HydrobiologiaThe International Journal of Aquatic Sciences
© Springer Science+Business Media B.V. 2007
10.1007/s10750-007-0790-7

Trophic interrelationships amongst cichlid fishes in a tropical African reservoir (Lake Chivero, Zimbabwe)

Tsungai Alfred Zengeya1, 2   and Brian E. Marshall1
(1)
Department of Biological Sciences, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe
(2)
Present address: University of Zimbabwe, Lake Kariba Research Station, P.O. Box 48, Kariba, Zimbabwe
 
 
Tsungai Alfred Zengeya
Received: 18 October 2006Revised: 04 April 2007Accepted: 19 April 2007Published online: 04 July 2007
Abstract
The diet of seven cichlid species of Lake Chivero, Zimbabwe, was investigated from October 2004 to April 2005 to determine the degree of dietary overlap and interspecific competition. These fish could be separated into four groups. Two microphages, Oreochromis niloticus and O. macrochir, fed mostly on blue-green algae and their diets, in all size classes, overlapped almost completely. This suggested a high degree of competition between the two species and may account for the fact that O. niloticus, introduced into the lake in the 1980s is now the dominant species. There was also a significant dietary overlap between the two macrophages Tilapia sparrmanii and T. rendalli, which were macrophages with macrophytes as a significant component of their diet. Interspecific competition between these two species may account for the increase in the abundance of T. sparrmanii and a decrease in the numbers of T. rendalli over the last two decades. The diet of two small omnivores Pharyngochromis acuticeps and Pseudocrenilabrus philander consisted of algae, zooplankton, plant material, fish and detritus and also overlapped extensively, but these species probably avoid competition by living in different habitats; P. philander is usually associated with vegetation while P. acuticeps tends to live in more open waters. The last species, Serranochromis robustus, was largely piscivorous. There were no ontogenetic variations in diet, apart from P. acuticeps where small fish fed mainly on zooplankton becoming omnivorous in larger ones, and S. robustus where the diet shifted from zooplankton in specimens <50 mm SL to fish in larger size classes. Data available from the 1960s and 1970s indicate that the diets of some species have changed with invertebrates such as chironomid larvae being previously more important in all diets, while most species now take a higher proportion of algae. This reflects changes in the lake, which is eutrophic and has experienced severe algal blooms since 1960.
Keywords
Cichlids Diet Diet overlap Ontogenetic changes Zimbabwe
Handling editor: C. Strumbauer

Introduction

This study investigated patterns in resource use amongst the cichlid species present in Lake Chivero, a man-made lake created in 1956 and located about 37 km southwest of the city of Harare, Zimbabwe. The lake is highly eutrophic, owing to the discharge of sewage effluent from Harare and satellite towns into its tributary streams, and suffers from algal blooms and the excessive growth of aquatic weeds such as water hyacinth Eichhornia crassipes and other plants (Marshall, 2005). In spite of periodic fish kills attributable to pollution (Moyo, 1997), the lake supports productive fisheries dominated by one or two cichlid species. The native cichlids include two small omnivorous species Pseudocrenilabrus philander (Weber) and Pharyngochromis acuticeps (Steindachner), two macrophages Tilapia rendalli Boulenger and T. sparrmanii A. Smith.
Five other species have been introduced into the lake, including a microphagous tilapia generally considered to be Oreochromis mossambicus (Peters) [Note: The Oreochromis species in this catchment, which is part of the middle Zambezi system, should be O. mortimeri (Trewavas) (Skelton, 2001) but the identity of these fish around Harare is uncertain owing to the widespread introduction of O. mossambicus. Most live specimens brought into breeding colours can be identified as the latter.]. Oreochromis macrochir (Boulenger) was stocked in 1956 and rapidly replaced O. mossambicus to become the most important commercial species (Marshall, 1982). Oreochromis niloticus (Linnaeus) was introduced some time in the 1980s and seems to be supplanting O. macrochir having now become the dominant commercial species (unpublished data). The piscivore Serranochromis robustus (Günther) was also introduced in the 1980s, while the molluscivore Sargochromis codringtonii (Boulenger) was stocked in 1978 but failed to establish itself.
Little is known about the feeding ecology of these cichlids in the lake. The only published work is a description of the diets of some of them in Munro (1967) and a more detailed investigation of the diet and feeding physiology of juvenile and sub-adult T. rendalli by Caulton (1975, 1976) and Caulton & Bursell (1977). This paper considers the diet of all of these species in Lake Chivero in an attempt to determine how much dietary overlap there was between different size-classes of each species and between different species. It was hypothesised that the greatest overlap would be amongst small species and the juveniles of large ones because they were likely to be feeding on the same resources, but the overlap would decrease amongst larger individuals because of a tendency to specialise on certain food items.

