The Functional Role of Fish Diversity in Lake Victoria, East Africa.

 

 

 

 

 

 

 

A Thesis Prospectus By

 

Jesse Schwartz

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Submitted to the Graduate School at Boston University

 

In partial fulfillment of the requirements for the degree of

 

Philosophiae Doctor

 

October, 2001

 

 

 

 

 

 

 

 

 

 

 

 

 

ABSTRACT

 

Lake Victoria is home and livelihood to millions of Africans in the countries of Kenya, Tanzania, and Uganda. It is the largest freshwater tropical lake in the world, and source of the world’s largest freshwater fishery. Before 1960 the lake was also home to some 600+ species of closely related indigenous fishes called “haplochromine cichlids”. The introduction of the predatory food fish Lates niloticus, Nile perch, coupled with major environmental perturbations, culminated in the extinction of hundreds of species of cichlids; an event noted as the greatest vertebrate extinction ever witnessed by the scientific community. The resultant L. niloticus fishery gained global popularity as an export commodity and food source for the East African nations, greatly overshadowing concerns regarding the conservation of biodiversity. However, the loss of some 300+ species did not occur without drastic consequences to ecosystem function and energy cycling. The once diverse fish community is now dominated by a handful of hearty fishes, and L. niloticus now feeds primarily on an atyiid prawn, a schooling cyprinid, and its own young instead of the previously dominant cichlid fishes. These concerns have been addressed incoherently over the past twenty years, and conflicting stories have emerged.

 

In my thesis I will investigate the functional role of fish diversity and its importance to sustainable fisheries in Lake Victoria. I will:

 

The importance of this collective body of work is many-fold. First, this will be the first attempt to comprehensively quantify the functional role of the remaining endangered Lake Victorian cichlids, and to describe the functional importance of their conservation. Second, this research will address the importance of species and functional conservation in the management of sustainable fisheries: topics not historically addressed simultaneously and often juxtaposed. Finally, this work will relate specific, metabolic, and behavioral diversity to ecosystem dynamics in an exploited species rich lake.

 

 

 

 

 

 

 

 

 

INTRODUCTION

 

            Lake Victoria, East Africa, is the largest tropical freshwater lake in the world. It is relatively shallow (maximum depth is 87m) with a surface area of 68,000km2. Its waters are shared by the nations of Tanzania, Uganda, and Kenya, with much of Burundi and Rwanda lying within the watershed. Until recently the lake was home to a rich endemic fish community dominated by about 600 species of haplochromine cichlids, 99% endemic, of which less than one third have been formally described (Greenwood, 1981; Kaufman et al., 1997; Witte et al., 1992).

            During the 1950’s and 1960’s the predatory Nile perch, Lates niloticus, was introduced into the lake in an attempt to augment the indigenous fishery for tilapiine cichlids, cyprinids, and other taxa that had been negatively impacted by over exploitation (Hamblyn, 1961; Welcomme, 1988). Nile perch established themselves in the lake where they fed heavily on haplochromines (Ogutu-Ohwayo, 1990; Ogutu-Ohwayo, 1994). The population remained at very low levels until the early and mid-1980’s, and then exploded around the lake.  Precipitous declines in haplochromine biomass and species diversity followed (Gophen et al., 1995; Kaufman, 1992; Kaufman, 1997; Kaufman and Cohen, 1993; Ogutu-Ohwayo et al., 1997; Pitcher and Hart, 1995; Witte and Densen, 1995). The resultant Nile perch fishery has gained global acclaim as the worlds largest freshwater fishery and a major export commodity for the East African nations, but remains as a sad textbook story of human induced mass-extinction (Ricklefs and Miller, 1999), p.15-17).

