Managing aquaculture for sustainability in tropical Lake Kariba, Zimbabwe

ECOLOGICAL ECONOMICS ELSEVIER

Ecological Economics 18 (1996) 141-159

Analysis

Managing aquaculture for sustainability in tropical Lake Kariba, Zimbabwe H~kan Berg

a, *,

Petra Mich61sen

a,

Max Troell

a,

Carl Folke a,b, Nils Kautsky

a

a Department of Systems Ecology, Stockholm Universi~, S-106 91 Stockholm, Sweden b The Be(jer International Institute of Ecological Economics, The Royal Swedish Academy of Science, Box 50005, S- 104 05 Stockholm, Sweden

Received 5 July 1995; accepted 23 January 1996

Abstract In Lake Kariba, Zimbabwe, small-scale pond farming and experimental cage-culture of Tilapia fishes have been running for some years and there are now plans for large-scale aquaculture. As a basis for deciding on how aquaculture could be developed to improve the chances for sustainable resource use and long-term maximised fish production in the lake, we compare the potential ecological life-support demand of two alternative aquaculture methods. First, the economic and ecological resource demand, expressed in industrial and solar energy units, respectively, were estimated for semi-intensive pond farming and intensive cage farming. Next, the ecosystem areas appropriated by the two farms for production of feed, oxygen, and phosphorus assimilation were estimated. Our estimates indicate that intensive cage farming would require about 17 800 MJ solar energy (Gross Primary Production) to produce l kg of fish. The industrial energy input would be more than 1.5-times higher (about 85 MJ/kg) compared to semi-intensive pond farming (about 50 MJ/kg). Intensive cage farming must be supported by ecosystem areas that are all substantially larger than the area of the farm itself. The ecosystem area for feed production is the largest (21 000 m 2 m-Z), but the areas required for oxygen production (160 m e m -2) and nutrient assimilation (115 m 2 m 2) are of special importance since they must be located close to the farm, For semi-intensive pond farming, oxygen production and nutrient assimilation could probably be provided within the pond system, and no external life support from Lake Kariba would be needed. At least from an ecological point of view, semi-intensive pond farming is more sustainable than intensive cage farming because it needs a smaller input of external resources to survive. However, a moderate level of intensive cage fanning should not be ruled out in Lake Kariba. Aquaculture has potential to become successful in Lake Kariba, but only if it is developed within a linked economic, social, and ecological framework. Keywords: Tropical freshwater; Aquaculture;Energy analysis; Life support system; Ecological footprint; Resource management

1. Introduction Aquaculture is rapidly expanding in developing countries and is generally regarded as an efficient

* Corresponding author. Tel.: (46-8) 161-358; Fax: (46-8) 158417; e-mail: [email protected] 0921-8009/96/$15.00 Publishedby Elsevier Science B.V. PII S0921-8009(96)00018-3

means of increasing protein production and generating income (Balarin, 1988; Pullin, 1993). Often, however, aquaculture has not met the original expectations and in some cases it has even led to environmental disasters (Meltzoff and LiPuma, 1986; Bailey, 1988; Pullin, 1993; Santiago, 1994). This has often been due to a general lack of ecological under-

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H. Berg et al. /Ecological Economics 18 (1996) 141-159

standing about the strong complementarity between the supporting environment and the cultivation. Aquaculture relies on many resources and must be viewed in the broad context and not as an isolated sector (Folke and Kautsky, 1992; Edwards, 1993; Pullin, 1993). It is embedded in the economy and heavily dependent on natural capital (Folke, 1988). If aquaculture development is to be ecologically sustainable, efforts must be directed towards methods that make use of the natural environment without severely or irreversibly degrading it. The benefits derived from ecological processes and the life-supporting ecosystems must be recognized and play an important part in aquaculture development (Folke and Kautsky, 1989). In Lake Kariba, Zimbabwe, fish is very popular and an important food item, but due to the increasing

human population, the present fish catches may not be sufficient to satisfy the future demand. To increase the fish supply, small-scale fish pond farming has taken place on the shore of the lake, and recently experimental cage-culture has been initiated. The fish cultivation has proven to be successful and there are now plans for large-scale aquaculture (approx. 2000 tonnes/year) (Sanyanga, 1989; Lasserre and Hough, 1990; Gabriel, 1991). There may be potential for aquaculture development in Lake Kariba, perhaps even on a relatively large scale, also when environmental considerations are taken into account. However, the development of aquaculture should not follow the exploitive path of western countries (cf., Edwards, 1993; Gowen and Rosenthal, 1993; Pullin, 1993). We believe that it should be developed in a more diversified way which would benefit both the 28 E

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Fig. 1. Lake Kariba is a man-made lake situated on the Zambezi River between Zambia and Zimbabwe. The lake consists of five basins and is 320 km long with an surface area of 5364 km2. It is monomicticwith a mean surface temperature of about 25°C (Balon and Coche, 1974). The mean and maximum depth are 29 and 120 m, respectively (Ramberg et al., 1987).

tt. Berg et al./Ecological Economics 18 (1996) 141-159

economic and the ecological system of Lake Kariba in the long run. The aim of this study is to compare the ecological life support demand of two alternative aquaculture methods in Lake Kariba, as a basis for deciding how aquaculture could be developed to improve the chances for long-term sustainable resource use and maximised fish production in the lake. The choice of culturing system is often governed by economic incentives, but in this study we also try to account for the economically 'hidden' resource demands, and discuss some environmental aspects of aquaculture. These aspects must be taken into account when evaluating how to increase fish production through aquaculture in a multi-use water body, such as Lake Kariba. If aquaculture is not designed to stay within the ecological thresholds of the lake, one severe risk is that it might become a substitute and not a complement to the existing fisheries. It is therefore neces-

143

sary to reveal the dependence of different farming methods on external ecosystems. Focus must be put on linkages, which a priori appear to be ecologically important, between aquaculture and its environment. One such approach is to estimate the ecosystem area - - t h e ecological footprintIappropriated by aquaculture (Larsson et al., 1994; Rees and Wackernagel, 1994). This ecological footprint is estimated for two alternative aquaculture methods in Lake Kariba-large-scale intensive cage culture and small-scale, semi-intensive pond farming. In the beginning of the paper we make a first order comparison of the economic and ecological resource demand in intensive cage farming and semi-intensive pond farming, expressed in industrial and solar energy units. In the next section we estimate the ecosystem areas required for production of feed, oxygen, and phosphorus assimilation to sustain 1 m 2 of the two farms. As far as we know, no earlier attempts have been made to estimate the latter two

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Fig. 2. A simplified conceptual model describing the major flows in the ecological and economic system involved in the production of fish in Lake Kariba, Zimbabwe. The natural aquatic system can be divided into two subsystems: the pelagic system where the kapenta is found, and the littoral system where most inshore fish species are found. Two more or less separated fisheries exploit these resources; the capital-intensive kapenta fishery and the locally-based inshore fishery. Development of semi-intensive aquaculture is assumed to be linked to the inshore fisheries while intensive cage farming probably would be more closely linked to the pelagic system. For the simplified foodchain of the aquatic system, values are given in g C m -2 year- L (Machena et al., 1993). The total inshore fish production is estimated to be 1.3-times the production of the most common species (1.4 g C m 2 y e a r - t ) (cf., Machena et al., 1993), as these make up about 75% of the total yield (Anonymous, 1993d). The yields from the pelagic and inshore fisheries are estimated from Machena et al. (1993) and are given both as tonnes year-i from the whole lake, and as g C m - z year-i (values in parentheses), assuming a fresh fish carbon content of 10% (Beveridge, 1984). The feed requirements of intensive cage farming and semi-intensive pond farming are based on an annual production of 50 tonnes.

