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Aquaculture Research, 1997, 28, 527–544
Cage fish farming in the tropical Lake Kariba,
Zimbabwe: impact and biogeochemical changes in
sediment
M Troell & H Berg
Department of Systems Ecology, Stockholm University, Stockholm, Sweden
Correspondence: Dr M Troell, Department of Systems Ecology, Stockholm University, 106 91 Stockholm, Sweden
environment in the vicinity of the farm (Phillips,
Abstract Beveridge & Ross 1985; Gowen & Bradbury 1987;
Accumulation of nutrients in the sediment under a Rosenthal, Weston, Gowen & Black 1988; Beveridge
tilapia cage farm (2.8–4.4% C, 0.26–0.49% N and 1996. Waste materials in aquaculture effluents have
0.04–0.26% P) seemed to follow a seasonal pattern, been compared with industrial and domestic
with highest concentrations prior to winter water effluents, adding large amounts of carbon, nitrogen
turnover. However, in April 1994 (and for P also in and phosphorus to the environment (Persson 1991;
April 1992) the surface sediment contained Cripps 1994). Effluents from fish farms, i.e. water
significantly higher nutrient concentrations com- incorporating fish faeces, urinary waste and uneaten
pared to controls. Generally, significantly higher food enter watercourses and particulate material
pore water concentrations were found under the settles and accumulates in bottom sediments. The
cages compared to controls. However, only in April organic accumulation increases the oxygen
1992 were these concentrations of the same consumption of the sediment (Tsutsumi & Kikuchi
magnitude as those found in temperate studies. The 1983; Hall & Holby 1986; Samuelsen, Torsvik,
average flux of particulate material under the cages, Hansen, Pittman & Ervik 1988; Hargrave, Duplisea,
20–49 g m
–2
per day, was up to 22 times greater Pfeiffer & Wildfish 1993; Santiago 1994), which in
compared to controls. Carbon accumulated only in some cases has led to the development of anoxic
April 1994, implying rapid decomposition. This was lake and sea bottoms, that create comprehensive
supported by a 4–25-fold higher outward flux of negative ecological changes (species shifts,
ammonium and phosphate from the farm sediment ecosystem functional changes etc.) (Gowen,
but sediment O
2
consumption was only 29–40% Rosenthal, Ma
¨kinen & Ezzi 1990; Persson 1991).
higher compared to control sites. It was concluded The release of dissolved nutrients (mainly phosphate
that intensive fish farming in the tropics can generate and ammonium) can stimulate phytoplankton
similar eutrophication effects that are observed in production. The resulting increased algal biomass
temperate regions. However, the results also may further increase oxygen consumption during
indicated that a tropical lake system may be able to decomposition (Persson 1991).
process local deposition of organic wastes better There have been few assessments of the
than a temperate one, suggesting that microbial environmental impacts of tropical fish cage
decomposition may be a rapid and prominent aquaculture (Costa-Pierce & Roem 1990; Santiago
process. 1994; Angel, Krost & Gordin 1995). Environmental
conditions are different in the tropics; aquaculture
wastes do not necessarily have the same impact on
Introduction the environment as in temperate regions (Beveridge
& Phillips 1993). Despite reports of deteriorating
Expansion of intensive cage aquaculture systems is
usually accompanied by degradation of the natural ecological conditions, resulting from intensive tilapia
© 1997 Blackwell Science Ltd. 527
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
Figure 1 Map of Lake Kariba, a man-made lake on the border between Zambia and Zimbabwe. Lake Kariba is 320 km
long and has a surface area of almost 5400 km
2
. The insert shows the study area with control sites (R1 and R2) situated
at a distance of 300 m from the tilapia fish farm (F).
cage farming in tropical systems (Santiago 1994), farm waste in a tropical freshwater ecosystem, i.e.
studying biogeochemical changes in the sedimentit is possible that organic waste and nutrients from
aquaculture activities can be more rapidly and to compare the results with similar
investigations from tropical and temperate areas,incorporated into a tropical aquatic ecosystem,
compared to a temperate one. Costa-Pierce & Roem with a focus on phosphorus.
(1990) investigated waste production from floating
carp net cages situated in a tropical reservoir, and
found significantly lower sedimentation (per kg fish Materials and methods
m
–2
) and lower nutrient levels in settling matter Study area and fish farm management
compared to data from temperate areas. Angel et al.
(1995) found that no organic carbon accumulated Lake Kariba (17°S, 28°E) is one of the largest
man-made lakes in the world, covering an area ofin the sediment under a large-scale cage fish farm
situated in the Red Sea. However, data supporting 5000 km
2
(Fig. 1). It was constructed in 1958 for
the purpose of electricity production. The lake hasthe general applicability of these arguments are
insufficient, and as the development of aquaculture gone through some dramatic ecological changes
since its formation (reviews in Coche 1968; Balonin the tropics increases rapidly, the need for further
research on this subject is urgent. & Coche 1974; Ramberg, Bjo
¨rk-Ramberg, Kautsky
& Machena 1987; Machena & Kautsky 1988) andFor some years land-based, small-scale fish ponds
and an experimental cage farm have been operating is now described as a nutrient-poor (mesotrophic to
oligotrophic) soft water lake (Ramberg et al. 1987;in Lake Kariba, and there are now plans for a large
scale development of cage farming in the lake. Berg, Marshall 1994) with phosphorus probably being
the limiting nutrient (Magadza, Heinanen & DhlomoMiche
´lsen, Troell, Folke & Kautsky (1996) estimated
the ecosystem area needed to support an intensive 1989). The lake is monomictic with a turnover of
the water mass in the winter (July to August) (Baloncage cultivation in Lake Kariba was between 115
and 160 times larger than the farm area itself. The & Coche 1974). The theoretical water retention time
is 3 years and the water level fluctuates with aaim of this study was to investigate the fate of fish
528 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
Table 1 Fish production, feed input, feed conversion ratios Table 2 Gross composition (%) of the feed pellets used for