Materials and methods

Fish were collected with gill nets with stretched mesh sizes ranging from 12 mm to 120 mm, an electrofisher, and a beach seine net. Specimens were sorted according to species, weighed, measured to standard length and put into 50 mm size-classes with a minimum of 20 fish in each size class wherever possible and with scarce species, all available specimens were dissected. Their stomachs were dissected out and preserved in 5% formalin for 24 h to allow fixation of tissues and then transferred to 75% alcohol for storage. The contents of each stomach were suspended in 100 ml of water per gram of stomach contents and examined under an inverted microscope. Each item in the diet was identified to the lowest possible taxonomic level and counted.
The diet was first determined by the frequency of occurrence method, which records the percentage of stomachs containing a particular food item out of the total stomachs containing food (Hyslop, 1980). The food items were then combined into broader taxonomic categories for quantitative comparisons. The most practical way to estimate the contribution of each food category in each gut was modified from Platell and Potter (2001) by evenly spreading all contents from each gut in the counting cell chamber and examining under microscope. The area occupied by each food category in the chamber was later converted to the percentage of each food category.
The percentage of empty stomachs was determined for each species while the relative fullness of the stomachs was assessed on a scale from zero (empty) to four (full). Diet diversity was estimated by an index of niche breadth, calculated by:
$$ B_{i} = \frac{1} {{n - 1}}{\left[ {{\left( {\frac{1} {{{\sum {p_{{ij}} } }}}} \right)} - 1} \right]} $$
where B i = the index for species i, P i ′ = proportion of diet of species i that is made up of food item j and n = number of prey categories (Levin, 1968)
The dietary overlap between size classes of a single species or between species was calculated from:
$$ O_{{kj}} = \frac{{{\sum {(P_{{ik}} P_{{ij}} )} }}} {{{\sqrt {({\sum {P_{{ik}} ^{2} {\sum {P_{{ij}} ^{2} )} }} }} }}} $$
where O kj = niche overlap coefficient between species j and k, P ik and P ij = the proportion of food item i in the diet of species j and k (Pianka, 1974). The values of this index may range from zero (indicating a specialized diet or no overlap) to 1.0 (indicating an even use of food resources or complete overlap) with values >0.6 being considered a biologically significant overlap (Langton, 1982). A similarity matrix was constructed using a PRIMER statistical package version 6 and a one-way analysis of similarities (ANOSIM) was performed to test for diet differences between the species and their respective size classes. The similarity percentage (SIMPER) was constructed using the Bray–Curtis similarity index to access which food item makes the greatest contribution to diet similarity.

Results

A total of 555 specimens comprising seven of the nine cichlid species known to occur in Lake Chivero were collected during this study. Oreochromis mossambicus, now rare and possibly extinct in the lake, and Sargochromis codringtonii, which was introduced in 1978 but has rarely been caught since were not represented. Two of the larger species, T. rendalli and S. robustus, were represented only by small specimens, none of which were more than 100 mm SL and 200 mm SL, respectively. The average proportion of empty stomachs was 26.3% but this varied considerably from 10.8% in T. sparrmanii to 49.8% in P. acuticeps (Table 1). The relative fullness of those stomachs with food ranged from 2.3 in P. acuticeps and P. philander to 4.0 in T. rendalli, with a mean of 3.1.
Table 1
The numbers of fish collected (N), the proportion with empty stomachs and the relative fullness of those stomachs containing food in cichlid fishes from Lake Chivero
Species
N
Empty stomachs (%)
Relative fullness
Oreochromis niloticus
171
25.8
3.4
O. macrochir
79
25.9
3.6
Tilapia sparrmanii
33
10.8
3.9
T. rendalli
37
16.8
4.0
Pharyngochromis acuticeps
93
49.8
2.3
Pseudocrenilabrus philander
53
20.8
2.3
Serranochromis robustus
89
34.0
2.5
Oreochromis niloticus was a microphage and the most important items in its diet, over all size classes, were blue-green algae, predominantly Microcystis sp. (Table 2). Green algae were next in importance with diatoms, macrophytes and zooplankton being taken in small quantities. There were no significant differences (ANOSIM, Global R = −0.138, P > 0.05) in diet among the different size classes and the niche overlap coefficients ranged from 0.94 to 1.00 indicating an almost complete overlap in diet. The diet of O. macrochir was virtually identical to that of O. niloticus with blue-green algae making up >50% of its diet in all size classes, followed by green algae and diatoms with macrophytes and zooplankton making a small contribution. There were no significant differences (ANOSIM, Global R = −0.115, P > 0.05) among the different size classes and the dietary overlap coefficient ranged from 0.93 to 1.00 indicating an almost complete overlap in diet. The index of niche breadth ranged from 0.7 to 0.8.
Table 2
The proportion (by volume) of food items in the diet of seven cichlid species in relation to standard length, Lake Chivero
Species
Length (mm)
N
Blue-green algae
Green algae
Diatoms
Macrophytes
Microfauna
Fish
INB
O. niloticus
51–100
23
0.40
0.32
0.15
0.05
0.08
 