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H. Berg et al. / Ecological Economics 18 (1996) 141-159

services in relation to ecological footprints. These footprints should be regarded as rough first-order estimates, illustrating that aquaculture depends on several different services provided by the ecosystem, and that the capacity of ecosystems to maintain these services is limited in both time and space.

2. Study area

The Kariba Dam was built on the Zambezi River for the production of hydroelectric power (Fig. 1). After the lake was formed in December 1958, it underwent changes in its physico-chemical characteristics and a marked succession in plant and animal development occurred (Coche, 1968; McLachlan and McLachlan, 1971; Balon, 1978; Machena, 1989). In the first years nutrients leaching from the submerged soil increased the productivity of the Zambezi water, but due to the short water replacement time of 2.5 years (Balon and Coche, 1974), the initial nutrients were soon lost through the hydro-electric turbines. Today Lake Kariba is considered to be oligotrophic (Machena and Kautsky, 1988; Marshall, 1994) with phosphorus probably being the limiting nutrient (cf., Magadza et al., 1989). The supply of nutrients influences the phytoplankton productivity, and a first peak occurs during the hot rainy season in December-February, and a second peak occurs in July-August, just after turnover. The average primary productivity in the lake is 1.9 g Cday -~ m -2, of which more than 90% is within the littoral zone (Fig. 2) (Machena et al., 1993). The zooplankton production varies with the phytoplankton fluctuations (Ramberg et al., 1987). The sardine Limnothrissa miodon, locally called 'kapenta', was introduced from Lake Tanganyika in 1968 and proved to be a success (Marshall et al., 1982). As the pelagic zone seemed to be little exploited by the already existing riverine fish species, the pelagic Limnothrissa filled an empty niche in a structurally simple food chain: phytoplankton--zooplankton--Limnothrissa and finally the carnivorous tigerfish (Ramberg et al., 1987) (Fig. 2). The sardine propagated rapidly (Marshall et al., 1982) and commercial fishing started in 1974 on the Zimbabwean side. In 1988 the total catch was 18000 tonnes on the Zimbabwean side of the lake and 12 000 tonnes

on the Zambian side (Machena and Konondo, 1991). The peak in catches follows the phytoplankton and zooplankton maxima, which suggests that growth and survival of Limnothrissa is mainly regulated by the availability of food (Marshall, 1982; Ramberg et al., 1987). In addition to the pelagic kapenta fishery there is a locally based inshore fishery, which is smaller and less capital-intensive than the former (Machena and Konondo, 1991) (Fig. 2). Currently, the yield from both the Zambian side and the Zimbabwean side is about 3400 tonnes, which is much lower than earlier predictions (Machena et al., 1993). The decline over the last 20 years is probably due to natural causes, such as nutrient depletion of the submerged soils and changes in the trophic structure of the fish populations from a dominance of herbivores to invertebrate foragers (Ramberg et al., 1987). Today there are some 900 fishermen exploiting the inshore fishery resources on the Zimbabwean side, and another 1900 on the Zambian side (Machena and Konondo, 1991). Artisanal fisheries are generally considered a poor credit risk because of the nature of their fishing operations. Catches and income are often very seasonal and variable. There is a tendency to overfishing, which is a growing problem since the number of fishermen is slowly increasing (Machena and Konondo, 1991). It is now hoped that aquaculture will become another means of increasing the production of fish protein. During the last few years, plans have been made for an expansion of aquaculture on the Zimbabwean side of Lake Kariba, both on an artisanal basis and through intensive farming (Dam and Aquaconsult SpA, 1986; Gabriel, 1991). At present there is small scale pond farming and a pilot cage farm in basin 5 close to the Kariba town (Fig. 1). The main species farmed are Tilapia rendalli and Oreochromis mossambicus, but also Tilapia niloticus and some species of omnivorous carps have been kept in the ponds (Gabriel, 1991; F~ilster, 1994). In the near future the pilot cage farm will probably be reconstructed for large-scale cage farming with an annual production of 2000 tonnes. So far, no permits have been issued since the consequences of these activities have not yet been fully evaluated. The environmental impacts from the cages have been monitored over the last 3 years (StenstriSm, 1993; Troell and

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H. Berg et al. / Ecological Economics 18 (1996) 141-159

Berg, in preparation) and the ponds have been studied by Snook (1987), Langerman (1990) and Frlster (1994).

Table 1 Intensive cage farming (4.5 X 5 X 2 m) and semi-intensive pond farming (100 X 50 X1 m) of a mixture of Tilapia species in Lake Kariba, Zimbabwe

3. Methods

Stocking density (fish m 2) Growing-out period (days) Av. initial weight (g) Av. final weight (g) Survival rate (%) Daily growth (gm -2 ) Yield (kgm -2 c y c l e - i )

As the aim of this paper is to draw attention to some potential consequences of different aquaculture systems before they are introduced at full scale in Lake Kariba, the described farms are hypothetical. However, cage farming is at an advanced planning stage (Lasserre and Hough, 1990; Gabriel, 1991) and pond farming has been considered in earlier studies (Dam and Aquaconsult SpA, 1986). Thus, based on the best available data, simplified conceptual models were drawn to clarify how intensive cage farming and semi-intensive pond farming could be interlinked with the Lake Kariba ecosystem in the future. Semi-intensive pond farming is assumed to be developed in connection with the inshore fisheries, whereas the more capital-intensive cage farming probably would be closer linked to the pelagic fisheries (Fig. 2). The analysis is divided into three parts. In the two first parts the economic and ecological resource demand to produce 1 kg of farmed fish is expressed as human-made capital (or manufactured capital) input and natural capital input (Costanza and Daly, 1992; Berkes and Folke, 1992). By converting all resource inputs into industrial energy and solar energy, respectively, the total energy demand for production of fish was estimated. In the last part we try to elucidate the services provided by the ecosystem by estimating the ecosystem area (Odum, 1975) that would be required to support the two aquaculture activities. Such estimates are helpful in indicating ecological constraints on economic activities in order to avoid disrupting ecosystem functions. Detailed descriptions of the calculations are found in the notes to the tables and figures in the paper.

3.1. Human-made capital input The human-made capital input to aquaculture (materials, labour, machines, fuel, and so on) was estimated from a farm located in basin 5 in Lake Kariba, which is being reconstructed from a semi-intensive pond farm to a more intensive cage farm

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Ponds

725 280 20 200 85 380 125

2 360 20 250 90 1.3 0.5

(Gabriel, 1991). Investment costs and running expenses were based on an annual production of 50 tonnes (Gabriel, 1991). In intensive cage farming, 39 tonnes of fish are assumed to be produced every 280 days in 14 cages (14 X 46 m3), which would give an annual production of 50 tonnes (Table 1). However, to be economically viable, the minimum production size for intensive aquaculture is around 200 tonnes (Gabriel, 1991). In semi-intensive pond farming it is assumed that 50 tonnes will be produced in 20 ponds (20 X 5000 m 3) built close to ten of the larger fishing villages along the lake (cf., Dam and Aquaconsult SpA, 1986). All costs were estimated in year 1990 prices. To express the human-made capital demand as industrial energy use (cf., Hall et al., 1986), costs of different inputs were multiplied by the ratio between the industrial energy use and the gross national product (GNP) for Zimbabwe in 1990. Although this is an imprecise way of estimating industrial energy use (Hall et al., 1979; Cleveland, 1992), it provides a first rough estimate, as there were no data available for more precise calculations.