growing tilapia at the Kariba cage farm (data supplied byand waste emission for the Kariba farm from September
1990 to August 1994. Values in tonnes the manufacturers)
Moisture 9.7 Phosphorus 0.7Period Production Feed FC-ratio Waste
Protein 30.9 Salt 0.6
Fats 3.2 Copper 21.0
Sept 90–Aug 91 3.9 29.5 7.6 25.6 Cellulose 4.7 Hardness 4.3
Sept 91–Aug 92 4.3 15.9 3.7 11.6 Minerals 8.0 Calcium 1.4
Sept 92–Aug 93 10.5 36.7 3.5 26.2
Sept 93–Aug 94 23.2 51.0 2.2 27.8
and fry meal were used, containing 0.7 and 1.4%
phosphorus, respectively (Table 2). Three different
tilapia species were reared together; Oreochromisyearly amplitude of about 3 m. The Zambezi River
contributes about 70% of the water inflow. Next in mortimeri (Trewavas, 1966), Tilapia rendalli
(Boulenger, 1896) and Oreochromis niloticusimportance is the Sanyati river which discharges
into the lake basin close to the dam. During the (Linnaeus, 1758). A total of 250 fingerlings (20 g
each, unknown species proportion) were stockedmaximum stagnant period (December–April),
nutrients accumulate in the hypolimnion (Ramberg (2.5–7.5 kg m
–3
at the start and with a maximum
of 65 kg m
–3
at harvest) giving a growth rate ofet al. 1987), whereas dissolved oxygen concentration
is reduced. The suspended load into the lake is most 1.10 g day
–1
. Stocking was performed continuously
ona2to3months basis.profound during the period following the rainy
season. However, as most of the nutrients in the Both control stations had similar water depth,
submerged trees and type of sediment to the fishZambezi river is lost on the Barotse floodplain
(Mitchell 1973), the phosphorus loading into the farm site. Selection of control sites, considering the
influence of farm wastes, was based on divinglake is low (Marshall 1984). The most recent data
available for typical pelagic (1–5 m depth) mean observations in October 1991, current speed
measurements at two metres depth in October 1992nutrient concentrations in basin 5 ranged
between: total-P; 7–24 µgl
–1
, phosphate phosphorus (,0.05 m s
–1
of varying direction using a propeller
current meter) and observations of cage movements(PO
4
-P); 3–4 µgl
–1
, nitrate nitrogen (NO
3
-N);
2–20 µgl
–1
and ammonium nitrogen (NH
4
-N); by the farmers. Somewhat lower oxygen levels
compared to controls were found in almost all cages5–17 µgl
–1
(Lindmark, 1997).
The cage fish farm studied is situated in the containing larger fish, but the dissolved oxygen
depth profiles in the whole water column indicatedsouthern part of Lake Kariba, close to the south
coast of Antelope Island (Fig. 1). During the study similar conditions around the farm and the controls
(and out in the lake), implying sufficient waterit consisted of 9 3(4.8 34.8 32 m), 5 3(4.8 3
4.8 31.5 m), 6 3(2.4 32.4 31 m) and 1 3movement to prevent poor dissolved oxygen
conditions overall. The sediment under the fish farm(2.4 32.4 32 m) cages, anchored 30–40 m from
the shore, at sites at a water depth of 20–30 m, and the control sites consisted of soft clay with a
flocculent surficial sediment, a characteristic thatdepending on annual water level fluctuations (being
around 10 m). together with the measured and observed water
movements suggested accumulation bottoms. WaterHarvesting, stocking and feeding routines
gradually improved during the study period, temperature and dissolved oxygen profiles were
measured twice (separated by one week) duringresulting in decreased feed conversion factors and
increased monthly harvest (Table 1). Feed input each sampling period, using an Oximeter Oxi 196
(WTWD-82362, Weilheim). Water transparencyduring the sampling months was as follows: October
1991, 0.7 t; April 1992, 1.8 t; October 1992, 1.7 t was estimated with a Secchi disk.
Due to a collapse of the anchoring system 1and April 1994, 7.4 t. In 1991–1992 feeding was
performed manually (3% of body weight per day, month before sampling in April 1994, the cages
were moved some 10–20 m from their originaleight feeding occasions a day), but in 1993 this
method was replaced by automatic feeders. position. However, sampling of sediment was still
performed at the original position.Throughout the study the same brands of feed pellets
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 529
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
Field work and analysis giving undisturbed cores. On each sampling
occasion, ten to fifteen cores were taken both at the
Field work was carried out on four occasions during control sites and under the fish farm. The sediment
the period 1991–1994; October 1991, April 1992, cores were immediately sliced in an uppermost
October 1992 and April 1994. Samples were taken 0–2 cm layer and the 5–8 cm level and collected
at the tilapia farm (F) and at two control sites (R1 for analysis. The sediment samples were kept cold
and R2) situated 300 m East and 300 m West of and dark, in a cooler with ice packs, during
the cages, respectively (Fig. 1). sampling and the 1 h transport to the laboratory,
Two sediment traps (Håkanson 1984), mounted where pore water was extracted by centrifugation
in a horizontal position by cardanic suspension, at c3000 rpm (1500 g) for 10 min followed by
were placed at 10 m depth for 48 h below the farm filtration through a 0.45 µm millipore filter. Pore
site and at the control sites. The 10 m depth was water samples were then kept frozen (approximately
the maximum depth due to submerged standing 0.5–1 months at –23°C) in plastic containers for
trees. Measurements were repeated two to three later analysis of the total dissolved phosphorus
times during each sampling occasion. The sediment by persulphate oxidation (Valderama, 1981),
traps at the fish farm were attached to the centre molybdate reactive phosphate (MRP) (Murphy &
of the cage structure itself, while at the control sites Riley 1962), and ammonia determined as
they were suspended from branches of submerged indophenol blue (Koroleff 1972). Organic matter in
trees. The cages moved about 10 m back and forth the sediment was defined as the sediment weight
depending on the wind direction (the wind shifts fraction lost after combustion at 460°C for 3 h.
once or twice a day), and in October 1992, when Carbon and nitrogen were determined by a LECO
the water level was very low, the sediment traps CHN-900 elemental analyser (Leco Co., USA). Total
became repeatedly entangled with submerged trees, phosphorus was extracted from the sediment using
resulting in failure to obtain sedimentation data hydrochloric acid (Aspila et al. 1976).
