4. BIOLOGY OF CHAMBO AND OTHER COMMERCIALLY IMPORTANT FISHES

4.1 INTRODUCTION

From 1976 to 1990, the total fish production of Lakes Malawi and Malombe and the Upper Shire river has been estimated at 27–45,000 tonnes (CAS & Commercial Fisheries returns: Table 4.1). Cichlids account for 80–90% of the total catch. Between 1976 and 1985, tilapiine cichlids and haplochromine cichlids were of approximately equal importance, each providing around 40% of the total yield, but subsequently haplochromines have increased to around 60% of the catch, with tilapiines declining to less than 20%. However, since beach prices for tilapiines average about three times as much as those of haplochromines, the two groups are of approximately equal economic importance.

Non-cichlid fishes are of much lesser importance. Here too there has been a switch over from larger species, such as catfishes, to the small pelagic cyprinid Engraulicypris sardella (usipa). Most usipa are caught by open-water chirimila seines, operating with light attraction: this fishing technique has little by-catch and so does not directly interact with the cichlid fisheries.

Table 4.1 Total fish landings in Malawi, from Lakes Malawi and Malombe and the Upper Shire River.

Notes: data from commercial fisheries returns and 3-year running averages of CAS estimates for artisanal fisheries catches. Domira Bay area is excluded.

4.2 BIOLOGY OF CHAMBO & OTHER TILAPIINE CICHLIDS

Chambo is a general term for three closely related species of tilapiine cichlids, presently placed (Trewavas 1983) in the subgenus Nyasalapia of the genus Oreochromis.

Systematics and identification were studied by reanalysis of the morphometrics and meristics of 206 fish collected on the ODA-funded Oreochromis lidole project. Distribution, diet and reproductive biology were investigated in a year-long programme of stratified sampling of both the artisanal and commercial fisheries throughout the project area. 11,510 chambo were collected, identified, measured and weighed. Sex and gonad state were recorded for all fish above 20cm, and subsamples were taken for fecundity determination, dietary analysis and ageing through opercular bone reading. A three-day experimental trawl survey in July 1991 was used to investigate depth preferences of different sizes of fish, while a twelve-month experimental gillnetting (thrice weekly) and fyke-netting (daily) programme was used to study movements of fish through the Upper Shire river.

Apart from the chambo group, there are also two other species of tilapiine cichlids found in Lake Malawi: Oreochromis shiranus and Tilapia rendalli. Both are inshore species, favouring shallow swampy areas.

O. shiranus supports important fisheries in Lakes Chilwa and Chiuta, and is caught in substantial numbers in Lake Malombe (in the present report it comprises the majority of what is recorded as ‘Other Tilapia’). This species is extremely common in swamps, and temporary pools and streams, but it is also found throughout the lake, including offshore islands such as Mumbo, and the Maleris. It is occasionally taken by pair trawlers, which fish in shallow water. O. shiranus is easily distinguished from chambo by its golden body colour, strong horizontal bars, deep caudal peduncle and four anal spines. Like two of the chambo species, ripe males are black, but the margin of the dorsal fin in O. shiranus is red, rather than white. Lowe (1952) records a size at first maturity of 17cm for females and 22cm for males. It feeds on epiphytic algae, plankton and detritus.

Tilapia rendalli is of less commercial importance: it is a deep-bodied fish of a generally reddish-brown colour, with a white belly. Mature fish are greenish with pink or reddish bellies. A prominent spot in the dorsal fin is present, even in large fishes. Unlike the Oreochromis species, there is no difference in colour between the sexes, even when breeding. It is not a mouthbrooder. Both male and female guard the eggs which are laid in a burrow, and continue parental care for some time after the fry become free-swimming. The diet consists of macrophytes and detritus.

4.3 CHAMBO SYSTEMATICS AND IDENTIFICATION

The three chambo species, although closely-related and difficult to distinguish, differ in a number of features of their life history and must thus be considered separately, unless it can be shown that such differences are irrelevant for stock assessment and management purposes.

Oreochromis lidole was originally recognised by Trewavas (1942) by its slender lower pharyngeal bone, which supports a small toothed area. Morphologically, the species shows little variation between locations (Turner, ms). In the field, it could be easily distinguished at sizes above 15cm. Characteristics are given in Table 4.2.

Table 4.2 Identification of chambo species

Oreochromis squamipinnis is distinguished from O. lidole and O. karongae by its distinctive male breeding colours (Table 4.2). Morphologically, females and immature males are very similar to O. karongae. In Lake Malawi, fish below 20cm were difficult to distinguish, while in Lake Malombe, only adult breeding fish could be reliably identified.