3.2. Natural capital input Natural resource demand from ecosystems was estimated as the annual food input required to sustain the production of 1 kg fish, and is expressed in megajoules per kg fish produced. For intensive cage farming this input was based on a feed conversion ratio of 2.5, which is in the lower range of what was found at the cages between 1991 and 1994 (Troell and Berg, in preparation). The fish meal (50%) and

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H. Berg et al. / Ecological Economics 18 (1996) 141-159

fish oil (10%) in the pellets (cf., Folke and Anerr, 1988) were assumed to be derived from dried kapenta, as this may become economically competitive with commercial fishmeal in the future (see Discussion) (Lasserre and Hough, 1990). The solar energy content of the net primary production required to sustain the production of the kapenta used for fish meal was estimated from the energy ratio between net primary production and total fish production in Lake Kariba (cf., Folke and Anerr, 1988; Larsson et al., 1994). Production data, trophic relationships and community structure of the Lake Kariba ecosystem has been earlier analysed using the ECOPATH II model (Machena et al., 1993). Although this is a simplistic model of the lake, it is to our knowledge the only available synthesis of such data for Lake Kariba.

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14. Berg et al. / Ecological Economics 18 (1996) 141-159 a s s u m e d to b e p r o v i d e d b y the p o n d s y s t e m itself. T h e e n e r g y c o n t e n t o f the n e t p r i m a r y p r o d u c t i o n ( N P P ) r e q u i r e d to s u s t a i n the p r o d u c t i o n o f f i s h / f i s h w a s t e w a s c a l c u l a t e d as for the k a p e n t a a b o v e . T h e i n p u t o f a g r i c u l t u r a l p r o d u c t s in f e e d pellets w a s e s t i m a t e d b y a s s u m i n g t h a t the c o m p o s i t i o n o f t h e s e p e l l e t s w o u l d b e s i m i l a r to t h a t o f p e l l e t s f r o m t e m p e r a t e areas ( 2 0 % c e r e a l s a n d 10% s o y b e a n ) (personal communication, Statkorn, Norwegian grain; F o l k e a n d A n e r r , 1988). T h e total n e t p r i m a r y prod u c t i o n w a s c a l c u l a t e d u s i n g k n o w n f i g u r e s f o r the p r o p o r t i o n o f c r o p s t h a t is n o t h a r v e s t e d ( P i m e n t e l a n d P i m e n t e l , 1979; Z u c c h e t t o a n d J a n s s o n , 1985). H o w e v e r , s i n c e n o d e t a i l e d r e c i p e s f r o m the m a n u f a c t u r e r s w e r e a v a i l a b l e , the e x a c t n e t a n d gross p r i m a r y p r o d u c t i o n r e q u i r e d to p r o d u c e t h e s e c o m p o n e n t s are d i f f i c u l t to c a l c u l a t e a c c u r a t e l y . T h i s is f u r t h e r c o m p l i c a t e d b y the v a r i o u s c l i m a t i c a n d n u trient r e g i m e s u n d e r w h i c h t h e s e c r o p s m a y b e cultivated. A r o u g h e s t i m a t e m a y still b e p l a u s i b l e , s i n c e the a g r i c u l t u r a l part in pellets is u s u a l l y r e l a t i v e

147

c o n s t a n t , a n d since a g r i c u l t u r e N P P is a m i n o r c o m p o n e n t o f t h e total N P P r e q u i r e d in i n t e n s i v e c a g e fanning.

3.3. Ecosystem support areas T h e e c o s y s t e m areas t h a t w o u l d b e a p p r o p r i a t e d b y a q u a c u l t u r e w e r e e s t i m a t e d for f e e d p r o d u c t i o n ( d i v i d e d into terrestrial a n d a q u a t i c support), o x y g e n p r o d u c t i o n , a n d p h o s p h o r u s a s s i m i l a t i o n . T h e s e areas w e r e c a l c u l a t e d as the area o f p r i m a r y p r o d u c t i o n r e q u i r e d to s u s t a i n 1 m 2 o f i n t e n s i v e a n d s e m i - i n t e n sive a q u a c u l t u r e , a n d w e r e b a s e d o n a daily fish p r o d u c t i o n o f 3 8 0 g m - 2 a n d 1.3 g m - 2 , r e s p e c t i v e l y (see T a b l e 1). F o r f e e d production the e c o s y s t e m s u p p o r t area for the d i f f e r e n t c u l t u r e s y s t e m s w a s c a l c u l a t e d as the area r e q u i r e d to p r o d u c e 3 8 % (0.38 k g fish m - 2 ) a n d 0 . 1 3 % ( 0 . 0 0 1 3 k g fish m - 2 ) , r e s p e c t i v e l y , o f the n e t p r i m a r y p r o d u c t i o n g i v e n in Fig, 3 (1 kg f i s h m - 2 ) . T h e s e e s t i m a t e s w e r e b a s e d o n a daily net

Fig. 3. Input of natural capital and solar energy flows in intensive cage farming (top) and semi-intensive pond farming (bottom) in Lake Kariba, Zimbabwe. Estimates of energy flows (MJ) and material flows (kg) are estimated from a production of 1 kg fish. Figures may not add up due to rounding. The calculations are as follows: (a) The feed convertion ratio for the intensive fish farm is assumed to be 2.5 (cf., Gabriel, 1991). (b) Pellets would consist of approximately 50% fish meal and 10% fish oil (cf., Folke and Anerr, 1988; Anonymous, 1991). This is assumed to be derived from dried kapenta (35% of wet wt.), (c) The energy content of kapenta is estimated to be 6.2 MJ kg-1 wet weight, using the same energy content as in Clupea harengus, (Anonymous, 1988). (d) The kapenta feed 99% on zooplankton, and the production of 1 kg kapenta requires approximately 14 kg zooplankton (cf., Machena et al., 1993). (e) The energy content of zooplankton is about 2 MJkg-l wet wt. (cf., Larsson et al., 1994). (f) The energy content of net primary production supporting the aquatic food-web is estimated from an energy transfer efficiency (ETE) of 0.6% between total fish production (4.1 g C m-2 year- I) and primary production (680 g C m -2 year i) (Machena et al., 1993; see Fig. 2). (g) The net primary production, expressed in wet weight, is back-calculated from the energy content. It is assumed that the carbon content in primary production is 50% by dry weight (Jansson and Wulff, 1979) and that the energy content is 42.5 MJ kg-I carbon (Folke and Anerr, 1988). The dry weight is around 10% of the wet weight (Larsson et al., 1994). (h) Aquatic gross primary production (GPP) is estimated to be 4-times the net primary production (NPP) (Melack, 1976; Silva and Davies, 1986) and terrestrial GPP to be 2-times the NPP (Odum, 1983; Larsson et al., 1994). (i) The pellets are assumed to contain approximately 20% wheat or maize (87% dry wt.) and 10% soybean meal (88% dry wt.) (cf., Folke and Anerr, 1988; Anonymous, 1991). (j) The avarage energy content of the crops used is 13.2 MJ kg- l (Anonymous, 1988). (k) About 25% of the total energy content of agriculture net primary production is in the edible parts, 30% in the waste products and the rest in below ground parts (Zucchetto and Jansson, 1985; Pimentel and Pimentel, 1979). (1) Fish produced in semi-intensive pond farming are assumed to require the same amount of energy (37.9 MJ) as fish produced in intensive cage farming. More than 80% of this energy is suggested to come from fish waste (see Discussion). (m) The average energy content of the inshore fish species (including offal) in the lake is estimated at 4.6 MJ kg t. This estimate is based on the average energy content in similar species from temperate areas, as no other data were available (Anonymous, 1988). (n) Based on data from temperate areas, it was assumed that 65% of the landed fish from the inshore fishery can be used for human consumption, and the remaining 35% is waste products, such as fish offal (Anonymous, 1988). (o) Due to the low agriculture potential in the area, it is assumed that agriculture waste only can provide some 10% of the total energy requirement in the pond farm. (p) 10% of the feed demand is assumed to be provided by the primary production in the pond. Using the same ratio between carbon content of fish and net primary production as in the lake, a production of 10 g carbon in fish corresponds to a net primary production of approximately 1.7 kg carbon or 33 kg wet wt. (5% carbon in phytoplankton wet wt.). (q) The energy content of bream is around 4.3 MJ kg-t(estimated from the energy content in Abramis brama) (Anonymous, 1988).

