recorded where possible. A
weighing scale and a measuring board were used.
Fishermen were interviewed in order to determine
the types and quantities of bait used for each canoe
landing, as well as the frequency of fishing, using
the specified baits. This information was later used
to work out the average number of baitfish used per
canoe and per week.
Results
Estimation of the importance of the bait
fishery in the Tanzanian waters
of Lake Victoria
The fisheries statistics of Lake Victoria, Tanza-
nia are partly based on the results of Frame Surveys.
Reports of these surveys show a marked and contin-
ual increase in the numbers of fish hooks used be-
tween 1992 and 2002 (Table 1). The first enormous
increase was observed in the middle of the 1990s
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460 Mkumbo and Mlaponi /Aquatic Ecosystem Health and Management 10 (2007) 458–466
Tab l e 1 . Trend in the hook and line fishery, 1992–2002 (Source: Fishery Statistics, Tanzania).
Year 1992 1995 2000 2002
n. hooks 369,444 1,419,687 2,212,571 c.4,000,000
% increase 284% 56% 81%
as gillnet yields decreased and fishermen adopted
additional long-line fishing.
A total number of 4,684 fishing canoes were
recorded on the 45 landing beaches surveyed. These
represent about 30% of all fishing canoes (15,489
canoes) recorded during the Frame Survey of 2000.
From the sampled canoes, 54% (2,552 canoes) took
part in the baited hook fishery. In prominent landing
sites for long-line fishing, 80–90% of all the canoes
use baited hooks.
The 83 long-line fishermen interviewed reported
to use 150–2000 hooks per canoe. In total 48,022
hooks were recorded targeting Nile perch and us-
ing endemic species as bait, with approximately 600
hooks per canoe. The hook size used depends on the
size and type of bait and ranges from 5 to 13. The
most commonly used were sizes 9 to 11; sizes 5 7
are used with Mormyrus and Labeo baits from pad-
dled canoes. Sometimes, as little as size 1–3 hooks
are used.
Types and sources of bait
Bait species
Juveniles of Clarias species rank first as bait for
the Nile perch hook fishery (used by 41% of the ca-
noes). These included juvenile Clarias gariepinus,
Clarias alluaudi and Clarias wermeri. They are used
mostly during the dry season (May to October). At
some sites and in most of the islands they are used
throughout the year.
Haplochromines from the rocky areas, mostly
Haplochromis nyererei,Neochromis nigricans, and
Paralabidochromis ‘yellow rockpicker’ and those
from the inshore areas, which include H. nigricans,
H. laparogramma and H. microdon,contributed
about 37% of the bait reported in the hook fishery
for Nile perch. These are used whenever Clarias is
not available and in the minor landing sites.
Dagaa (Rastrineobola argentea)was also re-
ported to be used as bait for the Nile perch fishery
with 10% frequency, but only when the other two
bait species are not available.
Other endemic species are also used but only
depending on their availability. Synodontis species
were recorded (3%). Mormyrus kannume (2%) and
Labeo victorianus (1%) were recorded at beaches
with adjacent rocky areas or rocky islands. At only
one landing site, chopped pieces of Oreochromis
niloticus and chicken were reported to be used.
Source of baitfish and collection techniques
Juvenile Clarias are collected in almost all
water bodies where they occur naturally, while
some reportedly originate from aquaculture abroad.
Beaches bordering Kenya received additional bait
from across the border. On the eastern and south-
ern sides of the lake, Clarias baitfish are collected
in main rivers, their tributaries and swamps as well
as in seasonal rivers. Sometimes, passenger seats
are removed from minibuses which transport 70–80
30-litre buckets, holding about 1000 baitfish each.
At some beaches juvenile Clarias are reported
to be transported from hatcheries in Rwanda and
Uganda. Trucks from Rwanda reportedly carry up
to 200 containers of 30 litres (referred to as diabas),
each with up to 1000 baitfish. These were to be dis-
tributed by canoe to different sites. Six to eighteen
diabas were reported to be transported daily across
the Nyakaliro ferry, each with 1000 baitfish, except
during heavy rains and very dry spells. By 1998, 70
to 80 20 litre buckets, each holding about 500 fish,
were received on a daily basis. Traps, cloth like bed
sheets, mosquito seine nets or handpicking are used
to catch/collect baitfish. These are kept in 20 or 30
litre buckets of which the water is exchanged once
aday. They are fed biscuits and can stay alive while
waiting to be used.
Haplochromines are collected from the main lake
either by using beach seines or gillnets of 3
/
3-1 mesh
sizes or by handlining in the rocky areas, using earth-
worms and Caridina nilotica.