0.7
101–150
27
0.52
0.25
0.09
0.08
0.07
 
0.6
151–200
29
0.56
0.21
0.09
0.09
0.05
 
0.6
201–250
34
0.58
0.22
0.07
0.09
0.04
 
0.6
251–300
28
0.57
0.25
0.06
0.08
0.04
 
0.6
>300
30
0.58
0.24
0.07
0.10
0.03
 
0.7
O. macrochir
51–100
22
0.46
0.24
0.20
0.04
0.06
 
0.8
101–150
24
0.55
0.27
0.07
0.05
0.06
 
0.8
151–200
14
0.52
0.27
0.06
0.06
0.08
 
0.8
>200
19
0.66
0.16
0.04
0.07
0.07
 
0.7
T. sparrmanii
51–100
23
0.31
0.18
0.08
0.38
0.04
 
0.7
>100
10
0.28
0.14
0.11
0.44
0.03
 
0.7
T. rendalli
51–100
27
0.05
0.07
0.13
0.69
0.07
 
0.8
>100
10
0.04
0.08
0.12
0.71
0.05
 
0.5
P. acuticeps
<50
24
  
0.25
 
0.75
 
0.6
51–100
69
0.19
0.37
0.16
0.04
0.15
0.08
0.7
P. philander
<50
24
0.08
0.21
0.33
0.01
0.35
0.02
0.5
51–100
29
0.10
0.20
0.31
0.02
0.35
0.02
0.7
S. robustus
<50
25
0.07
 