H. Berg et ai. / Ecologic al Economics 18 (1996) 141-159

148

p r i m a r y p r o d u c t i o n o f 1.9 g C m - 2 in t h e lake a n d 2.3 g C m - 2 in t h e p o n d s . A l t h o u g h r e l i a b l e d a t a o n p r i m a r y p r o d u c t i o n in t h e p o n d s y s t e m w e r e l a c k i n g , it w a s a s s u m e d to b e h i g h e r t h a n in t h e lake b e c a u s e o f t h e h i g h e r n u t r i e n t c o n c e n t r a t i o n s in t h e p o n d

c a l c u l a t i o n w i t h p h o t o s y n t h e s i s as the o n l y s o u r c e o f o x y g e n , as o n l y a m i n o r p a r t o f t h e o x y g e n d i s s o l v e d in w a t e r o r i g i n a t e s f r o m air ( D e l i n c 6 , 1992). T h e p h o s p h o r u s assimilation capacity o f t h e local e n v i r o n m e n t w a s a s s u m e d to r e m a i n u n c h a n g e d d e s p i t e

w a t e r ( S n o o k , 1987; L a n g e r m a n , 1990; F~ilster, 1994). F o r i n t e n s i v e c a g e f a r m i n g t h e r e q u i r e d agriculture support area was estimated from the average

the increased nutrient inputs from the fish farms, and was based on the uptake of phosphorus through p h o t o s y n t h e s i s . T h e e c o s y s t e m a r e a s r e q u i r e d to p r o -

a n n u a l y i e l d o f w h e a t , m a i z e a n d s o y b e a n s in Z i m -

vide these services for the fish farms should be r e g a r d e d as r o u g h l o w e r e n d e s t i m a t e s s i n c e t h e c a l c u l a t i o n s d o n o t t a k e into a c c o u n t o x y g e n c o n sumption and nutrient excretion from other organisms within the Lake Kariba ecosystem.

b a b w e (0.3 g m - 2 , w e t w t . ) ( A n o n y m o u s , 1990). T h e a r e a n e e d e d f o r oxygen production, c o m p e n sating for fish respiration and biological oxygen demand (BOD) from waste products, was based on

cages lm2

assimilation

ponds lm2

Oxygen production

,~. ~ ^ . ^^".****%^^%~%%~.%% "^%^.%*.

0.5 m2

115 m 2

~

Oxygen production

~

.:+:^:^:-:, F~d p~od~ct,o~ ^~,'^'^^,~, 2-2^:^2^2~2-:420 m2 22:222222:2

..........o iiiiiii!iiiii!iiii)ii!i!iiiiiii!iiii!iii)i!!i i Agriculture ecosystem

Phosphorous aa.tmllaUon

0.9 m 2

Enlarged pond surface

Fig. 4. Left panel (cages lm2): Ecosystem support areas required to sustain 1 m 2 of intensive cage farming. Estimates are based on an average daily production of 380 g fish m -2 (Table 1). The fi)od demand is 38% of the figures given in Fig. 3, as these are estimated for a production of 1 kg. Based on an aquatic primary production of 1.9 g C m -~day- ~, the required net primary production, 40 kg C clay- i, is produced from a lake surface area of about 21 000 m 2. The agriculture life support area is about 420 m 2. This estimate is calculated from an avarage yield of 0.3 kgm 2 year- i for maize, wheat and soybean in Zimbabwe in 1990 (Anonymous, 1990). Respiration and biological oxygen demand (BOD) are equivalent to a daily oxygen production, through photosynthesis, from areas of approximately 60 and 100 m -2, respectively. Oxygen consumption, 290 g 02 m 2 clay ~, is estimated from a respiration rate of 0.2 g 02 kg- i h - i for Tilapia (Beveridge, 1991; Stenstri3m, 1993) and an average fish density of 60 kgm -2. A feed input of 1 kgm-2day - i and an FCR of 2.5 result in a BOD of about 500 g m -2 day t (Bergheim et al., 1991; Michelsen, 1991). For each gram of carbon fixed in photosynthesis, 2.6 g oxygen is produced (Pruder, 1986), which gives an average oxygen production of 4,8 g 02 m -2 day- i. The phosphorus load, 5.4 g, can sustain primary production over an area of about 115 m 2. The pellets used in Kariba have a phosphorus content of 0.7% (Gabriel, 1991 ; Stenstr~m, 1993). With an FCR of 2.5 some 80% of the P content in the feed is lost to the environment (Beveridge, 1991; Stenstr~m, 1993; Troell and Berg, in preparation). According to the Redfield ratio (Redfield et al., 1963), between P and C (1/40), a primary production of 1.9 g C m -2 day ~ is equivalent to an assimilation of phosphorus of about 50 mg m -2 day- ~. Right panel (ponds lm2): Ecosystem support areas required to sustain 1 m 2 of semi-intensive pond farming. Estimates are based on a avearage daily production of 1.3 g fish m -2. Respiration and biological oxygen demand (BOD) are equivalent to a daily oxygen production, through photosynthesis, from areas of approximately 0.2 and 0.3 m -z, respectively. Oxygen consumption, 1.2 g 02 m -2 day -~ , is estimated from a respiration rate of 0.2 g O 2 kg- i h i for Tilapia (Beveridge, 1991 ; StenstrSm, 1993) and an average fish density of 0.25 kgm 2. A production of 1.3 g fish would require about 3.2 g feed (dry wt.) (FCR approx. 2.5), which is equivalent to approximately 1.6 g BOD (see panel a). The phosphorus load, 0.05 g, can probably be assimilated within a pond area of approximately 0.9 m e. The fish waste from the inshore fisheries has a phosphorus content of 0,7% (wet wt.) (Gabriel, 1991) and it is assumed that some 80% of the P content in the feed is lost to the environment. A primary production of 2.3 g C m-2 day-~ assimilates approximately 60 mg phosphorus m-2 day-I.

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H. Berg et al. / Ecological Economics 18 (1996) 141-159

Quantitative estimates of photosynthesis-related processes are derived from the empirical relationship developed by Redfield et al. (1963) where each gram of fixed carbon is equivalent to a production of approximately 2.6 g oxygen (Pruder, 1986) and an assimilation of 25 mg phosphorus.