under the farm during that sampling period. In October 1992 the oxygen consumption and
The collection tube had an inner diameter of nutrient exchange of PO
4
-P and NH
4
-N were
9.3 cm and an aspect ratio (H : D, 6 : 1) within the measured under oxic conditions (i.e. overlaying O
2
range recommended for reliable trapping (Blomqvist concentration of water above 2.5 mg l
–1
) over
and Håkanson 1981). No preservatives were added the sediment–water interface. Sediment cores were
to the sediment traps because of the short sampling incubated in darkness at in situ temperature (22°C)
period. Sedimented material was suspended in 1 l and with continuous stirring. Samples were taken
of overlying trap water and aliquots from this at intervals of 4–8 h over a 12–24 h period by
homogenised suspension were then filtered through means of syringes, and filtered through a 0.45 µm
pre-combusted Whatman GF/F (0.7 µm) glass cellulose acetate filter and kept frozen (approximately
microfibre filters (Whatman Ltd, UK). Total 0.5–2 months at –23°C) until analysed for nutrients.
particulate material was measured after drying the Dissolved oxygen and nutrients in the overlying
filters at 100°C until no further decrease in weight water was followed during incubation using an
was obtained. Analyses for carbon and nitrogen oxygen meter and the methods described for pore
were made using a LECO CHN-900 elemental water.
analyser (Leco Co., USA). An ultrasonic bath was Redox potential was also measured, in the same
used for separating the material from the filter and cores as used for the sediment incubation in October
then the total particulate phosphorus was extracted 1992. Readings were taken at each centimetre from
using hydrochloric acid (Aspila, Agemian & Chau the sediment surface down to 10 cm depth using a
1976). platinum electrode with a reference calomel
Sediment cores were collected by scuba diving in electrode (Hallberg, Bågander, Engvall, Lindstro
¨m,
October 1991 (using 5.5 cm diameter Plexiglas Ode
`n & Schippel 1973). These values were corrected
tubes) and with a Kajak core sampler (Blomqvist to give the redox potential (Eh) results.
and Abrahamsson 1985) in April 1992, October Carbon and nutrient levels in the sediment,
1992 and April 1994 (8 cm diameter Plexiglas nutrients in pore water and benthic fluxes were
tubes). The change in method was due to the statistically analysed using one-way
ANOVA
. To avoid
appearance of crocodiles around the cages. However, comparison variability between the controls a
planned comparison (contrast analysis) was madethe methods were regarded as equivalent, both
530 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
(except for sediment incubation). Data not following 4.4% C, 0.26–0.49% N and 0.04–0.26% P under
the cages and between 2.6–3.9% C, 0.22–0.40% Nthe assumptions for analysis of variance (using
Cochran’s test) was transformed (log and square and 0.04–0.06% P at the control sites (Fig. 3). The
farm sediment had significantly higher concentra-root) to achieve a homogenous variance. In only a
few cases (Pvalues marked with * in text) did tions compared to controls only in April 1994 (and
for P also in April 1992) (N: P,0.01; C: P,transformation fail to remove heterogeneity of
variances. The sediment trap data was not analysed 0.001; P: P,0.001*).
Carbon and nutrients in the 5–8 cm sedimentstatistically due to the small number of samples.
layer were on average lower compared to surface
sediment layer (both under the farm and at the
control sites), however this was not tested
Results statistically due to few samples in the 5–8 cm layer.
Analysis of pore water samples revealed that inTemperature and dissolved oxygen profiles in the
water column changed seasonally, with similar October 1991, only PO
4
-P was significantly higher
at the farm compared to controls (P,0.05). Inprofiles at the fish farm and the control sites. In
October 1991 and October 1992, the whole water October 1992 and April 1994 the mean pore water
nutrient concentrations under the cages was 13–column was oxygenated at all the sites. The lowest
dissolved oxygen concentration was found just above 81 µgl
–1
PO
4
-P, 22–135 µgl
–1
tot-P and 690–
3213 µgl
–1
NH
4
-N. These concentrations werethe bottom (3 mg O
2
l
–1
, 25°C, 60% saturation),
whilst 9 mg O
2
l
–1
(22°C, 100% saturation) was significantly higher (P,0.001) than those
measured at the control sites (5–20 µgl
–1
PO
4
-P,measured at the surface. In April 1992 and April
1994, hypoxic conditions prevailed at greater depth 13–32 µgl
–1
tot-P and 303–1409 µgl
–1
NH
4
-N)
(Fig. 4). The greatest difference in pore waterthan 13–15 m (,0.5 mg l
–1
), both at the control
sites and under the cages. The temperature ranged concentration between the farm site and controls
was, however, found in April 1992. Then PO
4
-Pfrom 28–29°C at the surface to 26–27°C at the
bottom, with a weakly developed thermocline and tot-P (1100 µgl
–1
PO
4
-P, 1700 µgl
–1
tot-P)
was up to 60 times higher (P,0.001) underapparent at 15 m (1992) and 8–10 m (1994) depth.
The Secchi depth during 1991 and 1992 varied the cages compared to controls, and NH
4
-N
(4750 µgl
–1
NH
4
-N), six times higher than controlsbetween 3.1–3.5 m at the cage site, 3.0–3.75 m at
control site 1, and 2.3–3.6 m at control site 2. (P,0.001).
The sediment under the cages consumed inSedimentation rate of particulate material, carbon,
nitrogen and phosphorus was higher at the fish October 1992 305 mg O
2
m
–2
per day, the O
2
uptake being 30–40% higher than at the controlfarm than at the control sites (Fig. 2). The mean
sedimentation rate of total particulate material sites (P,0.05) (Fig. 5). PO
4
-P was released from
the sediment under the fish cages (4 mg m
–2
perunder the cages, 20–28 g m
–2
per day, was up to
10 times higher than the controls in October 1991 day), at rates some 22–25 times higher (P,0.001*)
than from control cores (Fig. 5). The release rate ofand April 1992. Due to the reasons described earlier
in the text no data was obtained for sedimentation NH
4
-N from the fish farm cores was 31 mg m
–2
per
day, which was 25 and four times higher (P,0.01*)under the fish farm in October 1992. In April 1994,
sedimentation of particulate material under the fish than from cores taken from sites R1 and R2,
respectively (Fig. 5).farm had increased compared with previous years
(49 g m
–2
per day), but decreased compared to 1991 All the sediments were highly reduced and at site
R2 the redox potential ranged from 1110 mV atand 1992 by a factor of 2–4 at both control sites
(Fig. 2). the surface to –230 mV at 4 cm depth, while at the
fish farm redox potential ranged from 1142 at theMean percentage carbon and nutrients in settling
material under the cages ranged between 20–60% surface to –320 mV at 4 cm depth (Table 3). Site
R1 sediments showed the smallest changes, rangingC, 2–6% N and 0.5–0.7% P, with values in April
1994 up to three times higher than previous years. from 1150 at the surface, down to –140 mV at 4
cm depth. The redox potential, at control sites andThese parameters were consistently lower at the
control sites; 4–13%C, 0.5–1.0% Nand 0.1–0.6% P. under the fish cages, changed most from the surface
down to 4 cm depth, reaching a minimum ofThe mean percentage of carbon and nutrients, in
surface sediment (0–2 cm) ranged between 2.8– –400 mV at 10 cm depth.