O. karongae is a very variable species (Table 4.3). Note that the most variable characters are the number of tooth rows and the shape of the lower pharyngeal bone. These features are feeding structures, and it is likely that differences in the types of food eaten and in the kind of substrate the fish feed from could be responsible for this variation. In other cichlid species, if a single brood of fry is split in two and each given a different diet, similar differences in feeding structures are produced (Meyer 1987, 1990). Thus, there does not need to be a genetic basis to such differences: alternatively similar morphological differences can be genetically-based, but the two morphs may interbreed (Kornfield et al. 1982). For this reason, the southern populations of O. karongae, which had been believed to be a different species (Oreochromis saka - Lowe 1952, 1953; Trewavas 1983) are now regarded as variants of O. karongae (Turner & Robinson 1990; Turner ms).

Species identification was difficult and involved consideration of a number of characteristics, none of which was in itself diagnostic of all specimens of each species. In addition to the characteristics listed in Table 4.2, the arrangement of lower jaw teeth was found to be of considerable value in identification. In O. lidole, the small number of tooth rows are clearly separated by wide gaps. The first row of teeth in O. squamipinnis is usually clearly demarcated from the inner teeth which are closely-packed and usually smaller. Oreochromis karongae generally has longer teeth, which are, like the inner teeth of O. squamipinnis, closely-packed and not arranged in clear-cut rows. However, O. karongae in Lake Malombe has a tooth arrangement more like O. squamipinnis than is typical of Lake Malawi forms.

Figure 4.1 Breeding seasons of chambo, expressed as percentage of ripe adults taken from all gears sampled. High levels of breeding activity were recorded from all months, except April to June. Reproduction begins earlier in Lake Malombe than in Lake Malawi, but north of Boadzulu Island O. karongae has a longer, and possibly bimodal breeding season. The main breeding area for O. lidole is north of Boadzulu Island.

Figure 4.2 Oreochromis squamipinnis from Lake Malombe have higher fecundity than conspecifics from the south-east arm of Lake Malawi. Fecundities of Oreochromis karongae are not different in the two water bodies, and are lower than O. squamipinnis in Lake Malombe. Oreochromis lidole has a lower fecundity at all sizes.

Table 4.3 Variation in characteristics of Oreochromis karongae. Measurements given as means and range (from Turner, ms).

4.4 REPRODUCTION

4.4.1 Breeding behaviour

Chambo, like other Oreochromis species are maternal mouthbrooders. That is, females carry the eggs and fry in their mouths until the young have developed to a stage where exogenous feeding is possible.

Males construct nests on sandy or muddy bottoms and display their conspicuous colours to passing females. Eggs are laid on a central raised platform of the nest and are immediately taken into the female's mouth. The female then starts mouthing the male's elongate genital papilla, which is believed to have developed as an egg-mimic: fertilisation takes place inside the female's mouth. Females rapidly leave the nesting areas after spawning. Males play no part in parental care, and will begin to court another female immediately after spawning. Thus, fishing on nesting sites mainly catches ripe males and, since males are polygamous, high mortality of nesting males makes little difference to total population fecundity.

4.4.2 Size at Maturity

As with many other tropical species with long breeding seasons, the proportion of ripe chambo caught is generally small, even at the largest size-classes during the height of the breeding season. Consequently, the size at 50% maturity is taken as the size at which the percentage of ripe fish is half of the maximum for the species (leaving out size-classes with small sample sizes). It was found that there were no consistent differences in sizes of ripe males and females. O. squampinnis matures at a smaller size than the other species (Table 4.4). Note that females of both O. squamipinnis and O. karongae are slightly larger in Lake Malombe than in the south-east arm. Thus, there is no justification for the present regulations where minimum mesh sizes for gillnets and chambo seines are smaller in Lake Malombe.

Table 4.4 Size (cm) at maturity of chambo (means, ranges and sample sizes of ripe fish examined are given).

4.4.3 Nest Sites

In a study of chambo nesting sites at Cape Maclear (Nankumba Peninsula), Turner et al. (1991b) found that Oreochromis karongae nests at depths of 0.5 to at least 30m, while all O. lidole nests were found at depths in excess of 17m. During the present project, commercial trawl catches from as deep as 48m were found to have caught large numbers of running ripe male O. lidole.