4. Results The results indicate that intensive cage f a n n i n g in Lake Kariba would appropriate substantially larger ecosystem areas for producing its food and processing its waste than semi-intensive pond farming (Fig. 4a,b). Intensive cage f a n n i n g is estimated to require 17800 MJ GPP to produce 1 kg fish, while the corresponding figure for semi-intensive pond farming is estimated at 58 800 MJ (Fig. 3). However, the latter estimate includes not only the fish waste assumed to be used in pond farming but also the other parts of the fish which are consumed by other economic activities. Thus, since fish waste is a by-product from the inshore fisheries, it does not appropriate additional ecosystem areas and is therefore not accounted for (Table 3). The industrial energy input to cage farming (about 85 MJ kg -~ ) is 1.7-times higher than the energy demand in semi-intensive pond farming (about 50 M J k g - l ) (Table 2), mainly because there are no additional costs associated with the feed used in the ponds. The ponds are also more separated than the cages from the lake, and services such as oxygen production and phosphorus assimilation can probably be provided within the pond system itself because of the much lower rearing density of fish (Table 1 and Fig. 4b). Intensive cage farming, on the other hand, must be supported by ecosystem areas that all are much larger than the area of the farm (Fig. 4a). The ecosystem area for feed production is as much as 21 000-times larger than the area of the cages. The areas required for oxygen production (160 m 2 m - 2 ) and phosphorus assimilation (115 m 2 m - 2 ) are much smaller, but of special importance, since they must be located close to the farm (Fig. 4a). 4.1. H u m a n - m a d e capital input

The industrial energy demand for the production of 1 tonne of fish in intensive cage farming and

Table 2 Estimated industrialenergy requirmentsa for intensivecage farming and semi-intensive pond farming in Zimbabwe, 1990 (MJ kg I) Type of expenses

Cages

Ponds

b Labour(wages, salaries management) c Investments(financial,depreciation) d Pellets/feed Fry f Other costs

17 2b 38 22 4

20 9 -16 5

Total costs

83

50

a The industrial energy requirments are estimated by multiplying the cost of different inputs with the energy use/GNP ratio. In 1990 this ratio was 0.0298 GJ US $-l based on an energy use of 185 1015 J (Anonymous, 1993a), GNP of 15 174 million Z$ (Anonymous, 1993b) and an exchange rate of 0.4085 US$ per Z$ (Anonymous, 1993c). b Cost of labour (cages approx. 1400 Z$, ponds approx. 1600 Z$) is estimated from Gabriel (1991). c Investmentcosts (cages approx. 150 Z$, ponds approx. 740 Z$) are based on a depreciation rate of 20 years (cf. Gabriel, 1991). Costs for capital financingexcluded. d Based on the cost for catching kapenta (Michrlsen, 1995). This is slightly higher than estimates based on the actual price for pellets (approx. 2700 Z$ or 32.8 MJ kg -L ) (Gabriel, 1991), but according to Larsson et al. (1994) the energy cost for pellet production in developingcountriesis often underestimated,and 38 MJ kg- i is probably also a lower end estimate. The productioncost of 20 g fingerlings(cages approx. 1800 Z$, ponds approx. 1300 Z$) was estimated from the feed cost (Lasserre and Hough, 1990), assuming that this cost accounts for approximately 20% of the total cost (Anonymous,199l). f Includescosts for electricity, medicine,antibiotics,boat fuel, etc (cages approx. 330 Z$, ponds approx. 410 Z$).

semi-intensive pond f a n n i n g is presented in Table 2. These are first-order estimates, to make a preliminary comparison of the resource demand of the two different fish-fanning techniques. In intensive cage farming, pellets will be the largest single input and make up almost 50% of the total industrial energy demand. In semi-intensive pond farming this cost will be zero if only waste products are used for feed. The labour costs are higher for the ponds than for the cages and make up 40 and 20% of the total production cost for ponds and cages, respectively. The higher input of labour needed in pond f a n n i n g is because this production is assumed to be divided into smaller production units than in intensive cage farming. Although the construction costs probably can be

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1t. Berg et al. / Ecological Economics 18 (1996) 141-159

Table 3 Natural and industrial energy inputs required to produce I J of fish cultured in cages and ponds in Kariba (average values calculate in J / J )

NPP/biomass GPP/biomass GPP/edible protein Industrial energy/biomass ~ Industrial energy/edible protein ~ Natural/industrial energy input ratios d

Cages

Ponds

1040 4140 10450 b 15 40 270

(3 420) ~ (13 670) ~ (34520) ~ 7 18 1960

a It should be noted that although only waste products are used as feed in semi-intensive aquaculture, this farming system relies to a large extent on the inshore fish production. b Edible parts of Tilapia were assumed to be 60%, of which 16.7% is protein. As no data on Tilapia were available, this was estimated from data on common bream, Abramis brama (Anonymous, 1988). The energy content of 1 g protein is around 17 KJ (Anonymous, 1988). Thus, 1 kg Tilapia corresponds to about 1.7 MJ edible protein. Excluding labour in Table 2. d GPP/industrial energy (excluding labour).

kept low for the ponds (cf., Lasserre and Hough, 1990), the investment costs will be high compared to the cages if powerful electric pumps are needed for the pumping of water. However, these costs can probably be substantially reduced by using winddriven pumps (cf., Dam and Aquaconsult SpA, 1986). The costs for fry are slightly higher in the cages because of the higher mortality rate expected in intensive cage farming than in semi-intensive pond farming (Table 2). The total industrial energy required to produce 1 kg fish is estimated at 50 MJ and 83 MJ in ponds and cages, respectively. If labour is excluded, about half as much energy is required in semi-intensive pond farming as in intensive cage farming (Table 2). This corresponds to an input of 18 and 40 J of industrial energy per joule of edible protein output in the two systems (Tables 3 and 4).

cage farming this energy input would be derived from 4.3 kg kapenta and 0.9 kg crop, with about 70% of the feed energy derived from the aquatic ecosystem and 30% from agriculture (Fig. 3). Calculated as gross primary production, 99% would come from the lake and 1% from agriculture (Fig. 3). The comparatively high proportion of energy from the aquatic system is a reflection of the kapenta being a predator in the food chain, whereas agricultural products are primary producers. The energy content of the gross primary production required to support 1 kg fish is 4140-times larger than the energy content of the farmed fish (Fig. 3 and Table 3). The corresponding figure based on edible protein is 10450 MJ (Table 3). In semi-intensiL'e pond farming the solar energy support expressed as gross primary production is 13 670 J per joule farmed fish, which is more than 3 times the energy used in intensive aquaculture (Fig. 3 and Table 3). However, such a comparison is deceptive, since the major part of the landed fish is used for human consumption, and only the fish waste (approx. 35% wet wt.) will be used as feed in the ponds (Fig. 3). Thus, semi-intensive aquaculture is dependent on a large support of fixed solar energy,

Table 4 Estimates of industrial energy inputs (excluding labour) per protein output for various food production systems ( J / J ) Aquaculture type

Industrial energy input/protein energy output ( J / J )

Seaweed culturing in West Indies Kapenta fishery in Lake Kariba Mussel farming in Scandinavia Convention sea ranching of Atlantic salmon Pond-farming of Tilapia in Lake Kariba Cod fisheries in US Cage-farming of rainbow trout in N. Ireland Atlantic salmon fishery

1~ 4 b 10 c 12 c 18 ~ 20 c 24 ~ 29 c 40 a 40 ~ 50 c 192 c

Cage-farming of Tilapia in Lake Kariba

4.2. Natural capital input Fig. 3 lists the estimated natural capital demand in a scenario of intensive cage fanning and semi-intensive pond farming in Lake Kariba. Added feed would amount to 37.9 MJ per kg fish produced. In intensice

Shrimp farming in Colombia Cage-farming of Atlantic salmon Lobster fishery in US results from this study. From Michrlsen (1995). Adapted from Folke and Kautsky (1992). From Larsson et al. (1994).

H. Berg et al. / Ecological Economics 18 (1996) 141-159

but does not compete for this fixed energy with other uses since it solely utilizes energy flows that otherwise would have been wasted.