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 531
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
Figure 2 Sedimentation (measured
during 48 h) under the fish farm (F)
and at control sites (R1 and R2)
at four sampling occasions; October
1991, April 1992, October 1992 and
April 1994. (a) total dry matter (Dw)
(g m
–2
per day), (b) carbon (c)
(g m
–2
per day), (c) total nitrogen
(tot-N) (g m
–2
per day) and (d) total
phosphorus (tot-P) (mg m
–2
per day).
Mean values 6SE n5number of
replicates.
Discussion trout farms (20 and 40 t yr
–1
; feed conversion
1.6 : 1), measured sedimentation rates amounting
Production factors and environmental accumulation to 17–26 g dw m
–2
per day. Other investigations
factors from this study and similar other fish cage report even higher and more variable sedimentation
studies being discussed are summarized in Table 4. rates (17–296 g dw m
–2
per day) (Merican & Phillips
1985; Hall & Holby 1986; Angel et al. 1995). This
indicates a large variation within and between
Sedimentation of particulate matter different investigations. Merican & Phillips (1985)
The sedimentation rates under the fish farm in Lake found a significant correlation between the waste
Kariba (i.e. 20–48 g dry weight (dw) m
–2
per day) output per unit biomass and the feeding rates, but
were of the same magnitude as under the fish farms concluded that this alone could not explain the
with significantly higher production (Enell & Lo
¨fvariation in solids output. Other factors i.e.
1983; Hansen, Pittman & Ervik 1991). Enell & Lo
¨fproduction intensity (fish density and total
production), feeding regime and the hydrodynamics(1983) who investigated two lacustrine rainbow
532 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
Figure 3 Mean values (6SE) (mg
g
–1
dry wt) of total phosphorus (tot-P)
(a), total nitrogen (tot-N) (b) and
carbon (tot carbon) (c) in the 0–2
and 5–8 cm sediment layers under
the fish farm (F) and at the control
sites (R1 and R2) from October 1991,
April 1992, October 1992 and April
1994. n56.
(e.g. currents, water depth) of the receiving water compared to 1991–92, it is assumed that the
increased sedimentation noted in 1994 originatedmust also be considered (Håkanson & Jansson 1983;
Hansen et al. 1991), as well as trap designs and from increased fish stocks.
Stocking density at the Kariba farm was 2.5–the frequency of sampling (Blomqvist & Håkansson
1981; Gowen, Weston & Ervik 1991; Hansen et al. 7.5 kg m
–3
, although prior to harvest, the fish
density could, due to tilapias tolerance to low oxygen1991).
The high load of surplus feed during October levels and little space, be reared at densities as high
as 65 kg m
–3
. This maximum stocking density per1991–April 1992 in Kariba, resulting in high feed
conversion ratios, probably explains why cubic metre is higher than in salmon and trout
farming, which was also reflected in the wastesedimentation under the cages was so high during
that period. The feed conversion ratio was 3.5–3.7 production under the cages. However, salmon cages
are usually about twice as deep, resulting in higherin 1991 and 1992, which is 2.0–2.5 higher than
that is generally found in intensive tilapia cage fish biomass and waste loads per m
–2
. The relatively
low sedimentation rate observed by Costa-Pierce &cultures fed on commercial dry feed (Beveridge
1984). As the feeding regime had improved in Roem (1990) under carp (Cyprinus carpio L.) cages
in a tropical reservoir (13 g dw m
–2
per day) is1994, (2.2 : 1) and natural sedimentation was low
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 533
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
Figure 4 Mean values (6SE)
(µgl
–1
) of total phosphorus (tot-P)
(a), PO
4
-P (b) and NH
4
-N (c) in pore
water from the 0–2 and 5–8 cm
sediment layers under the fish farm
(F) and at control sites (R1 and R2)
from October 1991, April 1992,
October 1992 and April 1994. n5
number of replicates.
probably explained by the relatively low fish biomass, Brycinus lateralis (Boulenger 1900)). Local fishermen
recorded increased catches of both bottom-dwellingranging only from 2–12 kg m
–2
. Sedimentation in
that study was measured just below the cage bottom. and pelagic fish species around the cages. These
wild fish could have influenced the sedimentationIt is likely that sedimentation would have been even
lower if measured at 10 m depth, as was the case pattern by removing the settling feed and releasing
faecal products. The net effect on sedimentationin Kariba.
At the Kariba fish farm, continuous stocking and would thus depend on the movement of these fish
within the studied area (i.e. adding to sedimentationharvesting was performed during the year, which
means that the sedimentation rate could not be if releasing their faeces within the cage area and
reducing sedimentation if releasing elsewhere). Theexpected to vary much due to changes in fish
biomass. In temperate regions, sedimentation under fact that faecal material is more easily broken down
than feed particles, may result in lower faecal settlingfish cages i.e. salmon and trout cages usually follows
a seasonal pattern corresponding to the increase in velocities and thereby less particulate material
reaching the bottom directly under the farm thanfish biomass and the low feed inputs during winter.
Stenstro
¨m (1993) observed that feed waste lost estimates based on theoretical calculations would
suggest (Gowen & Bradbury 1987). However, in anfrom the Kariba cages to the surrounding water
attracted large quantities of small fish (mainly area with slow moving waters, such as around the
534 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
Kariba cages, particles may instead accumulate in
the sediment adjacent to the cages. Bottom dwellers
(e.g. Mormyrus longirostris Peters, 1852) may assist
in releasing dissolved N and P from sedimented
waste to the water column.
