Most of the earth's ecosystems are experiencing slight to

catastrophic losses of biodiversity, caused by habitat

destruction, alien species introduction, climate change and

pollution (Wilcove et al., 1998). These human effects have

led to the extinction of native fish species, the collapse of their populations and the loss of ecological integrity and ecosystem functioning (Ogutu-Ohwayo & Hecky, 1991;

Witte et al. , 1992a; Mills et al., 1994; Vitousek et al., 1996).

Food webs are macro-descriptors of community feeding

interactions that can be used to map the flow of materials

and nutrients in ecosystems (Jepsen & Winemiller, 2002).

Comparative food web studies have been used to address theoretical questions such as 'does greater trophic connectivity increase stability?' (Cohen et al., 1990), and 'does the number of trophic levels increase with productivity?' (Briand & Cohen, 1987). Answers to such questions have obvious applications for natural resources management. From a multi-species fisheries standpoint, there is a need to understand consumer-resource dynamics within complex trophic networks

Campbell, L.M.

Hecky, R.E.

Mbabazi, D.

Ogutu-Ohwayo, R.

Aquatic Commons

Editors:
J. S. Balirwa,
R: Mugidde,
R. Ogutu-Ohwayo.
Challenges for Management of the
Fisheries Resources, Biodiversity
and Environment of Lake Victoria
Edited by: J.S. Balirwa, R. Mugidde and
R. Ogutu-Ohwayo.
Fisheries Resources Research Institute
Technical Document No. 2
First Edition 2004
FIRRI, Jinja Uganda.
119
5.2 Implications of changes in
trophic diversity
and food webs on fisheries
and the environment
D. Mbabazi, R. Ogutu-Ohwayo, R.E. Hecky
and L. M. Campbell
Introduction
Most of the earth's ecosystems are experiencing slight to
catastrophic losses of biodiversity, caused by habitat
destruction, alien species introduction, climate change and
pollution (Wilcove et al., 1998). These human effects have
led to the extinction of native fish species, the collapse of
their populations and the loss of ecological integrity and
ecosystem functioning (Ogutu-Ohwayo & Hecky, 1991;
Witte et al. , 1992a; Mills et al., 1994; Vitousek et al., 1996).
Food webs are macro-descriptors of community feeding
interactions that can be used to map the flow of materials
and nutrients in ecosystems (Jepsen & Winemiller, 2002).
Comparative food web studies have been used to address theoretical questions
such as 'does greater trophic connectivity increase stability?' (Cohen et al., 1990),
and 'does the number of trophic levels increase with productivity?' (Briand & Cohen ,
1987). Answers to such questions have obvious applications Tor natural resources
management. From a multi-species fisheries standpoint, there is a need to understand
consumer-resource dynamics within complex trophic networks.
Trophic interactions playa major role in aquatic ecosystems (Carpenter et al., 1985;
Vander Zanden & Rasmussen 1999),and the trophic habits of fish have consequences
for other aspects of their biology, including contaminant accumulation (Kidd et al.,
1995; Vander Zanden & Rasmussen 1996; Kidd et al., 2001) as well as patterns of
life history and growth (Martin, 1966). Fish are important consumers in aquatic food
webs (Bax 1991). At the whole system scale, predation by fishes alters community
structure and nutrient cycling, thus , both direct and indirect predation effects are
expressed in population structure, community composition and productive processes
at all trophic !evels (Kitchell et al., 1994). Fishers and related fishing activities can
alter food web interactions because they target certain species that have important
trophic effects on the entire food webs.
Challenges for Management of the Fisheries Resources .
Biod ivers ity and Environment of Lake Victoria
120
Like in any other African Great lakes, Lake Victoria has experienced dramatic changes
in recent times, including eutrophication and deoxygenation (Hecky 1993; Hecky et
a/., 1994), the extirpation of native cichlid species (Witte et a/., 1992b) an increase in
Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus), and Rastrineobo/a
argentea fisheries (SEDAWOG 1999), intensive fishery exploitation (Kitchell et a/.,
1997) and the increase and subsequent decline of water hyacinth, Eichhornia
crassipes (Twongo 1996). Equally dramatic shifts have been observed in food web
structure and trophic relationships in Lake Victoria. This chapter reviews the original
food web structure prior to the Nile perch introduction and other environmental shifts
and compares this against the current food web structure.
The Original food web structure prior to transformation
Prior to the stockings of exotic fish species (Nile perch and four tilapiines (0. niloticus,
Oreochromis leucostictus, Tilapia zillii and Tilapia rendalll) , Lakes Victoria and Kyoga
had a high fish species and trophic diversity with many fish species that were found
only in these lakes. The Victoria food web and fishery contained an ecologically and
phylogenically diverse group of fishes . The fish fauna was dominated by a diversified,
presumably monophyletic species flock of haplochromine cichlids (Meyer et af., 1990).