4.3. Ecosystem support areas Fig. 4 shows the estimated ecosystem support areas that are required in intensive cage farming and semi-intensive pond farming for the production of feed, oxygen, and nutrient assimilation. These services can be provided within the same life-support area and should therefore not be added together. However, it should be noted that the two latter are to some extent separated from the first in time and space. The food required in intensive aquaculture would appropriate an area of Lake Kariba that is about 21 000-times the area of the cages (Fig. 4a). Taking into account that the maximum sustainable yield of kapenta must be smaller than the total production, this life support is a minimum of what is actually needed. The agriculture support of 420 m 2 is much smaller than the aquatic support because of the lower proportion of agriculture products in the pellets and the comparatively low solar energy input required to produce the crop (Fig. 4a). The surface area of primary production required to produce the oxygen consumed by the fish in the cages was estimated to be 60 times the area of the cages. It should be noted that the estimated fish respiration is based on an average stocking density of 60 kg m - 2 , which will be twice as high at harvest (Table 1). Thus, 50 tonnes of Tilapia, which are assumed be produced in 14 cages with an approximate area of 320 m 2 (cf., Table 1), would require an average area of about 20 000 m 2 during production and 40 000 m 2 at harvest. In addition to the oxygen consumed directly by the fish much oxygen is also consumed by the organic waste produced at the farm. As a first rough estimate it was assumed that with a feed conversion ratio of 2,5 approximately 0.5 kg oxygen is needed for each kilogram of feed added (cf., Bergheim et al., 1991; Michelsen, 1991). Thus, based on a average daily fish production of 0.380 g m -2, approximately 1 kg feed would be required, which is equivalent to a biological oxygen demand (BOD) of 0.5 kg. From this it was estimated that the BOD from the farm

151

requires an area for oxygen production that is 100 times larger than the area of the cages, if the oxygen in the vicinity of the cages is not to decrease. In Kariba the average phophorus content of the pellets, currently in use, is around 0.7% (Gabriel, 1991). For each kilogram of added feed approximately 6 g of phosphorus are lost to the environment (cf., Beveridge, 199l; Troell and Berg, in preparation). A daily phosphorus load of 6 g could satisfy primary production over an area of approximately 115 m 2 (Fig. 4a). This estimate is based on the assumption that the primary production should not be severely affected by the addition of phosphorus. As discussed above, there is no need to estimate the ecosystem areas for food production required in semi-intensive aquaculture since only waste products from fisheries and agriculture are assumed to be used in the ponds. Based on similar calculations to those for cage farming, an average daily fish production of 1.3 g m -2, and a primary production of 2.3 g C m - 2 in the ponds, oxygen production (0.5 m 2 m - 2 ) and assimilation of phosphorus (0.9 m 2 m - 2 ) could probably be sustained within the pond system itself (cf., Edwards, 1993) (Fig. 4b). Hence, no external life support from Lake Kariba would be needed to farm Tilapia in semi-intensive ponds.

5. Discussion In this paper we have made two scenarios of fish farming in Lake Kariba as this may indicate how aquaculture could be developed to increase the chances for long-term sustainable resource use in the lake. Intensive aquaculture is characterised by the high stocking density of fish. Large energy and material flows from the ecosystem are channelled through the cages (Fig. 3) and increases both fish yield and output of waste in the limited area of the fishfarm. As a consequence, intensive cage farming appropriates larger surface areas than semi-intensive pond fanning for the production of feed, assimilation of phosphorus and production of oxygen (Fig. 4a,b). In the first part of the discussion we compare our results with those from earlier field studies of cage farming and pond farming in Lake Kariba. The different assumptions used in the conceptual models of the two systems are discussed. As the scale of

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H. Berg et al. / Ecological Economics 18 (1996) 141-159

production may increase in the near future this is considered in relation to the resource demand of the different farms. In the last part of the discussion we try to place the two systems in the context of sustainable development of aquaculture in Lake Kariba.

5.1. Intensive cage farming The high input of natural and human-made capital (Fig. 3 and Table 2) into intensive cage culture clearly shows that this system must be supported with large amounts of resources produced outside the cages to sustain its high production of fish. The solar energy and primary production in the cages is much too small to feed the unnaturally high fish biomass at the rearing site. In Lake Kariba, for example, phytoplankton and algae growing on the cages has been estimated to contribute to less than I% of the total food demand (Stenstr~m, 1993). Because of this the cost of feeding is significant and can account for more than 50% of the total production cost (cf., Coche, 1982; Beveridge, 1984). In Lake Kariba the high feed costs are further increased because of the low quality of the feed (Lasserre and Hough, 1990), which is a common problem when developing intensive cage culture in the tropics (Beveridge, 1984). Today the energy content of the pellets is low because they are produced from a pig pre-mix rather than from a specific fish diet (Lasserre and Hough, 1990). The feed conversion ratio has over the last 2 years been around 3.5 and 2.5 (Troell and Berg, in preparation) and it has been stressed that the quality of the feed must be improved to achieve commercially successful fish farming in the future (Lasserre and Hough, 1990). The conventional way to improve the situation would be to complement the available mix from manufacturers with imported premixes, but it has also been suggested that if licences for catching kapenta were available, kapenta for fish meal purposes could be provided at a cost that would be competitive with the price of commercial fish meal (Lasserre and Hough, 1990). The energy content of these pellets would be around 15 MJkg - I (Fig. 3), which is normal for pelletized feed (cf., Johnsen and Wandsvik, 1991). If kapenta were used for the production of fishmeal in pellets, about 4 kg of kapenta would be

needed for production of I kg of farmed fish (Fig. 3). From an economic point of view this could be feasible as the price for Tilapia can be much higher than for kapenta (Lasserre and Hough, 1990). This may also be the most competitive way to provide the area with fishmeal in the future, as the demand for feedstuffs is recognized as a constraint to aquaculture growth in a number of countries and aquaculture is likely to use 20-25% of world fishmeal supplies by the year 2000 (Beveridge and Phillips, 1993). However, from an ecological and natural resource point of view the use of kapenta for fishmeal production is very doubtful. Apart from using a prime, highly-demanded human protein resource as animal feed with more than 80% losses in the conversion from kapenta to Tilapia (Fig. 3), a production of 2000 tonnes of fish in cages would require as much as 8600 tonnes of kapenta, which amounts to almost 30% of the total annual yield from the lake (cf., Fig. 2). With increasing prices of fish feed on the world market, such a reliance on the kapenta fisheries could result in an increased risk for an overexploitation of the pelagic resources in Lake Kariba. The large input of resources required by intensive aquaculture is further illustrated by the fact that the aquatic surface area appropriated for the production of fish pellets is more than 20 000-times larger than the area of the cages. If this estimate were based on the actual yield instead of the production of kapenta, the life support area would be almost 80000-times the cage's area since the yield is about one-fourth of the estimated total production (cf., Machena et al., 1993) (Fig. 2). The agriculture support area is comparatively small, but must probably be provided by ecosystems far away from the lake since the agriculture potential is very low around the lake. Thus, intensive aquaculture requires resources from large ecosystem areas outside the farm to sustain its high production of fish (Fig. 4a). If based on locally produced fishmeal, such a production must be regarded as a bad substitute for the pelagic fisheries since these fisheries support direct human consumption. It is obvious that the only way that intensive cage culture can lead to an increase in the total production of fish from the lake is if it uses feed that is imported from other areas. The life support would then be provided by ecosystems outside the lake. However, such support should not be taken for