Very few ripe male O. lidole were collected in Lake Malombe, which is nowhere deeper than 17m. It is likely that this lake is shallower than the preferred breeding depth of the species.

Lowe (1952) observed catches of ripe O. squamipinnis from as deep as 22m, but Tweddle (pers. comm.) collected this species from 8m at Karonga, and Turner et al. (1991b) recorded a territorial male from 4m in Monkey Bay. Since large numbers of ripe O. squamipinnis are caught in Lake Malombe, it appears that this species, like O. karongae, nests at a wide range of depths.

In general, O. lidole builds larger nests than the other species (Turner et al. 1991b) - this may be a factor in preventing hybridisation.

4.4.4 Seasonality and Breeding Areas

Oreochromis karongae and O. lidole breed mainly during the hot season before the rains, September to December (Figure 4.1). Reproduction in O. karongae begins earlier in Lake Malombe than in Lake Malawi. Few ripe O. karongae were taken in Area A, south of Boadzulu Island, the main breeding areas being in Lake Malombe and north of Boadzulu Island. Ripe O. lidole were mainly found in the north of the south-east arm. Breeding seasons in both species continue until March in Lake Malawi, which is consistent with Turner et al.'s observations at Cape Maclear (1991b). In Lake Malombe, O. karongae ceases reproduction in December, as Lowe recorded (1952).

Reproduction in O. squamipinnis begins somewhat later than the other species, in November to December, and continues until April. This species breeds in all areas surveyed, although Turner et al. (1991b) found that it did not nest on the steeply-shelving rocky coasts of the Nankumba Peninsula.

4.4.5 Fecundity

Comparisons of egg production against length were made using analysis of covariance tests on logarithmically transformed data (Table 4.5).

Oreochromis karongae from Lake Malombe and Lake Malawi did not differ significantly in their fecundities (F=0.67, 1,74 df, P=0.42), but it was found that O. squamipinnis from Lake Malombe produce more eggs than similarly-sized individuals from Lake Malawi (F=43.9, 1,66 df, P<0.001).

There were significant differences in fecundity in all comparisons between species, including those made with each of the two O. squamipinnis populations. At all sizes O. lidole has the lowest and O. squamipinnis from Lake Malombe the highest mean fecundity (Figure 4.2).

Table 4.5 Length-fecundity relationships for chambo species.

4.4.6 Life history and movements

(a) Depth Preferences

After leaving the nesting sites, females continue to brood eggs and larvae for about two weeks (Lowe 1952). When the yolk sac of the fry has been fully absorbed, brooding females move into shallow water, where they release and protect their fry. During this period, the female continues to protect them and to allow them to return to her mouth if they are threatened. During this fry-guarding stage, females and young remain in shallow water, where they are highly vulnerable to beach seining.

Large numbers of spent female O. lidole, many with fry, appear in Lake Malombe from December to April. Since ripe adults are rarely found south of Boadzulu Island, these females must have migrated from the northern part of the south-east arm, through the Shire river into Lake Malombe: a distance of at least 50km.

Fry continue to return to the females' mouths until they have reached large sizes: 24mm for O. karongae, 15mm for O. squamipinnis and 58mm for O. lidole (Lowe 1952). Thus, the fry guarding period must last for several weeks.

Figure 4.3 Experimental trawling indicated that chambo move to progressively deeper water as they grow, until, at around the size at maturity (25 cm), they move back to shallower water. Data from 760 fish caught in July 1991 in the south-east arm of Lake Malawi.

Figure 4.4 Fyke netting in the Upper Shire River between September 1990 and September 1991 indicated that the peak abundance of kasawala in the river occurred during the cold season, as the flooded areas along the river began to dry out. Most fish headed towards Lake Malombe (data from 1420 fish from 297 sets).

Figure 4.5 Seining experiments indicate that chambo stocks in the Upper Shire River have been greatly reduced over the last 18 years. This has probably disrupted any migration patterns which may have previously existed (Green Beach Mangochi: 20 mm beach seine - 7 hauls in October 1973, 54 hauls in October 1991).

Observations made while snorkelling or diving indicate that chambo fry up to 7cm are generally found in large shoals at depths of less than 1m. Between 7 and 15cm, they remain in water of a few metres depth, being most abundant in the vicinity of weed beds or rocks.