The most recent estimates of Lake Victoria's pre-Nile perch species flock comprised
of over 500 species that occupied a spectacular variety of niches in the lake
(Greenwood 1974; Seehausen 1997). Prior to Nile perch introduction in Lake Victoria ,
the predators on adult haplochromines were: the catfishes such as Bagrus docmac,
C/arias gariepinus, Schilbe intermedius and the lungfish, Protopterus aethiopicus
(Olowo & Chapman, 1999), fish eating birds (Pha/acrocorax africanus, Pha/acrocorax
carbo, Ceryle rudis) and otters (Goldschmidt, et a/., 1990, Goudswaard & Wanink,
1993). A direct chain existed from phytoplankton to tilapiine species (Ligtvoet & Witte
1991). Lake Kyoga, downstream of L. Victoria and Lake Nabugabo, formerly connected
to Victoria also had endemic haplochromine species (Worthington, 1929;Trewavas
1933; Greenwood 1965; 1966; Kendal! 1969). The fishes especially the
haplochromines occupied most trophic levels and played an important role in the flow
of energy in the ecosystem of these lakes. The haplochromines converted numerous
protein sources such as detritus, algae, zooplankton, insect larvae, molluscs and many
others into fish protein for consumption by higher trophic levels (Christian, 1995).The
major food web started from detritus and phytoplankton run via the various trophic
groups to the piscivores and other top predators such as crocodiles and humans
(Fig. 5.2.1). Haplochromines were crucial in maintaining the ecosystem which supported
other fishes, as well as the high biodiversity associated with the lake basin. Studies of
haplochromines ~';Ja yed a major role in illustrating how organisms undergo ad~iptive
radiation to produce new species, and how a trophically diverse assemblage can
efficiently utilize an ecosystem.
Challenges for Manag" ment of th e Fisheries Resources,
8 '<:>d'verSlly and EnwJnmenl of Lake VictOri a
Fig. 5.2,1. A simplified food web of the pelagic Lake Victciria before Nile Perch introduction (Modified from Worthington, 1933;
Ligtvoet and Witte, 1991 , Witte and van Densen, 1995). Note the number of ecological niches that haplochromine species occupy.
()
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Schilbe, Barbus,
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Synodontis,
Haolochromines
Oreochromis
Haolochromines
Detritus IPhytoplankton
Synodonlis, Labeo,
122
Current food web structure from dietary gut content and stable isotope
analyses
Changes in the food web of Lake Victoria during the last 40 years are among the
most dramatic known to ecology (Kaufman 1992; Lowe-McConnell 1994). By the
1960's, stocks of the native tilapiines and other large species of Lake Victoria had
been reduced byoverfishing (Jackson 1971; Ogutu-Ohwayo 1990a;b).
With the establishment of introduced species in Lake Victoria , the trophic ecology of
the system changed, both through predator-prey and competitive interactions, and
through the removal of various trophic groups from the food web (Reinthal & Kling,
1994). The detritivore/phytoplanktivore group, which constituted 40% and 16%
respectively of the total dermersal fish biomass, disappeared in the early 1980's,wi th
the explosive :ncrease of the introduced Nile perch . Only after the haplochromines
densities had declined to near zero did the Nile Perch switch to other prey, such as the
atyid prawn Caridina nilotica (Goldschmidt et aI., 1993), native cyprinid R. argentea
and its own young (Witte et al., 1992a). In the presentfood web of Lake Victoria, the
Nile perch is the top predator, replacing all the original piscivores. The detritivorous/
phytoplanktivorous'haplochromines have been replaced bythe prawn C. nilotica while
the indigenous tilapiine species have been replaced by the introduced Nile tilapia.
Nile tilapia particularly the young ones, live in the littoral and sub-littoral regions of the
lake'and have rninimal overlap with the Nile perch (Lowe-McConnell 1997; Welcomme
1988; Ogutu-Ohwayo 1990a) and the two species also co-exist in lakes Albert and
Turkana. An important feature of the present system is the extensive cannibalism
within the Nile perch population, which starts at very young stages (CruI1998). The
introduced Nile perch severely disrupted the ecosystem with the simplification of
the food web through virtual eradication of haplochromines from many interactions
and the development of short food chains to the Nile perch. Whereas the
haplochromines converted numerous protein sources into fish protein for
consumption by higher trophic levels, Nile perch now eats quite a lot of its own
juveniles, which feed on the same food items as the haplochromines did previously
(Christian , 1995). Other changes were also observed at the top of the food pyramid.
The pied Kingfisher (C. rudis) , shifted from the diet of mainly haplochromines to
exclusively R. argentea. A similar shift in diet was also found for the Great
Cormorant P carbo and the long-tailed Cormorant P africanus (Goudswaard et
al., 1993). Currently humans are a dominant component of the Lake Victoria food
web, and the fisheries have the potential to further alter community structure and
dynamics (Kitchell et al., 1997).