The somewhat higher sedimentation rate at site
R2 compared to site R1 reflects natural variations
as influence from the cages is unlikely, given the
distance from the fish cages (Enell & Lo
¨f, 1983;
Brown, Gowen & Mclusky 1987; Lumb 1989; Hall,
Andersson, Holby, Kollberg & Samuelsson 1990).
Accumulation of nutrients and carbon
It is estimated that some 70–80% of nitrogen
supplied through the feed is lost to the surrounding
environment (20–30% in particulate form). Of the
added phosphorus, up to 50% ends up in particulate
form (Phillips et al. 1985; Gowen, Bradbury & Brown
1985; Enell 1987; Holby & Hall 1991). The transfer
of nutrients from overlying water to the sediment is
governed mainly by sedimentation of particulate
material (biogenic and minerogenic), adsorption of
nutrients by the sediment and the direct uptake by
assimilation from the water column (Jacobsen 1978;
Holdren & Armstrong 1980; Bostro
¨m, Andersen,
Fleischer & Jansson 1988). Sediment oxygen
conditions play an important role in regulating the
release and uptake processes of nutrients and the
levels of nitrogen and phosphorus in the sediment,
especially in pore water (Mortimer 1941; Frevert,
Figure 5 Mean benthic fluxes (measured during 12–24 h)
1979).
(6SE) of dissolved oxygen (mg m
–2
per day), NH
4
-N (mg
The loadings of nutrients from the fish cages was
m
–2
per day) and PO
4
-P (mg m
–2
per day) from sediment
in accordance with findings from temperate areas
under the fish farm (F) and controls (R1 and R2) in
(Penczak, Galicka, Molinski, Kusto and Zalewski
October 1992. n56–7.
1982; Enell and Lo
¨f 1983; Samuelsen et al. 1988;
Hall et al. 1990; Hall, Holby, Kollberg & Samuelsson
1992; Holby & Hall 1991). However, only pore
water samples from April 1992 contained PO
4
-P
and NH
4
-N concentrations of the same magnitude
as found in marine and lacustrine (both temperate
Table 3 Redox potential measured in sediment cores taken
and tropical) studies (Enell & Lo
¨f 1983; Hall & Holby
in October 1992 under the fish cage farm (F) and at controls
1986; Angel et al. 1995). The increased nitrogen
(R1 and R2). 0 cm is just above the sediment surface
concentrations found in the sediment under the
cages in April 1994 were significantly lower
Stations
compared to studies in temperate areas. Samuelsen
et al. (1988) found up to 80 times higher nitrogen
Depth (cm) R1 F R2
levels and Hall & Holby (1986) found 10–14 times
higher N levels in the sediment below fish cages
01153 1142 1113
than at controls. These farms were, however, of a
1256 2144 2191
larger size and had been in operation for a longer
42144 2316 2232
time than the Kariba fish farm.
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 535
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
Table 4 Table summarizing production factors and environmental accumulation factors from this study and similar other fish cage studies being discussed in the text
Location Production Density Cage depth P in Feed F:C ratio Sed. trap Sedimentation P content
(tonnes y–1) (kg m–3) (m) % depth m (g dw m–2 per day) sed. mtrl. %
This study 1990–1992 Freshw. 4–11 2.5–65 2 0.7–1.4 3.5–7.6:1 10 20–28 0.5–0.6
This study 1994 Freshw. 23 2.5–65 2 0.7–1.4 2.2:1 10 48 0.7
Enell & Lo
¨f 1983 Freshw. 20–40 1.1–1.4 1.6:1 7–9 7–32 1.4–5.1
Costa-Pierce & Roem 1990 Freshw. 2–12 2.5 1.0 cage bottom 13 0.02
Cornel & Whoriskey 1992 Freshw. 14 1–2 3.7:1
Penczak
et al
. 1982 Freshw. 27 1.3–1.8 0.9–2.3**
Kelly 1993 Freshw. 1–1.8
Merican & Phillips 1985 Freshw. 2.8–235 1.4–2.4 1.4:1–3.3 7.5 62–295 2.6–3.7
Hall & Holby 1986 Marine 40 1–10 4 1.1–1.6 2.1 12–15 50–200
Hall
et al
. 1990 Marine 1.1–1.6 2.1 12–15 50–200
Holby & Hall 1991 Marine 1.1–1.6 2.1 12–15 50–200 0.3–0.9
Angel
et al
. 1995 Marine 60–90 20–50 5–6 1.1 3.65 7–8 30–70
Hansen
et al
. 1991 Marine 190–1900 4–16 17–37
Samuelson
et al
. 1988 Marine 190–1900
Johnsen
et al
. 1993 Marine 80
Hargrave
et al
. 1993 Marine 100
536 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
Table 4 Continued
Acc. sed. P* Acc. sed. Sed. org. Acc. porew. Acc. porew. Sed. flux Sed. flux Sed. resp. Increased
(mg g–1 dry N* mtrl PO4-P* NH4-N* NH4-N*** PO4-P*** (mg m–2 per hour) Sed. resp.
per week) % % (mg l–1) (mg l–1) (mg m–2 per (mg m–2 per (x times
day) day) control)
This study 1990–1992 0.4–0.9 0.25–0.38 2.8–3.8 0.02–1.8 0.8–4.8 32 4 13 1.3–1.4
This study 1994 1.6 0.49 4.5 0.03 1.8
Enell & Lo
¨f 1983 0.2–4 2–20 8–10 4.2–8.5 33–90 3–9
Costa-Pierce & Roem 1990
Cornel & Whoriskey 1992 0.08–2.2 39–69****
Penzak
et al
., 1982
Kelly 1993 0.8–5.3 13–38 0.9–57
Merican & Phillips 1985
Hall & Holby 1986 1.6–2.1 15–27 9.1–12 1.1–4.9 1.4–19.6 32–64 120–240 12–15
Hall
et al
. 1990
Holby & Hall 1991 14–16 5.1–216
Angel
et al
. 1995 4–12 0.6–4.3 2.8–16.8 14–520 15–350
Hansen
et al
. 1991 15–47 220–580 11–29
Samuelsen
et al
. 1988 2.3 14–67
HJohnsen
et al
. 1993 16–26
Hargrave
et al
. 1993 7–25 80–520 70–130 3–10
Notes: * 0–5 cm sediment layer; ** in faeces; *** oxic release; **** control 44–68.