Synodontis species are collected from bays on
the Tanzanian side and from rivers and swamps
in Kenya. Beach seines and small meshed gillnets
are used in the rainy season along almost all river
mouths.
Mormyrus and Labeo are collected by gillnets of
11
/
2–3
 mesh size from the main lake in areas with
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Mkumbo and Mlaponi /Aquatic Ecosystem Health and Management 10 (2007) 458–466 461
submerged rocky substratum or near rocky islands,
mainly at the height of the rainy season.
Size range, numbers and estimated weight
of the endemic baitfish species used
Clarias
Sizes range from about 11 20 cm TL, with
weights of 8 34 g and a mean weight of 17.5 g.
Fishermen report that only half of the hooks in a ca-
noe would be baited with Clarias, due to scarcity of
the baitfish and its cost (they are sold at TShs. 70/=
to 120/=).
With an approximate number of hooks of
4,000,000, estimating that 41% of all baitfish used
is Clarias and only half of the hooks in a canoe be-
ing baited, the total number used per day would be:
4,000,000 ×0,41 ×0,5 =820,000 Clarias indi-
viduals. Full-time fishing takes place in the period
from May to October each year. The total estimated
number of Clarias used in a year would then be: 6
months ×30 days ×820,000 Clarias =147,600,000
individuals, and their total weight 2,583 tons.
Haplochromines
Sizes used range from 7.8 cm to 11.8 cm TL,
with weights of 7.0 g to 26.0 g and a mean weight
of 15.85 g.
When using haplochromines, all the hooks are
baited, but fishermen alternate, collecting bait one
day and fishing the next day. At an estimated fre-
quency of use of 37%, the total number of hap-
lochromines baitfish used in a day is: 4,000,000
hooks ×0,37 =1,480,000. Assuming that hap-
lochromines are used in the six months when Clarias
is not used, the total numbers of haplochromine bait-
fish per year would be: 1,480,000 ×6 months ×30
days =266,400,000; this represents a total weight
of 15.85 g ×266,400,000 =4,222.44 tons.
Dagaa
Little direct information is available on the use of
dagaa as baitfish. Their lengths are known to range
from 21 mm to 76 mm TL, which, according to
the relationship W =0.00007L2.4375corresponds to
weights of 0.072 g to 4.56 g, with a mean weight of
1.54 g. The frequency of use of dagaa as baitfish is
about 10% and its duration 4 months, but only during
dark moon days, as they are fished with light. The
estimated annual number used would therefore be:
4,000,000 hooks ×0.10 ×4 months ×17 days =
27,200,000 fish, weighing 27,200,000 ×1.54 g =
41.89 tons.
Other species
Synodontis are reportedly used at 3% and only
once a week during the rainy season, so the estimated
number of fish used is 0.03 ×4,000,000 hooks ×5
months ×4days=2,400,000 individuals per year.
At an estimated mean weight of 200 g, the biomass
estimate is =2,400,000 ×200 =480 tons yr1.
The size range of Mormyrus used was 27 to 34
cm TL, weighing 520 to 1000 g, averaging 737 g.
Only 2% of the canoes use Mormyrus as bait, and
only 1 hook out of 60 is baited with this species, as
canoes in this fishery do not employ more than 10
hooks. Assuming 4 days of lightning and thunder
per (rainy) month, the number of baitfish and their
weight can be estimated as: 0.02 ×4,000,000 ×1/60
hooks ×5 months ×4days=26,667 individuals,
weighing 26,667 ×737 g =19.64 tons yr1.
The frequency of utilisation of Labeo was only
10% and like the latter species only during the
stormy/rainy season. Estimates, therefore, are 0.10
×4,000,000 ×1/60 hooks ×5 months ×4days=
13,333 individuals. At a mean weight of 500 g, they
weigh 13,333 ×500 =6.67 tons yr1.
Total estimated biomass of the endemic
species used as bait
The combined estimates calculated for the indi-
vidual species groups (Clarias 2,583 t yr1, hap-
lochromines 4,222.44 t yr1,Dagaa 41.89 t yr1,
Synodontis 480 t yr1,Mormyrus 19.64 t yr1and
Labeo 6.67 t yr1) show that about 7,464.86 t of the
endemic species are used annually as bait in the Nile
perch hook fishery. Excluding dagaa, about 7,422.97
tyr
1of the threatened endemic species are, there-
fore, used in the bait fishery.