            Modern ecological thought predicts that the interconnectedness of ecosystem components (species, populations, and functional groups) makes their preservation important to system dynamics (Ehrlich and Ehrlich, 1981; Norberg, 1999). This belief stems from pioneering theoretical work (Elton, 1958; MacArthur, 1955b; Tilman, 1982) and a long history of experimental work (Connell, 1961; Dayton, 1975; McNaughton, 1977; Naeem et al., 1994; Paine, 1974; Tilman and Downing. 1994; Tilman et al., 1996) that demonstrated a significant relationship between community complexity (or functional diversity) and community stability. The nature of this relationship has been hotly contested (Leigh, 1975; May, 1973; May, 1975), and it can be argued that much of the early work (Connell, 1961; Dayton, 1975; Paine, 1974), both experimental and theoretical, dealt with systems that were unrepresentatively simple. It can also be argued that the definition of stability greatly confounds the diversity-stability hypothesis (Doak et al., 1998; Hughes and Roughgarden, 1998; Takeuchi and Adachi, 1986), and this has dampened motivation to pursue these questions in a modern setting. Instead the most recent research has made poignant that, with few exceptions, the conservation of biodiversity is mandatory to the preservation of ecosystem function (Cairns, 1993; Costanza, 1999; Covich et al., 1999; Duarte, 2000; Engelhardt and Ritchie, 2001; Grasso, 1998; Holmlund and Hammer, 1999; Lawton and Brown, 1993; Loreau, 1998; Norberg, 1999; Rapport, 1995; Strange et al., 1999).

            The loss of more then 300 species of cichlids from Lake Victoria was not without effect. On the contrary this major vertebrate extinction is believed to be both cause and consequence of dramatic changes in limnological conditions (Hecky et al., 1994; Livingston, 1980), fish guild structure (Goldschmidt et al., 1993; Gophen et al., 1995; Gophen et al., 1993; Kaufman, 1992; Kaufman et al., 1997; Lowe-McConnell, 1994; Schofield and Chapman, 1999; Witte et al., 1992), and both commercial and artisanal fisheries (Barel et al., 1991; Dache, 1991; Getabu, 1987; Kaufman et al., 1993; Kitchell et al., 1997; Ligtvoet and Witte, 1991; Manyala et al., 1994; Pitcher and Hart, 1995; Reynolds and Greboval, 1988; Witte and Densen, 1995). Molluscivores, epilithic algae scrapers, higher plant eaters, detritivores, paedophages, scale-eaters, and specialized insectivores have all been decimated.

            As the species components and food web anatomy have changed, so might energetic properties of the fish community: e.g., flow diversity (MacArthur, 1955a), ascendancy (Christensen, 1994; Rutledge et al., 1976), capacity for development (Ulanowicz, 1986), organization (Hannon, 1973), cycling (Finn, 1976), and redundancy (Lawton and Brown, 1993; and see Ulanowicz and Wulff, 1991 for a review). Shifts in these metrics can affect emergent system dynamics such as resilience, resistance, or return times (Pimm, 1991), and the flow rate to any particular compartment or population. The correlated changes in food web anatomy (connectance, food chain length, etc.) and dynamics will alter the distribution of biomass to functional groups, and thus affect the overall system services and functions.

            The future of Lake Victorian fishes is dependent upon the action of fishery managers in the region. Controlling Nile perch populations is possible through the use of gear limitations and the allocation of fishing effort, and these actions will directly effect both fishery yield and fish diversity (Kaufman and Schwartz, in press; Schindler et al., 1998). Managers are therefore left to balance between fishery yields and fish conservation. Given the current economic and human health situation in East Africa, the obvious answer is to lean heavily towards fishery yields. The purpose of my research will be to describe the functional importance of fish conservation in Lake Victoria. I will highlight the importance of a diverse indigenous fish community as food for Nile perch stock and as a nexus for energy cycling. I will then describe a natural experiment, the Lake Kyoga Satellite System, in Central Uganda for testing hypotheses regarding system diversity and ecosystem function.

 

 

CICHLIDS as PREY for NILE PERCH

 

            Like so many invading organisms, Nile perch experienced phenomenal growth during their initial twenty-five years in Lake Victoria. After their population explosion, Nile perch quite literally ate themselves out of house and home. As their preferred prey, haplochromine cichlids, were extirpated, L. niloticus turned to other prey including invertebrates, other fishes, and their own young (Hughes, 1986; Ogari and Dadzie, 1988; Ogutu-Ohwayo, 1990; Ogutu-Ohwayo, 1994); a diet more closely resembling that found in their native habitat. Concurrent with this trophic shift Nile perch experienced a decrease in growth rates and length-weight ratios to more closely resemble that of their indigenous stocks (Hughes, 1992).