The loss of species and trophic diversity, and associated alterations in food webs
have been accompanied by more frequent algal blooms (Ochumba & Kibara 1989;
Hecky & Bugenyi 1992; Mugidde 1993) and deoxygenation of the deeper water
Challenges for Management of the Fisheries Resources,
Biod iversity and Enwonment of Lake V ic tori a
123
of the lake, which sometimes has been associated with mass fish kills in Lake
Victoria (Hecky et al., 1994). The accumulation of the excess organic matter is an
indication that much of the organic matter produced in the lake is not being
channeled efficiently through the food web. Originally the native fishes occupied
virtually all trophic levels, including phytoplanktivores, zooplanktivores, insectivores,
molluscivores, detritivores, and piscivores and maintained an efficient flow of
organic matter in the system. The depletion in stocks of this trophically diverse
fish community by Nile perch changed the food web of the lake and seems to have
reduced grazing pressure and the overall ecological efficiency of the ecosystem
in the lakes.
Use of Stable nitrogen and carbon isotope ratios in food web studies
Trophic interactions in fresh water food webs have traditionally been defined using
dietary analyses of fish stomachs or gut contents or from the presence of organisms
within a system (Balirwa 1984; Balirwa 1998; Mbabazi 1999). The Lake Victoria
food web structure is shifting, based on information on the volume and composition
of the fish catch in the lake, as well as on the stomach contents offish (Balirwa
1998; Ogutu-Ohwayo 1995; Wanink 1998).
While dietary analysis provides valuable taxonomic information on fish diets, they
can be complemented by the analyses of stable nitrogen (d 15N) and carbon
( d13C) isotope ratios of biota to characterise food web structur,e and trophic
interactions (Peterson & Fry 1987). Stable isotopes integrate a longer temporal
scale offeeding, assimilation, and growth (Kling et al., 1992; Yoshioka et al., 1994,
Harvey & Kitchell 2000). Globally, stable isotope analysis has been used
successfully to describe aquatic food webs and their measurements have been
used to estimate feeding patterns and food web structure in several African lakes,
including Lake Kyoga in Uganda (Hecky & Hesslein, 1995); Lake Malawi in
southern Africa (Bootsma et al., 1996; Genner et al. 1999; Hecky & Hesslein
1995; Kidd et al., 2001) and in Napoleon and Winam Gulfs of Lake Victoria in
East Africa (Campbell 2003.).
Nitrogen isotopes consistently fractionate in organisms. 14N is selectively
eliminated while 15N is incorporated into body tissues. At every successive trophic
transfer, d ;sN values in the tissue of biota increase. Many studies find the average
d15N difference between an animal and its food source is approximately 3 to 4 0100
(Peterson & Fry 1987; Vander Zanden & Rasmussen 2001; Post 2002). This
consistent change provides a powerful analytical tool to quantify relative trophic
position or dietary changes in fish (Cabana & Rasmussen 1994).
Challenges for Management 01 the Fishenes Resources.
Biodi ve rsity and Environment of Lake Vlctona
124
In contrast carbon isotopes fractionate very little in biota, with 1 0/00 enrichment in
d13C per trophic level (Peterson & Fry, 1987; Vander Zanden & Rasmussen, 2001 ;
Post 2002). Because ofthese low fract:onation rates, the stable carbon isotope values
of organisms reflect the average d 13C of their diets. d13C values can vary at the base of
the food web due to differences in photosynthetic enzymatic fixation, growth rates,
CO2 and pH levels and these differences are passed up in the food chain, indicating
the origin of organic carbon in organisms at higher trophic levels (Hecky and Hesslein,
1995). In lakes, d13C is useful for differentiating between two major sources ofavailable
energy, littoral (near shore) production from attached algae and detritus, and pelagic
(open water) production from phytoplankton, because the d13C of the base of the littoral
food web tends to be enriched in 13C (less negative d13C) relative to the base of the
pelagic food web (F.rance 1995; Post 2002).
Campbell et f3l.) 2003 found out that the food webs of the widely separated gulfs
(Napoleon and Winam) of Lake Victoria with differing water qualities and
anthropogenic effects are similar despite the different d15N and d13C values at the
base of their food webs. In the current Lake Victoria, Nile perch depends on a
variety of macro-invertebrates and it's own young for its growth through maturity
and even into adulthood (Fig . 5.2.2).Young Nile perch and Caridina are isotopically
consistent as prey items for larger adults and there is an increasing preference for
piscivory with increasing size. Nile tilapia in Napoleon Gulf have isotopic signatures
consistent with strong preference for detritus throughout its life history, while in
Winam Gulf, there are indications of shifting dietary patterns. R. argentea and
Yssichromis /aparograma cannot be contributing to somatic growth of t\lile perch
(Fig . 5.2.2) as the pelagic fish have d15N signatures indistinguishable from Nile
perch (and in Napoleon Gulf, significantly different d13C vaiues). Common fish