H. Berg et al. / Ecological Economics 18 (1996) 141-159

granted in the light of a rising future world shortage of fishmeal, and fish stocks worldwide balancing on the edge of overexploitation. Although the total area of 160 m 2 is a necessary rough estimate of the ecosystem support required to sustain 1 m 2 of cage farming with oxygen (Fig. 4a), it indicates that there are physical limits to how large the farm can be. A significant drop in oxygen levels found around the cage farm in Lake Kariba (Mhlanga, 1994) indicates that the fish in the cages consume considerable quantities of oxygenated water. Furthermore, much oxygen is also required to compensate for the increased oxygen demand from the organic waste produced at the farm (Troell and Berg, in preparation). As the size of production units increases (e.g., 2000 tonnes), the size of this area (approx. 2 000 000 m 2 ) may become critical because the mixing of water may not be sufficient to provide the cages with the oxygenated water. At the cages in Lake Kariba, for example, there was only a weak current ( < 0.05 m s- 1), and despite a low production ( < 10 tonnesyear-~), low oxygen saturation levels in the water ( < 25%) were recorded during shorter periods (Mhlanga, 1994). Furthermore, the sediment respiration rate was increased by 30-40%, and anaerobic conditions under the cages were indicated by the occurrence of hydrogen sulfide and methane gas in the sediments (Troell and Berg, in preparation). Thus, an uncontrolled expansion of intensive aquaculture could be accompanied by anoxic bottoms and a degradation of the environment in the vicinity of the cages. This has happened in temperate areas (Beveridge, 1984; Gowen and Bradbury, 1987; Gowen et al., 1990). Although little is known from tropical areas, fish farming could be expected to cause comparable changes in these environments given that the release of waste will be broadly similar (Beveridge and Phillips, 1993). Oxygen demanding waste could even be expected to be a worse problem in tropical lakes because of their comparatively higher water temperature (Fryer, 1972; Lewis, 1987). Another important service provided by the ecosystem, but seldom accounted for, is the assimilation of nutrients. In Lake Kariba, an area more than 100 times the area of the cages was estimated to be required for the uptake of phosphorus originating

153

from the cage farm (Fig. 4a). If the nutrients were released beyond the assimilative capacity of the ecosystem, this would lead to eutrophication effects. On a local scale, this could result in changes in algae species composition and blooms of harmful algae with high toxin production (Gowen et al., 1990). Excess nutrients would be accumulated in the sediments (cf., Troell and Berg, in preparation), which could have implications for the long-term recovery of the local environment following the closing of aquaculture activities. On the regional scale the increased nutrient input could increase the fish yield if, as in Lake Kariba, the water body is of oligotrophic type and nutrients are limiting to natural fish production. However, there could also be a risk of important fish stocks being affected negatively due to the shift in plankton dominance. In Lake Kariba this is especially true considering the pelagic fishery, which is entirely based on the zooplanktivorous kapenta (Marshall, 1994). However, in general, an increased input of nutrients from aquaculture is probably of minor importance for the trophic status of the whole lake (cf., Marshall, 1994). At a production of 50 tonnes the phosphorus load from the cages is only 0.07% of the total estimated amount of phosphorus entering the lake through the Zambezi river (cf., Magadza et al., 1989). Likewise, the increased oxygen demand from aquaculture activities would mainly be a problem for the local environment. Therefore, from a management perspective, these problems could to a large extent be avoided by adjusting the scale of production to the capacity of the local environment to generate oxygen and absorb nutrients.

5.2. Semi-intensice pond farming The degree of dependence on external ecosystems as well as the degree of negative environmental impact generally increases from extensive to intensive techniques (Pullin, 1989; Primavera, 1991; Folke and Kautsky, 1992). While intensive cage farming has been characterized as a throughput-based system, where inputs and outputs become more important as internal cycling of energy, nutrients and materials are reduced, semi-intensive pond farming is characterized by a more complete utilization of material and energy by organisms occupying different trophic lev-

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H. Berg et al. / Ecological Economics 18 (1996) 141-159

els and a higher recycling of organic matter and nutrients within the pond system (Beveridge, 1984; Folke and Kautsky, 1991). Contrary to intensive aquaculture, semi-intensive aquaculture relies both on natural food production in the ponds and on supplementary feed, usually from locally available plants or agriculture by-products (Coche, 1982; Beveridge, 1984; Beveridge and Phillips, 1993; Edwards, 1993), and the fish could probably obtain a major part of their nutritional requirements from natural sources when reared at low densities (Hepher and Pruginin, 1982). Conversion efficiencies from net primary production to Tilapia production can be more than 1% (Lowe-McConnell, 1987), and a primary production of 6001200 g C m -2 year-~ could probably sustain a fish yield of 60-150 g m - 2 year- i (cf., Beveridge, 1984). This is 12-30% of an annual fish production of 500 g m -2, and it seems reasonable to assume that the pond system in Fig. 3 at least could provide some 10% of the total energy requirements for the fish (Fig. 3). However, to obtain an annual production of 5 tonnes per hectare (Table 1) supplementary feed is needed for the ponds. Tilapia larger than 4 to 5 cm take up supplementary feed readily (Hepher and Pruginin, 1982) and many different feedstuffs could be used, such as different grasses, maize waste or other locally available materials of agricultural origin (cf., Hepher and Pruginin, 1982; Middendorp and Verreth, 1991 ; Noble and Costa-Pierce, 1991). However, as the agriculture potential is very low in the Lake Kariba area (Hutton, 1991), only a minor part of the energy demand could probably be provided through crop waste (Fig. 3). It is therefore suggested that the fish silage produced from slaughtering and processing of wild fish in the fishing villages could constitute an important part of the feed, complemented with waste from agriculture. The fish silage from Kariba contains 50% more proteins, 85% more fat, and the same amount of fibres as the pellets that are used today (Gabriel, 1991). In many semi-intensive fish cultures natural food production is supplemented with lowquality feed and the addition of high-quality feed, such as fish silage, could probably increase the fish yield significantly. Although only a minor part of the fish silage may be utilized directly by the fish,

excess feed would probably be used and recycled by other organisms within the pond system, which in the end are eaten by the fish (Hepher and Pruginin, 1982). This recycling of nutrient and materials within the pond system in combination with species utilising different food niches, could result in a feed conversion ratio that is even lower than in the cages (F~51ster, 1994). However, as only waste products would be used in ponds, semi-intensive pond farming would depend on large quantities of landed fish (Fig. 3). But, as long as only waste products are used, these activities would increase the total production of fish from the lake (Fig. 2 and Fig. 3). In this sense, semi-intensive pond farming is a complement to the inshore fisheries. However, since there are no economic possibilities to substitute this feed with imported pellets, the scale of production in ponds would probably have to be much smaller than in cage farming because it would be constrained by the amount of fish waste available. An annual production of 50 tonnes would require a total catch of 950 tonnes of inshore fish, which is close to the total yield from the Zimbabwean side (cf., Fig. 3) (Machena and Konondo, 1991; Sanyanga et al., 1993). Because of the much lower fish density in the ponds than in the cages, the life support areas needed for oxygen production and nutrient assimilation would be very small (Fig. 4b). At a stocking density of 0.1-0.5 kg m -2, oxygen produced through photosynthesis would probably be enough to compensate for the respiration in the pond system (cf., Ftilster, 1994). However, plankton blooms must be avoided by an appropriate grazing pressure. Otherwise, dense blooms could result in high diurnal fluctuations in dissolved oxygen levels with short periods of lethally low levels (Snook, 1987; Langerman, 1990; cf., Delincr, 1992; F~ilster, 1994). Low oxygen levels could also result in decreased growth and feed conversion ratios (Beveridge, 199l; Chervinski, 1982; Coche, 1982). Similar to the cages, the water would be enriched by nutrients when passing the farms because external sources of nutrients are continuously being added to the system with the feed, while only a minor part is removed when the fish is harvested. However, if the waste water were used for irrigation, most of the excess nutrients would probably be assimilated by