            Work in our laboratory suggests that Nile perch may more easily predate upon Lake Victorian fishes then those found in its native habitat (Bocking, 1993). The Nile perch’s early success may thus have been tied to their acquisition of easily captured prey: a diverse cichlid community. I hypothesize that when L. niloticus later shifted to other prey types their feeding success greatly decreased.

            To test this hypothesis I will conduct feeding experiments on wild caught Nile perch. L. niloticus will be offered cichlids and schooling cyprinids (now their dominant prey) separately in 100 gallon aquaria. Prey will be limited to young individuals of similar (not significantly different) size. Both naïve and experienced prey will be offered to perch during different trials. Feeding experiments will be recorded using Hi-8 and Mini-DV camcorders, and the results will later be analyzed using EthoLog 2.25: software for ethological analysis. I will record the times of all strikes (attempts) and captures (successes), and analyze the results using ANOVA, Principle Components Analysis (PCA), and multivariate regression.

 

ACTIVE METABOLISM in FISHES and ITS IMPORTANCE in the NILE PERCH QUESTION

 

            With only rare exceptions, fish maximize their growth to the consequence of their growth efficiency (Cushing, 1981). In other words, a fish will do whatever it can to get the ration it needs to accomplish some genetically and environmentally predetermined rate of growth (Ali and Wootton, 2001; Hunt et al., 2000; Imsland et al., 2000; Nimi, 1981; Present and Conover, 1992; Rooij et al., 1995; Verbeeten et al., 1999; Xie et al., 2001; Zhu et al., 2001). Even in the presence of excess food, aquaculturists must alter environmental (Alekhin, 1983; Biette and Geen, 1980; Frank and Leggett, 1982; McCarthy et al., 1998; Salinas and McLaren, 1983) or biochemical (Hunt et al., 2000; Wang et al., 1998) conditions to overcome this general principle.

            Mass-balance theory defines the process by which a fish might maximize growth to the cost of growth efficiency since

                                                                                     (1)

growth (G) equals consumption (C) minus metabolic losses (M) excretion (Ex) and egestion (Eg). Excretion and egestion are direct fractions of consumption and metabolic losses, and so they can play no role in a fish maximizing its growth rate. Changes in growth efficiency, and growth rates for that matter, are therefore the result of trade-offs in consumption or metabolic losses.

            At the extremes of resource availability fish are left without trade-offs and growth rates will suffer. The observed declines in growth rates and deterioration of length-weight ratios of Nile perch in the mid-1990’s suggest that both consumption rates and metabolic losses were affected by the extirpation of a diverse prey base. I hypothesize that the observed changes in growth parameters can be explained by a modest increase in activity (active metabolism) resulting in an increase of metabolic losses.

            Given the long and broad history of research on this topic, I will devote considerable attention towards a comprehensive review of fish bioenergetics. I will then use the results of my feeding experiments coupled with previously published work to develop mass-balance models for Nile perch growth before and after the extirpation of the Lake Victorian cichlids. I will use t-Tests and regression analysis to examine the energy budgets of these two scenarios and to describe the importance of the Lake Victorian haplochromine cichlids as an efficient prey source for L. niloticus.

 

METABOLIC DIVERSITY in LAKE VICTORIAN CICHLIDS

            G.G. Winberg is famous, in part, for his attempt to summarize our (then) current understanding of fish metabolism in its entirety (Winberg, 1960). In his classic work, Winberg addresses the surface law in fish and, even at this early time of our understanding, describes significant variance from the simple relationship

                                                            M = AW2/3                                          (2)

where resting metabolism (M) equals the fishes weight (W) to the power of 2/3 times some constant (A). The physics of physiological processes define equation 2 since the rate of metabolic functions is in general determined by the ratio between surface area and volume (0.67). Across some broad size classes these constants can be used to roughly estimate metabolic functions. However, both Winberg (1960) and Hemmingsen (1960) pointed out that the relationship between resting metabolism and mass deviates from these constants with the exponent approaching 0.75. The resting metabolism of animals is a measurement of the energy needed to maintain the metabolic machinery, though some small aspect of growth often confounds this metric. With this definition in mind, there are several possible explanations for the discrepancy between the observed and theorized results for equation 2.