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 537
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
Phosphorus levels in the sediment varied concentrations. The severe drought during 1991–93
could have influenced accumulation and circulationseasonally, with significantly higher values under
the farm only in April 1992 and April 1994. pattern, thus amplifying the effects seen in April
1992. The relocation of the farm prior to samplingSeasonal variations were also found by Cornel &
Whoriskey (1992) who investigated a 14 t trout farm in April 1994 resulted in a decrease of particulate
matter reaching the sediment. This could explain(feed conversion 3.7 : 1), situated in an oligotrophic,
temperate lake. They found significantly higher the lower power water concentrations in April 1994,
compared to April 1992, if the pore water phaseavailable P levels in the sediment 20 m below the
fish cages [0.08–2.2 mg (gdw)
–1
] compared with an responded more rapidly to the decreased loading
than the particulate sediment phase.unaffected reference [0.03–0.3 mg (gdw)
–1
]. Kelly
(1993) found sediment under a freshwater farm Based on the relatively large sedimentation found
under the cages, the accumulation of carbon wascontaining 0.8–5.3% P, and Holby and Hall (1991)
14–16% P under a marine cage farm. Difference in surprisingly low, and except for April 1994 (P,
0.01) the values were of the same magnitude as theP content between the feed used in this study and
the studies it is being compared with, influence control sites, and significantly lower than those
usually found under fish cages in temperate areasdifferences in the accumulation of P in the sediment.
Cornel & Whoriskey (1992) and Kelly (1993) used (e.g. Samuelsen et al. 1988; Hall et al. 1990; Hansen
et al. 1991; Johnsen, Grahl-Nielsen & Lunestadfeed containing P ranging from 1–2% and 1.1–
1.8%, respectively, and in the study by Holby & Hall 1993). The sedimentation rate measured at 10 m
depth may, however, not reflect the actual amount(1991) the P content in the feed ranged between
1.1–1.6%. This should be compared with the feed of material reaching the bottom, as the organic
material has to sink 10 m further before settling.containing 0.7% P which was used in the present
study (1.4% P in feed for fry). During this time it may be further broken down, or
partially consumed by fish.At the control sites the sediment contained
nitrogen and phosphorus levels in the same ranges Our data conforms with Angel et al. (1995), who
investigated a net cage farm situated in the Red Seaas those found by Lindmark (1989) in the soft
littoral sediment (0.2–8 m depth) along the shores and could not, despite a large flux of particulate
organic material under the cages, find anof Lake Kariba, and can thus be regarded as
representative. accumulation of organic carbon. However, they
concluded that seasonal storms was more importantThe general increase in nutrient levels in the
sediment and in pore water under the fish cages in than microbial decomposition for carbon removal.
The absence of seasonal storms in Kariba implies thatApril 1992, were followed by a significant decrease
in October 1992. One possible explanation for this microbial activity may be important for removing
carbon from the sediment under the cages i.e. acould be that a stable thermocline developed during
the warmer months, i.e. November to April, considerable proportion of the carbon deposited may
be lost in gaseous form. Microbial removal of carbonpreventing a vertical mixing of dissolved nutrients
and resuspension of the organic matter. This implies may be aerobic or anaerobic, resulting in the release
of CO
2
or CH
4
, respectively. The fact that bacterialan accumulation of dissolved nutrients in the
sediment during these months. Even though the growth and other microbial processes in sediment
increase with temperature (Bostro
¨met al. 1988;seasonal variations in sediment nutrient
concentrations have been documented in other Holdren & Armstrong 1980), strengthen the
suggestion that microbial activity should be high inaquaculture impact studies(Enell & Lo
¨f 1983; Cornel
& Whoriskey 1992; Angel et al. 1995), the the Kariba sediment, thus, explaining the absence
of carbon accumulation during 1991–92. However,significant increases in pore water nutrient levels
(P,0.001) noted in April 1992 may be too large the importance of bacteria was not measured in the
present study.to be of a seasonal character. This argument is
supported by significantly lower pore water levels of The measurement of nutrient concentrations at
0–2 and 5–8 cm sediment depth was to comparePO
4
-P, tot-P and NH
4
-N (P,0.001) present in
samples from April 1994. The increased production conditions before and after any impact from the
aquaculture activity. However, high nutrientduring 1993–94 compared to previous years, is
reflected in increased nitrogen and phosphorus levels concentrations in the 5–8 cm core layer sample
below the fish farm, indicate that this was not deepin the sediment, but not in pore water nutrient
538 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
enough not to be uninfluenced by the cultivation macrofauna, when Eh values (i.e. the corrected
value calculated from the mV readings) droppedactivity.
below –150 mV at 4 cm depth. In the present study
the redox potential was around or below –150 mV
at 4 cm depth at both the control sites and at the
Sediment respiration and redox potential farm site during all measurements.
When the sediment cores were taken from underSediment respiration is an important sink for oxygen
in freshwater lakes, and it has been shown that fish the cages in April 1992 gas bubbles rose into
the water column. Similar ‘outgassing’ has beenfarming can increase sediment respiration (Enell &
Lo
¨f 1983; Hall & Holby 1986; Hansen et al. 1991; reported as being common under fish cages in
Scotland (Gowen & Bradbury 1988) and NorwayHargrave et al. 1993; Angel et al. 1995), sometimes
resulting in enhanced deoxygenation of the (Braaten, Aure, Ervik & Boge 1983). Samuelsen
et al. (1988), Hall et al. (1990) and Iwama (1991)hypolimnion (Tsutsumi and Kikuchi 1983; Santiago
1994). Increased chemical and microbial activity showed that the gas is methane, produced during
anaerobic decomposition of organic material andleads to increased oxygen consumption and
decreased sediment oxygen levels, which create may be an important transport mechanism for
mobilizing nutrients in the sediment to lake waterchanges in the redox potential and thus alter the
adsorption–desorption characteristics of the (Bostro
¨met al. 1988). The amount of sulphate in
freshwater sediments is small by comparison withsediment (Bostro
¨met al. 1988; Enell & Lo
¨fgren
1988). that in marine sediments, so that sulphate reduction
may only account for 2–5% of the organicSediment respiration under fish farms in temperate
areas has been reported to increase by 30–40 times breakdown. Methanogenesis and denitrification are
the main anaerobic processes in freshwaterdue to waste accumulation (Enell & Lo
¨f 1983; Hall
& Holby 1986; Hansen et al. 1991; Hargrave et al. sediments. However, the whitish layer on the top of
some of the sediment cores and the smell of hydrogen1993; Angel et al. 1995). The relatively low
respiration under the Kariba cages (only 29–40% sulphide (H
2
S) observed in April 1992 indicate the
activity of sulphur bacteria in the farm sediment.higher than the controls) correlates with the low
sediment organic content observed, but not with Angel et al. (1995) reported a high outward flux of
hydrogen sulphide under a fish farm in the Red Sea,the relatively large sedimentation. These findings
may indicate the importance of other processes for and they observed diverse microbial mat
communities, including Beggiatoa spp., in thedecomposing sedimenting material, either in the
water column or in the sediment. sediment.