0.04
0.03
0.83
0.02
0.6
51–100
29
0.06
 
0.03
0.04
0.30
0.58
0.4
>200
35
     
1.00
0.0
The dominant food items are shown in bold font, N = numbers of fish collected, INB = index of niche breadth, “microfauna’ is a functional prey category = zooplankton, insects and protozoa
The diets of the two macrophagous species T. sparrmanii and T. rendalli were very similar with macrophytes being the most important food item, comprising about 40% of the diet in T. sparrmanii and 70% in T. rendalli (Table 2). Although T. sparrmanii fed on macrophytes, it also took algae, especially cyanophytes, which were a conspicuous component of its diet. The dietary overlap among the respective size classes of both species was high (0.99 in sparrmanii and 1.00 in rendalli) with no significant differences (ANOSIM, global R = −0.148, P > 0.05) in diet.
There was a pronounced ontogenetic shift in the diet of P. acuticeps where fish <50 mm SL fed primarily on microfauna and diatoms while those 50–100 mm SL fed on a range of items with green algae being the most important, followed by blue-green algae, diatoms and microfauna (Table 2). Although the niche overlap was low (0.40), the differences in diet were not significant (ANOSIM, Global R = 0.163, P > 0.05). Such a shift was not evident in the other small species, P. philander, where zooplankton and diatoms were the major components of the diet in both size classes, with a niche overlap of 1.00.
Only 35 large specimens of the predaceous species S. robustus were caught and combined into one size class (>200 mm) but there was a clear ontogenetic shift towards piscivory. The diet of the <50 mm size-class consisted predominantly of zooplankton and insects but a distinct ontogenetic shift occurred in fish >50 mm SL which fed mostly on fish (Table 2). The niche overlap with fish of 51–100 mm was low (0.49) and even lower with those >200 mm (0.02). No specimens measuring 101–200 mm SL were caught but all fish greater than 200 mm SL were entirely piscivorous, and there was a high niche overlap of 0.88 with fish of 51–100 mm indicating the importance of fish in the diet of the latter. The index of niche breadth decreased with length in this species, reflecting the increasing specialisation of its diet.
Four major trophic groups can be distinguished amongst the cichlids in Lake Chivero using the similarity percentage analysis (SIMPER) (Fig. 1). The microphages (O. niloticus; O. macrochir) fed mostly on algae with Microcystis sp. contributing ≥50% by volume to their diet. The macrophages (T. sparrmanii, T. rendalli) fed mainly on macrophytes, which constituted 46% and 75% by volume of the diet in T. sparrmanii and T. rendalli, respectively. The omnivores (P. acuticeps and P. philander) had a catholic diet consisting of algae, zooplankton, plant material, fish and detritus but with zooplankton as the main component in the former (75% in fish <50 mm). The last species, S. robustus, was largely piscivorous from 51 mm SL although smaller individuals took zooplankton and other items.
/static-content/images/681/art%253A10.1007%252Fs10750-007-0790-7/MediaObjects/10750_2007_46r1_f1.jpg
Fig. 1
A comparison of the similarity percentage (SIMPER) of diets among cichlid fish species in Lake Chivero, October 2004–April 2005. Microfauna = zooplankton, insects and protozoa
There were only three species for which very small specimens (<50 mm SL) could be collected. For this size class, there was no significant dietary overlap between S. robustus and P. acuticeps (0.45) and S. robustus and P. philander (0.38) but there was a significant overlap between P. acuticeps and P. philander (0.82). All seven species were available in the next size class (51–100 mm SL) and there were significant differences (ANOSIM, Global R = 0.746, P < 0.05) in diet among them. The most pronounced overlap was between O. niloticus and O. macrochir whose diet was essentially the same (Table 3). The high overlap between these species and P. acuticeps reflected the high proportion of algae (62%) in the diet of the latter. There was also a high overlap between T. sparrmanii and T. rendalli reflecting the high proportion of macrophytes in their diet, and between P. acuticeps and P. philander brought about by the rather catholic diet of both species with no single item accounting for more than 40% of their diet. There were also some significant (>0.6) niche overlaps between P. philander and the two Oreochromis species, which reflected a high proportion (0.61%) of algae and diatoms in the diet of the former. The final significant overlap, between T. sparrmanii and P. acuticeps, resulted from the catholic diet of both species.
Table 3
Dietary overlap coefficients in seven cichlid species (51–100 mm SL) from Lake Chivero
 
S. robustus.
P. philander
P. acuticeps
T. rendalli
T. sparrmanii
O. macrochir
O. niloticus.
0.153
0.637
0.896
0.265
0.751
0.979
O. macrochir
0.144
0.611
0.815
0.228
0.738
 
T. sparrmanii
0.138
0.412
0.629
0.806
  
T. rendalli
0.119
0.246
0.277
   
P. acuticeps
0.347
0.795
    
P. philander
0.389
     
The most significant overlaps (>0.70) are shown in bold font
Dietary overlap between species >100 mm SL were generally very low except for O. niloticus and O. macrochir (0.99) and T. sparrmanii and T. rendalli (0.87) and this reflects the high proportion of algae, especially blue-greens, in the diet of the former and macrophytes in the diets of the latter. Serranochromis robustus (>200 mm SL) was piscivorous and its diet was significantly different (ANOSIM, Global R = 0.746, P < 0.05) from the other trophic groups.