Experimental trawling showed that this movement to progressively greater depths continues throughout the immature phase, with 20–22cm fish dominating catches at 40–50m. At around the minimum size at maturity (24cm), most chambo begin to return to shallower waters (Figure 4.3). This is the pattern for O. squamipinnis and O. karongae. Oreochromis lidole was not caught by the experimental demersal trawl, and is rare in commercial demersal trawl catches. Immature O. lidole at sizes of 10–25cm were rarely seen, even in mid-water trawl catches. The habitat preference of immature O. lidole could thus not be determined with certainty, but it seems likely that they may be in mid-water.

In addition to the effect of age on depth preference, there is also a seasonal effect. In all shallow water fisheries, the catch per unit effort for chambo declines during the cold season. Underwater observations indicated that few chambo over 10cm were to be seen at depths of less than 20m from May to August.

4.4.7 Movements through the Shire river

Experimental gillnetting in the Upper Shire river indicated that, like in other shallow water areas, few chambo were to be found during the cold season. There was a significant change in the direction in which fish entered the net: more fish were facing towards Lake Malombe during the earlier part of the breeding season, later most faced in the opposite direction. Thus there is evidence for a seasonal migration through the river. However, multivariate morphometric analysis indicates that more than 80% of adults of both O. squamipinnis and O. karongae from Lake Malombe could be distinguished from conspecifics in the south-east arm, arguing that the Malombe stocks of these species are largely sedentary. The presence of brooding females O. lidole in Lake Malombe, -but not ripe fish-, indicates that this species at least migrates through the river.

Experimental fyke-netting showed that large numbers of 6–10cm juveniles are present in the river from May to July: the majority of them moving towards Lake Malombe (Seisay et al., 1992a). The timing, and their relatively large size suggests that they have moved from the drying swamps around the river, into the main river channel (Figure 4.4).

If adults from Lake Malawi have moved into Lake Malombe to spawn or, in the case of O. lidole, release fry, there should be a return migration of immature fish. This was not observed. Comparison of experimental seining at Mangochi in October 1991 and October 1973 indicated that the density of chambo in the river had dramatically declined (Figure 4.5), and migration patterns which formerly existed may have been disrupted by the collapse of the Lake Malombe and Upper Shire chambo fisheries.

4.4.8 Diet

Like other Oreochromis species, chambo feed mainly on algae, detritus and zooplankton. Turner et al. (1991a) found that the diet of adult (>17cm SL) and juvenile chambo at Cape Maclear were dominated by filamentous green algae (Mougeotia), but juveniles ate more attached benthic filamentous species, such as Calothrix, Cladophora and Spirogyra. Underwater observations indicated that the smallest juveniles (5–7cm SL) obtained most of their food by scraping the surfaces of rocks, switching more to surfaces of submerged plants at 8–12cm SL, but feeding on sediment and plankton at larger sizes (Turner et al. 1991a).

From samples taken by commercial trawlers in the south-east arm, adult chambo of all species were found to feed mainly on the filamentous diatom Aulacoseira (= Melosira), while diets in Lake Malombe were more varied, but included a higher proportion of Mougeotia and zooplankton (Turner et al. 1991a). There was little difference between the diets of different chambo species caught in the same place at the same time of year, suggesting that if food competition is avoided it is by means of habitat preference rather than food preference.

Turner et al. (1991a) did not quantify seasonal fluctuations in diet, nor the proportion of zooplankton taken. During the present study, monthly samples of the stomach contents of O. squamipinnis were obtained from Lake Malawi and Lake Malombe. In the south-east arm of Lake Malawi (Table 4.6), Aulacoseira was again found to be the dominant species, although Stephanodiscus, Surirella and zooplankton (mainly Diaptomus) made up 35–70% of the stomach contents in August and September.

In Lake Malombe, zooplankton (mainly Bosmina) and the large solitary diatom Surirella were found to be the dominant items, zooplankton being especially important from August to December.

Comparison with plankton hauls (Table 4.6), indicates that chambo positively select cladocerans (such as Bosmina and Diaphanosoma) and large diatoms, while taking fewer Mesocyclops (a large fast copepod) and Closterium (a small alga). Further samples have been taken and will be analysed as part of the PhD work of N.C. Mwanyama.

Table 4.6 Chambo diet composition.

Note: Phytoplankton composition in diet is similar to that in the lake. Zooplankton accounts for 8% of the stomach contents by volume in the south-east arm and 38% in Lake Malombe. Chambo are positively selective for Bosmina.