Angel et al. (1995) found sediment respiration
under fish cages situated in the tropics to vary by a
factor of 20 during the year. Significantly less Benthic fluxes of ammonium and phosphorus
variation has been recorded in temperate areas (a
factor of 2 in the studies by Enell & Lo
¨f 1983 and Accumulation of organic material under fish farms
influence the exchange of nutrients betweenHargrave et al. 1993). This highlights that even if the
temperature is more stable in the tropics compared to sediment and overlying water, often resulting in an
increased flux from the sediment (Enell & Lo
¨f 1983;temperate areas, there is a need for measuring the
changes in sediment respiration throughout the Hall & Holby 1986; Kelly 1993; Hargrave et al.
1993). The phosphorus flux is also influenced byyear, rather just on one occasion as in this study.
In the present study respiration was measured in nutrient levels in the ambient water, redox potential
(Enell & Lo
¨fgren 1988), and the concentrations ofOctober. It is likely that the increased temperature
in April would result in higher respiration. inorganic solids in the sediment (Frevert 1979).
Deoxygenation of the sediment is not necessarily aThe low redox potential in the fish farm sediment,
and at the control sites, is probably the reason for prerequisite for P release. However, phosphorus
release is often found to be higher under anaerobicthe low abundance of benthic macrofauna (Troell &
Berg unpublished data). This is in accordance with conditions than under oxic conditions, due to the
reduction of Fe
31
to Fe
21
, with a subsequent releasePearson and Stanley (1979) who found a close
correlation between the redox potential and the of phosphate (Mortimer 1941; Gowen et al. 1991).
The net phosphate flux from the sediment underabundance of benthic fauna, with elimination of all
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 539
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
the fish cages was of the same magnitude as findings particulate form, whereas most other investigations
report higher values in the range 50–66% (Enell &by Enell & Lo
¨f (1983). Under oxic conditions they
measured an outward flux of 4.2–8.5 mg PO
4
-P Lo
¨f 1983; Phillips et al. 1985; Stigebrandt 1986;
Holby & Hall 1991).m
–2
per day while their control stations had an
inward flux of 1.0–2.0 mg PO
4
-P m
–2
per day. These The remaining phosphorus fraction lost to the
environment, 96 kg (53%) and 217 kg (46%) infigures are, however, low compared to those of Kelly
(1993) who found an outward PO
4
-P flux of 57 mg 1991–92 and 1993–94, respectively, must be
released to the water column in dissolved form. Thism
–2
per day (up to 15 times higher than controls)
from sediment beneath a freshwater cage farm. fraction is high compared to 25–30% reported from
studies on rainbow trout cages in temperate areasAmmonium is found to be released more rapidly
during anoxic than oxic conditions (Enell & Lo
¨f (Enell & Lo
¨f 1983; Holby & Hall 1991).
The magnitude of the phosphorus flux from the1983; Hall & Holby 1986). The oxic release of
ammonium from the Kariba fish farm sediment was sediment, can be estimated from measurements in
October 1992. This shows that efflux removes onlyhigher than that found by Enell & Lo
¨f (1983) and
Hall & Holby (1986) (8–10 and 1.4–19.6 mg NH
4
-N 2.2 kg of sediment phosphorus (1.2 and 0.5%) which
means that 67 kg (1991–92) and 168 kg (1993–m
–2
per day, respectively), but significant lower than
findings by Angel et al. (1995) who found 666 mg 94) of phosphorus is accumulated in the sediment.
However, the actual input of phosphorus to them
–2
per day being released from sediment under a
sub-tropical marine cage farm. These studies deal sediment under the tilapia farm during October
1991–October 1992, was only apparent as awith marine sediments where the flux of ammonium
is only a minor part of the total nitrogen and where moderate accumulation of phosphorus. In April
1994, a more profound sediment phosphorusdissolved organic nitrogen (DON) can comprise 95%
of the release (Hall & Holby 1986; Hall et al. 1992). accumulation was found, but still lower than
suggested by the mass balance calculations.
Impact studies of cage culture in tropical regions
have only just begun, giving few data for
Phosphorus mass balance comparison. However, it is likely that a higher
dissolved phosphorus fraction is released fromA phosphorus mass balance was produced for two
different periods (Fig. 6). The first, October 1991 to tropical fish farms compared to temperate ones.
Apart from variables such as food type, foodOctober 1992, represents a situation with improper
feeding practices, and the second, April 1993 to May composition, feeding regime and cultured species
affecting the waste composition from the farm1994, where feeding practices had been improved.
Production and feed inputs, during 1991–92 were (Beveridge, Phillips & Clarke, 1991, Beveridge &
Phillips, 1993), the higher temperature found in the4300 kg and 18 900 kg, respectively, while during
the period 1993–94 the corresponding values were tropics may play a major role by increasing the
decomposition of particulate material in the water26 500 and 66 700 kg. The mass balance for
phosphorus (Fig. 6) reveals that 180 kg and 477 kg column. This decomposition may also be facilitated
by the large amount of wild fish attracted to theof phosphorus were added during 1991–92 and
1993–94, respectively, and that only 15 kg (8%) Kariba cages, which mechanically break down
particulate waste material in the water column.(1991–92) and 90 kg (18%) (1993–94) were
removed through harvest (tilapia 0.34% P kg
–1
body A further explanation for the absence of
phosphorus accumulation in the sediment couldweight, Meske & Manthey (1983)), during the two
periods. The measured net phosphorus sediment- be that a much higher phosphorus flux from the
sediment occurs during periods of anoxia. Enell &ation at 10 m depth during the different periods,
and an estimated affected bottom area of 1350 m
2
Lo
¨f (1983) found a three- to five-fold increase in
phosphorus flux from the sediment under fish cages(approximately four times the cage area; Enell & Lo
¨f
1983; Holby & Hall 1991; Angel et al. 1995), during anoxic conditions compared to oxic.