Population structure of Nile perch caught
in the bait fishery
Length frequencies of Nile perch in the catch
was only recorded for three bait species, Clarias
spp., haplocromines and dagaa. Their combined fre-
quency distribution shows a mode at 55–56 cm TL,
while 49% of the catch is below 50 cm TL and 18%
is above 85 cm TL (Fig. 2a). The slot-size under
regulation is 50–85 cm TL.
Separated by baitfish type, Nile perch caught us-
ing Clarias had a mode at 60–61 cm TL (Fig. 2b),
with 41% of the catch below 50 cm TL and 20%
above 85 cm TL. Haplochromine and dagaa baitfish
caught relatively smaller Nile perch with a mode of
49–50 cm TL and 61% of the catch below 50 cm
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462 Mkumbo and Mlaponi /Aquatic Ecosystem Health and Management 10 (2007) 458–466
Figure 2. Length frequency distribution of Nile perch caught
with (a) all bait species (b) Clarias bait (c) Haplochromine bait
and (d) Dagaa bait.
TL, and only 7% (dagaa) 12% (haplochromines)
was above the 85 cm TL (Figures 2c and 2d). It
was not possible to get enough length distribution
samples for the other baitfish species Synodontis,
Mormyrus and Labeo,but fishermen report catch-
ing bigger Nile perch with these than when using
Clarias bait.
Other fisheries impacting
the endemic species
Other fisheries exist which target the endemic
species for domestic consumption and not for use
as bait. A hook and line fishery using earthworms,
haplocromines and pieces of washing soap targeting
Figure 3a. Length frequency distribution of C. gariepinus from
long lines.
Clarias gariepinus was also recorded. The size range
of the catch is shown in Figure 3a, with a mode below
39 cm TL.
Gillnets with mesh sizes below 5 inches (with
a majority of 21/2) catch a variety of the endemic
species in sheltered bays or river mouths. These in-
clude Clarias gariepinus with a mode at 38 cm TL
(Figure 3b), Oreochromis esculentus with a modal
length below 18 cm TL (Figure 3c) and lungfish
(Protopterus aethiopicus) with mode below 62 cm
TL (Figure 3d). Other species encountered in the
gillnet catches were Schilbe intermedius,Brycinus
sadleri and B. jacksonii,Mormyrus kannume, Syn-
odontis victoriae, Synodontis afrofischeri and Labeo
victorianus.
Discussion
Status of the baited hook fishery
in the Tanzanian part of Lake Victoria
The decline of species diversity in Lake Victoria
is usually associated with the increased predation of
Nile perch, overfishing and environmental change
(Goldschmidt and Wanink, 1993; Acere, 1988), but
the impact of the baited hook fishery had so far been
completely overlooked. This hook fishery was of lit-
tle commercial importance before 1994 and targeted
Figure 3b. Length frequency distribution of C. gariepinus from
gillnets.
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Mkumbo and Mlaponi /Aquatic Ecosystem Health and Management 10 (2007) 458–466 463
Figure 3c. Length frequency distribution of O. esculentus from
gillnets.
only the tilapiine species. The increased demand for
Nile perch led fishermen to diversify their fishing
techniques and since then there has been a continu-
ous increase in the number of hooks in the Tanzanian
part of Lake Victoria (Ligtvoet et al., 1995). In this
period, the number of fish processing factories in-
creased to five (Gibbon, 1997; SEDAWOG, 1999;
Medard, 2002). With the increase in gear theft, es-
pecially gillnets which are more expensive, some
fishermen have permanently engaged themselves in
hook fishing, despite the seasonal differences in the
availability of fish to different gears and in the sur-
vival time of baitfish. Some prominent fishermen
own both gillnets and hooks, which are used inter-
changeably during the periods of high catches of the
respective gear. Currently, 54% of all the canoes on
Nile perch fishery are using baited hooks.
The number of hooks actually used depends on
the availability of baitfish and the experience of the
baiter, as the time needed to set the hooks depends on
experience. However, due to scarcity of baitfish, fish-
ermen are now forced to use even dead specimens.
Following the imposition of a lower (and upper) size
limit in the Nile perch catch, fishermen currently
use hooks of numbers 7–11, as the smaller hooks of
numbers 12 and above catch small-sized Nile perch.
The small sized hooks, numbers 13 and above, are
Figure 3d. Length frequency distribution of P. aethiopicus from
gillnets.
only used for Oreochromis niloticus, sometimes us-
ing haplochromines as bait.