            In fish larger organisms (and species) often gain access to higher quality resources then smaller ones. However the acquisition of these resources may require a greater metabolic machinery since they may be more evasive or defensive then less desirable resources. In addition fishes may evolve (between generations) or develop (within generations) a more efficient metabolic machinery at larger size classes in order to defend or compete for vital resources. Resting metabolism can thus serve as an indicator of functional machinery in fishes which may in turn bring light to the true nature of equation 2.

            The 600+ species of Lake Victoria haplochromine cichlids are believed to have evolved from a relatively few common ancestors only 12,000 years ago (Stager et al., 1986). This surprising figure is supported by the remarkable similarity in size structure of the Lake Victoria flock when compared to other African cichlid lakes, but is contradictory to their remarkable functional diversity (Greenwood, 1981; Lowe-McConnell, 1995). Regional managers and the fishing community disregard the metabolic diversity of haplochromines under the premise that species are somehow replaceable in the lake.

            Given that there are no recorded metabolic rates for the Lake Victorian cichlids, these beliefs are difficult to contest. I hypothesize that the resting metabolism of the piscivore Pyxichromis orthostoma will be significantly different from that of the herbivore Xystichromis orthostoma. These two species are similar in body form, size, and habitat but differ greatly in their choice of food (Greenwood, 1981). I will use closed respirometry to estimate the resting metabolism of thirty specimens of each species across their size range. In addition I will estimate the resting metabolism of thirty specimens of a Pyxichromis orthostoma X Xystichromis orthostoma.hybrid to support the assumption that any observed interspecific differences are hereditary in nature. All specimens will be laboratory reared in order to eliminate environmental based variance, and I will follow closely the suggestions outline by (Jobling, 1994). I will use non-linear regression to determine the metabolic curves of the three taxa, and linear regression to compare the three curves.

FISH DIVERSITY and ECOSYSTEM FUNCTION in LAKE VICTORIA

            The effects of Nile perch on the Lake Victoria fish community were perhaps most eloquently illustrated by Ligtvoet and Witte (1991). In that work the authors diagram the pre and post Nile perch sub-littoral food webs, thus describing the loss of more than 300 species and several functional groups. Most notable is the disappearance of molluscivores, parasite eaters, detritivores, scale eaters, epilithic algae scrapers, paedophages, and specialized insectivores. However, as several authors have pointed out, Nile perch, Nile tilapia (Oreochromis niloticus), some catfish (Synodontis sp.), and the schooling cyprinid Rastrineobola argentea now dominate the lake and have diversified their diet (Batjakas et al., 1997; Hughes, 1986; Ogari and Dadzie, 1988; Ogutu-Ohwayo, 1990; Olowo and Chapman, 1999). Understanding the functional role of fish diversity in Lake Victoria therefore requires a system simulation using a common currency.

            Moreau (1995; Moreau et al., 1993) used mass-balance theory to describe the effects of Nile perch introduction. However, his work used general delineations of functional groups and he failed to address his results from a top down perspective. Instead he focused his analysis on the simple fact that since food chains in Lake Victoria had shortened as a result of L. niloticus introduction, the efficiency of the system must have increased. Moreau and his reviewers failed to consider the possibility that entire prey categories were no longer being predated despite sufficient field evidence to suspect so (i.e. Mothersill et al., 1980). They also did not consider the thought that biomass and energy were being drawn from the haplochromine standing stock at a rate that was faster then sustainable.