Furthermore, a high turnover activity in thesuggest that 69 kg (38%) and 170 kg (36%) of
phosphorus was lost as solid waste during 1991– sediment, due to the high temperature, together
with increased densities of deposit feeding fish, could92 and 1993–94, respectively. These values are in
accordance with findings by Persson (1986), who decrease accumulation in the sediment. Sediment-
ation may also have been overestimated as it wasfound 25–60% of total phosphorus input was in
540 © 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544
Aquaculture Research, 1997, 28, 527–544 Cage fish farming in tropical Lake Kariba M Troell & H Berg
Figure 6 Phosphorus budget for the Kariba farm during October 1991 to October 1992 and April 1993 to April 1994.
Sediment flux for budget October 1991 to October 1992 is based on measurements during oxic conditions in October
1992. Values in kg and percentage of total phosphorus input. Phosphorus input and removal through fry and fish
mortality is not included.
measured at 10 m depth, 10–20 m above the increased turnover of nutrients. It is suggested that
sediment. This would give more time for inputs of nutrient rich organic material in the lake
decomposition of materials in the water column, are quickly decomposed resulting in a rapid release of
and increase the chance of particles being eaten by nutrients that are assimilated by primary producers
fish under the cages. given the appropriate hydrodynamic conditions. It
should be remembered that Lake Kariba is a man-
made lake and that nutrient-rich deep water is
General conclusions continuously being drained through the electric
turbines. This decreases the risks of large-scale
Carbon and nutrients accumulated in sediment and eutrophication from additional inputs of nutrients
pore water under the cages. However, with the to the lake system.
exception of April 1992 the accumulation in pore The higher proportion of dissolved phosphate
water was significantly lower than in similar studies being released from the cages, compared to
in temperate areas. A significant accumulation of temperate studies, could be due to the higher
carbon and nutrients in the sediment was only temperature in the tropics, but this has to be further
recorded in April 1994 (and for P also in April
investigated.
1992). The low carbon accumulation agrees with
Although accumulation of carbon and nutrients
other investigations from tropical environments
was low compared with temperate areas it followed
(Angel et al. 1995).
a seasonal pattern. The slow build up of carbon and
The controls contained less nutrients than
nutrients in the sediment reflect the main factors
recorded in many temperate lakes, probably due
to higher temperatures in Lake Kariba resulting determining environmental impacts from any
© 1997 Blackwell Science Ltd, Aquaculture Research, 28, 527–544 541
Cage fish farming in tropical Lake Kariba M Troell & H Berg Aquaculture Research, 1997, 28, 527–544
quantitative and qualitative assessment of wastes from
aquaculture; the scale and the duration of farming
aquatic animal production. In: Advances in World
activity. The present study indicated that a 4 t
Aquaculture (ed. by D.E. Brune & J.R. Tomasso), Vol. 3,
cultivation did not result in accumulation of carbon
506–533. World Aquaculture Society, Baton Rouge.
or nutrients in the sediment, whereas an annual
Beveridge M.C.M. & Phillips M.J. (1993) Environmental
production of 23 t did.
impact of tropical inland aquaculture. In: Environment
and Aquaculture in Developing Countries (ed. by R. Pullin,
S.V. Rosenthal and J.L. Maclean), pp. 213–236. ICLARM
Conference Proceedings, 31.
Acknowledgments
Blomquist S. & Abrahamson B. (1985) An improved Kajak-
The authors wish to thank the staff at Lake Kariba
type gravity core sampler for soft bottom sediments.
Fisheries Research Institute and at the University
Schweizerische Zeitschrift fu
¨r Hydrologie 47, 81–84.
Lake Kariba Research Station for practical help
Blomqvist S. & Håkanson L. (1981) A review on sediment
during the field work and their great hospitality,
traps in aquatic environments. Archive Hydrobiologia 91,
101–132.
and Mr Moyo and colleagues at Freshnet Inc.
Bostro
¨m B., Andersen J.M., Fleischer S. & Jansson M.
(Willards Food, Pvt., Ltd.) for their kind courtesy in
(1988) Exchange of phosphorus across the sediment-
placing time and information at our disposal. Thanks
water interface. Hydrobiologia 170, 229–244.
to Jan-Eric Ha
¨gerroth for providing the sediment
Braaten B., Aure J., Ervik A. & Boge E. (1983) Pollution
sampling equipment with short notice. The authors
problems in Norwegian fish farming. ICES C.M./F:26.
thank Professor Nils Kautsky for help with sampling
Brown J.R., Gowen R.J. & Mclusky D.S. (1987) The effect
and preparing this paper, and Dr Per Hall for valuable
of salmon farming on the benthos of a Scottish sea loch.
discussions and comments on the manuscript. A
Journal of Experimental Marine Biology and Ecology 109,
special thanks to Dr Magnus Enell and Dr Sven
39–51.
Blomqvist for critical readings of the manuscript
Coche A.G. (1968) Description of the physico-chemical
and most valuable discussions. This work was
aspects of Lake Kariba, an impoundment in Zambia/
supported by SAREC (Swedish Agency for Research
Zimbabwe. Fish. Res. Bull. Zambia 5, 200–267.
Cooperation with Developing Countries) and Sida
Cornel G.E. & Whoriskey F.G. (1993) The effects of rainbow
trout (Oncorynchus mykis) cage culture on the water
(Swedish International Developing Cooperation
quality, zooplankton, benthos and sediments of Lac du
Agency).
Passage, Quebec. Aquaculture 109, 101–117.
Costa-Pierce B.A. & Roem C.M. (1990) Waste production
and efficiency of feed use in floating net cages in
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