The seasonality observed in the bait fishery is ex-
plained by the seasonality of the rainfall and of the
environmental parameters. Generally the dry sea-
son extends from June to September and the rainy
season from October to May, with a dry spell in
January/February (Mkumbo, 2002; Akiyama et al.,
1977). The rainy period before the dry spell is char-
acterized by light rains (October–December) while
the heavy rains are in March–May. Heavy rains are
associated with strong currents and abrupt mix-
ing. This may result in the sudden upwelling of
deep anaerobic waters, and lead to massive fish
kills (Ochumba, 1990; Kitaka, 1972). This phe-
nomenon is commonly reported in Lake Victoria
when Nile perch are found dead on the surface
after heavy, windy rains (Ochumba, 1987, 1990;
Mkumbo, 2002). Heavy rains can also cause high
turbidity, which is known to limit the shoreward dis-
tribution of some fish, avoiding the erosion of gill
filaments by accumulated sediments (Laevastu and
Hayes, 1981). This is also the period when bait fish-
ing is less practised, restricted to limited areas as the
bait dies once hooked, before a predator is attracted.
However this is the period when gillnets have high
catches as the fish are restless and migrate, thus in-
creasing their chance of encountering a net. The bait
fishery represents a latent threat to the stocks as the
estimated weight of the endemic species used repre-
sents 25% of that of the total haplochromis catch of
2000 (17,161 t), 235% of that of Clarias (1,101.5 t),
593% of Synodontis (81.4 t), 44% of Labeo (15.2 t)
and 20% of Mormyrus (99.6 t) catches of the pre-
vious year (Mkumbo 2002). These very high pro-
portions of Clarias and Synodontis in relation to the
recorded catch illustrate the potential negative im-
pact of this fishery on the recovery of these native
stocks and the high demand for these species in the
bait fishery. The threat is severe for Clarias gariepi-
nus (Lm50 =40 cm TL), as the majority of the ju-
veniles observed belong to this species. C. alluaudi
and C. wermeri originating from swamps, dams or
rivers outside the lake basin and even outside the
country, mature at much smaller sizes (Lm50 =14
cm TL and 21 cm TL respectively). The other bait
species Synodontis, Mormyrus and Labeo mature
first at respectively 9–10 cm FL, 34 cm TL and 15–
19 cm SL, and the majority of the fish observed are
within these ranges. Nevertheless, the techniques of
catching them by blocking river mouths with traps
or small meshed nets and in rocky areas during the
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464 Mkumbo and Mlaponi /Aquatic Ecosystem Health and Management 10 (2007) 458–466
rainy season interferes with spawning activities as
they are caught before releasing the eggs. This has
major implications for the replenishment and sus-
tainability of the stocks.
The import of species from outside the lake
basin poses the additional threat of introducing ex-
otics into Lake Victoria. Although this is indeed
against government regulations, traders either trans-
port them at night or bribe local authorities to
get through. With the increasing importance of the
baited hook fishery it is almost impossible to stop
the supply of baitfish. Therefore, management mea-
sures must be taken ensuring both the supply of the
baitfish and the sustainability of the stocks, taking
into consideration the risks of exotics finding their
way into the main lake.
Sizes of fish caught in the bait fishery
in relation to existing regulations
There are marked differences in the sizes of fish
caught with Clarias bait and with the other species.
The mode of the distribution (60–61 cm TL) of the
Nile perch catches from Clarias bait is within the
slot size range (50–85 cm TL), but 20% of the catch
is above the upper size limit. The latter can affect
the sustainability of the Nile perch fishery. As lit-
tle is known of its stock—recruitment relationship,
this needs to be studied before a bait fishery using
Clarias is encouraged.
When haplochromines and dagaa are used as bait-
fish, almost 61% of the Nile perch catches are be-
low the lower limit. Baitfish themselves are caught
in beach seines and gillnets of less than 3 inches,
which are all illegal gears. The Fisheries Depart-
ment could perhaps demarcate areas or bays to al-
low these gears to operate specifically for bait. This
would require considerable additional monitoring
and surveillance, as the catching and trading of ju-
venile fish are prohibited.
Conclusions
Recommendations for conservation
and management
The most used bait is Clarias spp., collected from
almost all rivers, tributaries, swamps and ponds in
the Lake Victoria watershed. They, together with
the other bait species: haplochromines, Synodontis,
Mormyrus and Labeo are either harvested immature
or their harvesting technique is illegal. There is a
serious threat of introducing exotic species into Lake
Victoria from outside the lake basin.
Any viable management measure to regulate the
hook and line fishery should take into account its
socio-economic implications for the 25,092 fisher-
men fully engaged in the bait fishery. An immediate
complete ban of hook fishing, aiming at the conser-
vation of the threatened species will be a catastrophe.