4.5 BIOLOGY OF OTHER FISH SPECIES

Most of the fisheries which exploit chambo also catch other species, and thus their biology and exploitation also demand consideration. Altogether 69 samples (38 mm midwater trawl: 35, 38 mm bottom trawl: 13, pair trawl: 6, seine nets: 15) and 36,500 non-chambo species were analysed. In addition, the project fish biologist also analysed 87 samples from experimental bottom trawls made as part of an ODA-funded survey of demersal fish stocks from May to November 1991. Species composition by weight and numbers was determined for all samples. Reference collections of all species were made. Most species were photographed fresh. Length, sex and gonad state were determined for 100 specimens of the most abundant species from the mid-water trawl bycatch, which accounts for more than 700 tonnes of the commercial fish catch, and is new to science.

Figure 4.7 Different species of Lethrinops and related genera have different depth preferences. This information can be used to define the stocks exploited by different trawling techniques. (Trawl data from ODA Demersal Fisheries Reassessment Project's samples from May to November 1991).

From artisanal seine net samples, species composition by weight and numbers were determined, and additionally the length distribution of all species was obtained from 8 samples. Length, sex and gonad state were recorded for some 430 specimens of the two most numerous haplochromine species from Lake Malombe, which accounted for an estimated 4,200 tonnes of the catch in 1991. Both species appear to be new to science.

4.5.1 Haplochromine cichlids

Haplochromine cichlids are recorded under a variety of names (utaka, kambuzi, chisawasawa, ndunduma, ncheni, mbaba, saguga), and the choice of name used is more governed by the size of the fish or the gear employed than by the species caught. All species are maternal mouthbrooders, with relatively low fecundity. The most recent key available (Eccles & Trewavas 1989) contains few of the most important commercial species: many have never been studied or named, appear in no museum collection and nothing is known of their biology. One factor contributing to this has been the increase in importance of small haplochromines, either through changing fishing practices or changes in community structure resulting from heavy exploitation. For this reason, brief notes on major or potential commercial species are included to assist future identification.

(a) Pelagic Species (Ncheni, Ndunduma)

There are more than 30 species of pelagic cichlids of the genera Rhamphochromis and Diplotaxodon. The systematics of this group are very confused. Different species appear to have different habitat preferences, some preferring inshore waters, while others inhabit the surface layers of open waters, and some appear to be largely demersal. Large numbers of small Rhamphochromis are caught in shallow waters by kambuzi seines in Lake Malawi: it is not known if all of the offshore or deep water species have inshore-living young or if these represent different species. Diplotaxodon species appear to breed in open waters, and young are not vulnerable to beach seining.

(b) Semipelagic Species (Utaka)

Copadichromis are relatively deep-bodied species, with highly protractile mouths. Although sometimes described as pelagic, they actually seem to be closely associated with the bottom, feeding on zooplankton, in large shoals one or two metres above the substrate. Many species are closely associated with rocky shores, and are particularly abundant at areas of upwelling around islands or submerged rocky reefs (virundu). Here, they are exploited by chirimila nets and handlines. Most of these rock-associated species do not appear to cross soft-bottoms or move in open water, and have never been recorded in trawl catches. These species may have restricted distributions and would thus be vulnerable to local overfishing. Other utaka species are also frequently taken over rocks, but are more mobile, breeding on shallow sandy bottoms in large nesting aggregations. Two small species are abundant in Lake Malombe, particularly in nkacha catches from the eastern part of the lake, and are also found in the Upper Shire river and swampy bays in southern Lake Malawi. A third group of species comprises elongate, more benthic forms, which are seined in large numbers from exposed sandy beaches, and sometimes taken in chirimila catches.

(c) Benthic Haplochromines

There are around 150 species of benthic haplochromine cichlids exploited in the south-east arm of Lake Malawi. Most have restricted depth distributions (Figure 4.7) and habitat preferences. Unlike chambo, most of the deep water species remain in the same habitat at all stages in their life histories. Thus, they are not vulnerable to seining of shallow waters. Heavy fishing with small-meshed trawls has brought about large changes in species composition of the benthic communities (Turner et al. 1992a), and many of the larger species which have been previously studied are now of little commercial significance. Few of the smaller species which are now dominant can be identified from existing keys. There is an urgent need for a simple field guide to these fishes, as misidentification can render biological information of little value. For example, FAO (1976) gave information such as growth parameters, size at maturity, fecundity and mortality for 9 species. Although a reference collection was not made, specimens collected at the same time have been examined in the field collection of the Monkey Bay Fisheries Station. Only 2 of the 9 are actually single species: all others were found to consist of several distinct species, often with different maximum sizes and habitat preferences. Thus, it is likely that the published information is misleading.