            I will use Fish Bioenergetics 3.0 (Jobling, 1994) and EcoPath (Christensen and Pauly, 1992) to develop a pre and post Nile perch mass-balance model for Lake Victoria. Fish Bioenergetics using the mass-balance model described in equation 1 and is a useful tool for estimating the rates of predation of fishes. EcoPath uses a community wide mass-balance formula

                                                                         (3)

such that Bi and Bj are biomasses of i the consumer and j its prey, P/B is the production to biomass ratio for i, EEi is the fraction of i consumed in the system, Yi is fishing mortality, Q/B is the relative food consumption for j, and DCij is the fraction of i in the diet of j (Allen, 1971). Though general in nature, the ECOPATH equations provide a rough estimate of flow rates between community components, sufficient for comparing the pre and post Nile perch systems, and for incorporating the results of this dissertation. I will calculate throughput (aggregate energy flow from each component), ascendancy (total throughput times average mutual information, Rutledge et al., 1976), relative ascendancy (the product of total throughput and flow diversity), and the Finn Cycling Index (fraction of total system throughput dedicated to the recycling of medium) for both pre and post Nile perch systems. I will use ANOVA, MANOVA, multivariate regression, PCA, and ARIMA models to compare energy metrics between systems, and will relate these results to future management and conservation in Lake Victoria.

LAKE KYOGA and its SATELLITES: A MODEL SYSTEM for LAKE VICTORIAN FUNCTIONAL DIVERSITY?

 

            The difficult reality of the Lake Victoria problem is that there is but one lake, and the extirpation of so much diversity occurred before researchers could sufficiently describe its impact on the lake and fisheries. Even with the most detailed road map a manager would, and should, be hesitant to conduct experimental management on the lake proper without preliminary results for risk of harming fishery yields or experiencing indirect effects of conservation initiatives.

            Lake Kyoga lies down river of the Nile headwaters of Lake Victoria. Throughout modern history the Kyoga system has received fish propagules from Lake Victoria which remained isolated from Kyoga via the Victorian falls (now a working hydroelectric dam). Similar communities thus evolved, and the Kyoga system has been described both as a refugium for Lake Victoria functional diversity (Kaufman, 1997; Kaufman et al., 1997), and as a significant fishery resource (Kudhongania et al., 1992; Ogutu-Ohwayo, 1995). The Kyoga system is really one large papyrus swamp whose topography allows for scores of lake isolates to form. The lakes share water via the papyrus swamps, but fish transport is limited for the less hypoxia-resistant species to extremely rare and grandiose flood events (Chapman and Chapman, 1998; Chapman et al., 1995; Schofield and Chapman, 2000).

            Within the Kyoga system lakes Kyoga, Nyaguo, and Nakua contain Nile perch and fish communities that closely resemble that of Lake Victoria, whereas lakes Agu, Basina, Gigate, Kawi, and Nawampasa maintain diverse fish populations free from Nile perch. I hypothesize that lakes with Nile perch will resemble the modern Lake Victoria, whereas lakes without Nile perch will contain functionally diverse assemblages of haplochromine cichlids.

            In 1999 we visited and collected fish and prey specimens from the above mentioned seven lakes using variable mesh experimental gill-nets, dredges, hand nets, and beach seines. I will use stable isotope analysis (d15N) to describe the functional diversity of all seven lakes. This approach has been used successfully to estimate the realized trophic roles of fishes (Bizerril, 1996; Forsberg et al., 1993; Genner et al., 1999; Gu et al., 1996; Gu et al., 1997; Jaarsma et al., 1998; Jennings et al., 1997; Mizutani et al., 1992; Thompson and Furness, 1995; Wainright et al., 1993; Wainright et al., 1996; Walker et al., 1991; Yoshii et al., 1999). I will then compare the fish communities of all lakes using simple and multiple regression, and finally define the importance of the Kyoga lakes as a model system for testing adaptive management scenarios for Lake Victoria.

CONCLUSION

 

In this increasingly populated world, resource managers will continue to struggle to balance the importance of species conservation and resource extraction. In Lake Victoria this problem is particularly troublesome in that managers view these goals as juxtaposed. In my work I will elucidate the functional importance of species conservation in Lake Victoria. My novel approach to this now classic ecology story will demonstrate that conservation and resource extraction are simultaneously obtainable goals. I will demonstrate that the maintenance of a diverse fish community is vital to the production of Nile perch, the cycling of energy into the fish community, and the dynamics of the Lake Victoria ecosystem.

 

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