H. Berg et a l l Ecological Economics 18 (1996) 141-159

plants or accumulate in the sediments on its way to the lake (Beveridge and Phillips, 1993; Edwards, 1993; FiSlster, 1994). This would not only reduce the output of organic waste and nutrients to the lake to almost zero (FiSlster, 1994), but could also increase agricultural output significantly from this region (cf., HuRon, 1991) In conclusion, the potential environmental effects from the ponds are lower than from the cages because the cages are open subsystems placed in the aquatic environment. In a pond there is less degree of interaction with the lake ecosystem (Edwards, 1993; Beveridge and Phillips, 1993) and more of the wastes can be trapped and recirculated within the pond system itself. 5.3. Sustainable aquaculture in Lake Kariba In this paper we have tried to illustrate that it is possible to develop aquaculture in different ways in Lake Kariba. Our intention was not to present a detailed study of how these farms should be run, but to indicate that these systems would be linked differently to the ecological and economic systems of the Lake Kariba region (Fig. 2). Like all economic ventures, development of aquaculture must be supported by an input of human-made capital, and in Lake Kariba this input is higher for intensive cage farming than for semi-intensive pond farming (Table 2). It has been argued that for intensive cage fanning in Lake Kariba the minimum production size to be economically viable is 200 tonnes, and 500-2000 tonnes would be an adequate production each year (Gabriel, 1991). The quality of the farmed fish must be high to realise a sufficiently high price. Thus, the lowest level of 'economically sustainable' aquaculture is constrained by demand for both a quantitatively and qualitatively high production. The problems with an expanding production in cages is that it is followed by a 'hidden' increase in external support from ecosystems (Fig. 3). This is seen when comparing the life support areas required by ponds and cages (Fig. 4a,b). As the scale of production increases relative to the life support ecosystem, the capacity of ecosystems to produce goods and services is more easily disrupted (Folke and Kautsky, 1992). This is especially true when

155

considering services such as oxygen production and nutrient assimilation which must be provided by ecosystems close to the farm. Thus, there are physical limits to cage farming expansion, and if these are not perceived and accounted for, the existence of both aquaculture and the fishery industry in Lake Kariba may be jeopardized. It is therefore necessary to find ways to direct aquaculture development towards a path where fish production can not only be maximised, but also sustained, and where environmental quality is maintained or enhanced. This is the 'ecological' part of a sustainable aquaculture. An operational definition of an ecologically sustainable development of aquaculture in Lake Kariba is that it must stay within the dynamic carrying capacity of the life supporting system. Stocks of environmental goods, such as the kapenta and inshore fisheries, and services, such as oxygen production and waste assimilation, produced by the system should not be allowed to deteriorate due to aquaculture activities. Based on sustainability principles (Costanza and Daly, 1992), minimum conditions for this to be achieved are that: (1) the scale of fish production is limited within the Lake Kariba's carrying capacity; (2) culture methods applied should be efficiency-increasing rather than throughput-increasing; (3) harvesting rates of fish for e.g. pellet production should not exceed regeneration rates; and (4) waste emission should not exceed the assimilative capacity of the environment. As indicated in this study large-scale intensive aquaculture in Lake Kariba would probably fulfill few of the criteria for sustainability. It needs a high degree of both ecological and economic support to survive (Table 2 and Fig. 3). Increasing economic returns to scale would be a driving force to continuously increase the scale of production. When the scale of production increases it is only a matter of time until aquaculture will be limited by the impacts of its own actions on the environment. At this stage the environmental support must be substituted, if possible, by expensive energy-consuming devices. If the increased costs cannot be compensated by an increased production, this venture has to come to an end (Folke and Kautsky, 1992). In contrast to the cages, the pond farm makes use of services and goods produced by the pond system and does not need the same degree of economic

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support as the cages (Tables 2 and 4). Therefore, it is possible with a lower scale of production, at least from an ecological point of view, semi-intensive pond fanning would fulfill more of the requirements for sustainability than intensive cage fanning in Lake Kariba. This is because: (1) the pond system is more separated from the lake ecosystem than the cages, and therefore has less impact on the lake and on other activities in the lake; (2) production of fish in ponds is less resource-demanding than in cages since only waste products are used; (3) the scale of production will presumably stay within the carrying capacity of Lake Kariba because it will be limited by use of fish waste generated from the inshore fisheries. However, intensive cage farming should not be ruled out in Lake Kariba. The important message is that intensive farming is acceptable if it is managed in the context of the combined ecological economic system--i.e., if it takes into account sustainability (scale), efficiency (allocation) and equity (distribution). In this paper we have largely analysed the issue of scale and, preferably, the latter aspect should be further considered before reaching final conclusions about the development of aquaculture in Lake Kariba. Artisanal fishing communities have developed around Lake Kariba and depend on fish for their livelihood. Intensive large-scale cage farming may, unless carefully managed, lead to conflicts of interests. According to Beveridge (1991) the unchecked expansion of the privately owned fish cage industry in Laguna de Bay in the Philippines disrupted the traditional livelihoods, such as fishing, which lead to both poaching and vandalism by the local communities. At other lakes in the Phillipines, development was controlled and the villagers were encouraged to set up their own farm units, aided by government loan shemes. In these areas problems were largely avoided (Beveridge, 1991). At present the local fisherman in Lake Kariba are restricted to the inshore fisheries as the capital investment required to take part in the kapenta fishery is much too high, Large-scale cage farming could for the same reason exclude the rural communities from taking part in aquaculture. The local fisherman cannot compete with private, high-income investors. The only venture that is economically feasible for

the fishing communities is extensive or semi-intensive aquaculture. Both the self-organizing ability of the pond system and the possibility to use water and nutrients for irrigation and fertilization of small-scale agriculture projects make pond farming for increased local food production an attractive alternative, although there might also be other competitive low-cost methods that should be considered.

6. Conclusions

There are plans to expand aquaculture in Lake Kariba. In this article we have estimated the ecological life-support demand from scenarios with intensive cage farming and semi-intensive pond farming of Tilapia. Not surprisingly, the analysis shows that intensive cage farming would appropriate a large spatial ecosystem support, whereas pond farming could be sustained basically by waste products. In contrast to cage farming, it does not demand extra ecological support areas. Although intensive cage farming in the short-term perspective seems to be the only economically feasible method, semi-intensive pond farming or other less capital-intensive farming methods may be competitive alternatives if ecological and social aspects are also taken into account. In this article we have mainly analysed the ecological aspects, and have shown that aquaculture cannot exist without the support of natural capital. This support is hardly ever taken into account in economic decision-making. Also, the social aspects need to be further investigated prior to any decisions on how to develop aquaculture. What is important is to avoid an aquaculture development based on a short period of prosperous growth followed by collapse with severe ecological, social, and economic problems, as has been the case in shrimp farming (Bailey, 1988; Primavera, 1991). A sound combination of semi-intensive and intensive aquaculture has potential to become successful in Lake Kariba, but only if the plans and implementations of aquaculture strive towards optimizing the long-term production of fish within a linked ecological, social, and economic framework.

H. Berg et al. / Ecological Economics 18 (1996) 141-159

Acknowledgements This p r o j e c t w a s in part s u p p o r t e d by the S w e d i s h International Development Cooperation Agency ( S I D A ) . T h e authors w o u l d like to t h a n k the s t a f f at L a k e K a r i b a F i s h e r i e s R e s e a r c h Institute, F r e s h n e t Inc, ( W i l l a r d s F o o d , Pvt., Ltd), a n d U n i v e r s i t y L a k e K a r i b a R e s e a r c h Station for practical h e l p and their hospitality. M a n y t h a n k s to A l e j a n d r o B u s c h m a n n a n d t w o a n o n y m o u s r e v i e w e r s for v a l u a b l e c o m m e n t s o n the m a n u s c r i p t .

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