Discussion

The extensive feeding overlaps between closely related species suggest a considerable degree of interspecific competition that might explain some of the changes that have taken place in the cichlid fauna. The dietary overlap between O. niloticus and O. macrochir was almost complete and the possibility that niloticus might be able to out-compete macrochir could partly explain its success in the lake and elsewhere where it is rapidly replacing indigenous Oreochromis species (De Vos et al., 1990; Schwank, 1995; Chifamba, 1998; Goudswaard, et al. 2002). The success of the O. niloticus is attributable to its opportunistic and versatile feeding behaviour (Moriarity & Moriarity, 1973; Getabu, 1994; Njiru et al., 2004), high fecundity and fast growth rate (Balirwa, 1998) and its ability to utilize a wide range of habitats for spawning and nursery purposes (Twongo, 1995). It is also an aggressive competitor with large males often out-competing other species when spawning places are limited (Lowe-McConnell, 2000). Interspecific competition might also explain the success of T. sparrmanii, almost unknown in the lake 20 years ago (Marshall, 1982) but now apparently more numerous than T. rendalli. The other species that might compete with each other are the two small omnivores, P. acuticeps and P. philander, but their unspecialised diet may reduce competition, as would their rather different habitat preferences with philander being more numerous in vegetated areas while acuticeps occurs amongst rocks or in areas without plants (Hustler & Marshall, 1990).
A striking feature of all of these species in Lake Chivero, with the exception of S. robustus, was the high proportion of algae in their diets, in contrast to tilapiine cichlids in the upper Zambezi, where algae seldom constituted more than 10% of their diet (Winemiller & Kelso-Winemiller, 2003). In Lake Liambezi, phytoplankton was a major component of the diet in O. macrochir, O. andersonii (Castelnau) and T. sparrmanii but not that of T. rendalli or P. acuticeps (Van der Waal, 1985). Many cichlids are adaptable and will feed on a wide variety of food items, depending on what is available; in Lake Kariba during the 1970s algae were not recorded in the diets of P. acuticeps or P. philander and insects were the main food items (Mitchell, 1976). By the 1990s algae were recorded in 82% and 64% of their stomachs, respectively, and the only important insects were chironomid larvae (Mhlanga, 2000). This difference in diet reflects the fact that Mitchell sampled amongst extensive Salvinia mats while Mhlanga collected fish from areas with little vegetation.
Similarly, the diet of cichlids in Lake Chivero seems to have changed along with environmental changes in the lake. When it was last investigated in 1961–62 the diet of juvenile O. macrochir (43–106 mm) consisted mainly of diatoms and some Cladocera and filamentous green algae, while larger fish (154–336 mm) contained some Microcystis in their stomachs as well as Cladocera, diatoms and filamentous algae. One individual had fed exclusively on chironomid larvae and pupae (Munro, 1967). This is a striking contrast to the present situation where blue-green algae (principally Microcystis) made up 50% of the diet of O. macrochir, with green algae amounting to another 25%. This probably reflects the continuing eutrophication of the lake, which has led to blooms of blue-green and other algae (Marshall, 2005). Interestingly in other lakes were eutrophication occurred during the past decades: e.g. Lake Victoria and Lake Nabugabo, O. niloticus became more omnivorous with high proportions of detritus and invertebrates in its diet (Gophen et al., 1993; Balirwa, 1998). The difference in diet change between these studies and the present study might be explained by the disappearance of other competitors due to the Nile perch boom (Bwanika et al., 2006) and O. niloticus has widened its traditional niche to encompass food resources previously utilized by various cichlids and non-cichlids which no longer exist in the lake (Njiru et al., 2004). A similar change to omnivory was observed for another cichlid fish Haplochromis pyrrhocephalus in the lake (Katunzi et al., 2003), a former zooplankivore which now feeds on fish, shrimps and molluscs.
In Lake Chivero most of the T. rendalli captured in 1961–62 were larger (170–340 mm) than any captured in this study, so a direct comparison of their diets is less easy. Higher plants, mainly Nymphaea and Lagarosiphon spp., made up 64% of the diet over 1 year although the diet was highly seasonal with chironomids and zooplankton making up about 50% of the diet between November and March. In the present study plant material still accounted for about 70% of the T. rendalli diet although the macrophyte community in the lake has changed dramatically over the years with Eichhornia and Hydrocotyle now being the most important species (Marshall, 2005). The diet of P. acuticeps in 1961–62 consisted mostly of chironomid larvae and pupae and Chaoborus larvae, as well mayfly and dragonfly larvae and other insects and occasional gastropods. The fry fed primarily on Cladocera. Their diet was similar in this study although with a larger proportion of algae, which may also reflect the increased density of algae in the lake.
Ontogenetic diet shifts have been noted in some tilapias (Le Roux, 1956) but these cichlids are opportunistic feeders during periods when resources are plenty and switch to a more specialized diet when resources are scarce (Winemiller, 1991; Winemiller & Winemiller, 2003; Lowe-McConnell, 1987). In Lake Chivero, algae and organic detritus are probably the most abundant food resource and all species will feed on whatever is available.
Acknowledgements
This project was funded by the Government of Belgium, through the University of Zimbabwe/Flemish Universities link of which the “Aquatic Ecology Project” was one component. We are grateful to the National Parks and Wild Life Authority of Zimbabwe who allowed us to work in Lake Chivero and use their facilities at the McIlwaine Research Centre. Trevor Dube, Taurai Bere, and Itai Tendaiupenyu are thanked for their assistance with fieldwork.
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