It is therefore recommended:
to start an immediate detailed investigation of
the selectivity of hooks, bait type and size so that
clear advice can be formulated to manage this
fishery;
to conduct a study of the stock—recruitment re-
lationship to monitor the impact of the significant
percentage of Nile perch above the slot size, so
as to enable early warnings of any threat to re-
cruitment overfishing;
to ensure a functioning system on Monitoring,
Control and Surveillance (MCS) in Lake Victo-
ria so that illegal fishing of juveniles, including
baitfish are controlled;
to ensure a sustainable supply of Clarias baitfish,
of which 62% of the Nile perch catch falls within
the slot size, by disseminating the technology of
culture/farming and artificial propagation of the
Clarias species endemic to Lake Victoria.
Acknowledgements
The authors wish to express their gratitude to
the Lake Victoria Environment Management Project
(LVEMP) for financial support. The Director Gen-
eral of Tanzania Fisheries Research Institute is ac-
knowledged for permission granted to the scientists
to conduct the study. The efforts undertaken by the
Project Coordinator, Fisheries Research Component
to solicit funds are much accredited. The coopera-
tion extended by all stakeholders especially long-
line fishermen, BMUs and leaders at the beaches
visited is much appreciated.
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... We also estimated the unreported bait catches for the Nile perch longline fishery (Cowx et al. 2003;Mkumbo and Mlaponi 2007). The Nile perch longline fisheries in Kenya and Tanzania are considered to be similar (P. ...
... obs.). Therefore, we used the bait to hook ratio and a modified taxonomic composition of bait catch for Tanzania's Lake Victoria longline Nile perch fishery to estimate Kenya's bait catch (Mkumbo and Mlaponi 2007; see the 'Methods' section of the Supplementary material for details). We recognise that many Kenyan fishers operate in Tanzanian and Ugandan waters, but these are still deemed Kenyan catches because international 'flag-state' rules apply to catch data (Garibaldi 2012). ...
... Cowx, pers. obs.), it was excluded from the taxonomic composition data taken from Mkumbo and Mlaponi (2007). Furthermore, since 2000, an increasing amount of bait used by Kenyan fishers consists of farmed catfish, which is not wild caught fish. ...
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Trawl surveys to assess the stocks of Lake Victoria for estimates of biomass and yield, together with the establishment of exploitation patterns, are being undertaken under the Lake Victoria Fisheries Research Project. Preliminary surveys to establish the sampling stations and strategy were carried out between October 1997 and February 1998. Three cruises to cover the whole of the Tanzanian waters were undertaken with a tota.l of 133 samnling stations. Data on catch rates, species composition and distribution were collected.
Chapter
Understanding of the functioning of a diverse ecosystem, and of the possible impacts that species changes have on the system as a whole, calls for quantification of the trophic relationships between its different groups. This is one reason why Polovina (1984) developed ECOPATH, a steady-state model of trophic interactions in ecosystems. Using ECOPATH the system is partitioned into boxes comprising species having a common physical habitat, similar diet and life history characteristics (Polovina, 1984; Christensen and Pauly, 1992). The model produces estimates of mean (annual) biomass (annual) biomass production and (annual) biomass consumption for each of the boxes in the ecosystem. The ECOPATH model assumes that the ecosystem considered is at equilibrium, which means that input to a group should equal output from it for the period considered.
Chapter
Lake Victoria, the largest tropical lake in the world (Fig. 6.1), was until recently a typical cichlid lake. Eighty percent of the demersal ichthyomass of this East African lake consisted of haplochromine cichlids (Kudhongania and Cordone, 1974a, b). The more than 300 haplochromine species, of which 99% were endemic, exploited virtually all food sources in the lake (Greenwood, 1974, 1981; van Oijen et al., 1981; Witte and van Oijen, 1990). Each species had its own unique combination of food and habitat preferences (e.g. van Oijen, 1982; Goldschmidt et al., 1990). The haplochromines were only locally important to fisheries (Kudhongania and Cordone, 1974a). In the 1970s a small-scale trawl fishery on these fishes was developed in the southern part of the lake near Mwanza. At that time the mean catch rate was more than 1000 kg h-1 (Goudswaard and Ligtvoet, 1988; Barel et al., 1991; Witte et al., 1992b).
Article
Based on ecological and morphological features, the 300 or more haplochromine cichlid species of Lake Victoria are classified into fifteen (sub)trophic groups. A key to the trophic groups, mainly based on external morphological characters, is presented. Of each trophic group a description is given comprising data on taxonomy, ecology and fishery. As far as possible data from the period before the Nile perch upsurge and from the present situation are compared. A list of described species classified into trophic groups is added.