species as Schi/be intermedius and Synodontis afrofischeri, which coexist with
Nile perch in the modem lake, are trophically isolated from Nile perch. This suggests
that only a narrow group offishes is available to sustain growth of larger Nile perch
and the stability of prey populations is crucial to the sustainabillity of Nile Perch.
Cha llenges (or Management o( the Fisheries Resources .
BiOdiversity and Environment of Lake Vi ctoria
Fig. 5.2.2. A simplified food web of the pelagic Lake Victoria befo re Nile Perch introduction (Modified from Campbell
in press). Note the number of ecological niches that haplochromine speci es occ upy. The dotted lines indicate that theeri!H"
occasionally include s that prey in its diet and the dot-das h lines indicate that the fish changes its diet as it grows.
~•........••••
Young Nile tilapia
« 5 em)
.......,.-<.Zooplankton
126
Effects of fishing on trophic interactions and food webs
Currently humans make a dominant comp.onent of the Lake Victoria food web, and
fisheries have the potential to further alter community structure and dynamics. The
complex nature of fishing effects on ecosystems became entrenched in the
environmental consciousness when it became clear that fishing could cause
imbalances in the most basic of species interactions, predator/prey relationships.
Like in many other aquatic bodies fishing down food webs occurs as the fishery
concentrates on the top predators and subsequently more abundant prey species of
these predators (Pauly et al., 1998; Steele & Schumacher 2000). This decreases
both structural and functional diversity of ecosystems, presumably also decreasing
their resilience (Coleman & Williams 2002).
In Lake Victoria today, the dominant components of the fish community are Nile perch,
Nile tilapia, and the pelagic R. argentea (Ogutu-Ohwayo 1994). The life history
strategies of Nile tilapia and R. argentea allow their persistence. Nile tilapia is sympatric
with the Nile Perch and appears resistant to predation due to its rapid growth and
large adult size (>30 cm total length). From a socioeconomic perspective, the Lake
Victoria fisheries are in a highly desirable state due to the high fish exports. However
from a fisheries mahagement perspective, the current state is problematic. This is
because regulatory policies are difficult to define as they require international
agreements however, fish populations change more rapidly than the policy-making
process, and regulations are difficult to implement due to the dispersion of fishing
effort and responsibility for regulatory enforcement (Kitchell et al., 1997).
From an ecological perspective , the trophic system is now relatively simple .
Increased nutrient loading fuels algal blooms., increasing the detritus base
consumed by the extraordinary abundant C. nilotica . This prawn is the main prey
of juvenile Nile perch , which is the primary prey of adult Nile perch. R. argentea
grows rapidly on the increased abundance of large zooplankton and emerging
insects (Kaufman & Cohen 1993 , Campbell et al. , 2003.). Based on experiences
in great lakes else where, this much-simplified ecosystem is unlikely to remain in
its current state (Kaufman & Cohen 1993).
The fisheries of Lake Victoria have undergone massive changes in the past and are
likely to change in the future. Environmental conditions are in flux (Mugidde 1993;
Hecky et a/.,1994), fish populations and predator-prey interactions are changing rapidly
(Ogutu-Ohwayo 1990a; 1994), and humans continue to become more efficient top
predators in this ecosystem and their effects extend beyond the lake shores (Riedmiller
Of 994) . Conserving the remnant species flock of haplochromines depends in many
ways on the success of the ~~ile Perch fishe ry (Ribbink 1987; Kitchell et al., 1997) .
Challenges for Management of the Fishenes Resources.
Biod iversi ty and Environm ent of Lake V ictona
127
Although restoration of Lake Victoria's biodiversity and its historical food web
could be impossible, fi sheries management can control exploitation in ways that
should benefit both the people that depend on the Nile Perch fishery for their
livelihood and those that are concerned with conserving and sustaining a diverse
fish community (Bruton 1.990; Kitchell et a/., 1997; Schindler et a/., 1998).
Conclusion
Trophic interactions and food webs have crucial implications for both community
and ecosystem patterns and processes, such as the regulation of species diversity,
energetic efficiencies and the biomass of trophic levels, community stability,
contaminant levels in the biota and biogeochemical fluxes from ecosystems.
Understanding of trophic interactions and food weds of aquatic systems is long
over due and can be enhanced through accurate quantification of these processes
through the complementary use of stable isotopes and dietary gut content analyses.


Implications of changes in trophic diversity and food webs on fisheries and the environment

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Implications of changes in trophic diversity and food webs on fisheries and the environment

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