Catches sampled
Table 1 provides a summary of the total recorded catches by region for both 2020 and 1998 biomass assessment surveys. The 2020 biomass assessment survey recorded 27,261.6kg of fish against 37,853.2kg registered in the 1998 survey thereby representing a decrease of 28%. At regional level, the recent survey recorded 713.3kg of fish in Karonga which represented a drop of 58.1% from 1,700.6kg reported in 1998. Another big decline was recorded in the SEA where catches of 9,233.1kg were registered in 2020 against 17,912kg recorded in the previous survey translating into a 48.5% drop. The central part and the SWA reported declines of 13.4% and 4.6%, respectively. Nkhata Bay was the only site that reported an increase of 12.3% between the two surveys.
Table 1
Comparative results of total fish catch and mean catch rates between 2020 and 1998 surveys
| 2020 | 1998 |
Region | No of Stations | Total Catch (kg) | Mean Catch Rates (kg/0.5Hr) | Total Catch (kg) | Mean Catch Rates (kg/0.5Hr) | Change (%) |
Karonga | 7 | 713.3 | 101.9 | 1,700.6 | 242.9 | -58.1 |
Nkhata Bay | 14 | 4,267.7 | 304.8 | 3,800.9 | 271.5 | 12.3 |
Central | 29 | 7,130.0 | 245.9 | 8,234.0 | 283.9 | -13.4 |
SWA | 27 | 5,917.5 | 219.2 | 6,205.6 | 229.8 | -4.6 |
SEA | 39 | 9,233.1 | 236.7 | 17,912.0 | 459.3 | -48.5 |
Overall | 116 | 27,261.6 | 235.0 | 37,853.2 | 326.3 | -28.0 |
Fish species composition
The 2020 survey recorded fishes comprising 7 families, 45 genera and 149 species. The catch from the 1998 survey on the other hand consisted of 7 families, 45 genera and 158 species. The family Cichlidae that contributed 86% in 2020 and 83% in 1998 survey was dominant in both surveys (Fig. 2). The Cichlidae dominance, was followed by Clariidae, Bagridae, Mormyridae and Mochokidae with 7.1%, 3.4%, 1.4% and 1.2%, respectively in the 2020 survey. The family Cyprinidae had smaller contribution of less than 1%. In the former survey, family Bagridae Clariidae, Mochokidae and Cyprinidae with 6%, 5%, 5% and 1%, respectively followed the dominant display of the Cichlids. Family Mastacembelidae and Mormyridae had insignificant contributions of less than 1% each in the earlier survey.
At genus level, Fig. 3 shows fish that contributed to more than 1% of the total sampled catches. The recent survey recorded Lethrinops (23%) Copadichromis (9%), Placidochromis (9%), Aulonocara (8%), Otopharynx (7%) and Diplotaxodon (7%), as the most common with a combined contribution of more than 60%. In the former survey, Lethrinops (22%) Otopharynx (10%), Copadichromis (9%), Aulonocara (8%), Diplotaxodon (7%), Rhamphochromis (6%) Alticorpus (5%) and Bagrus (4%) were the most important genera in the sampled catches. The genera Taeniolethrinops, Stigmatochromis and Placidochromis were the least in the 1998 survey.
In the category of fish genus that contributed less than 1% in the recent survey, the genera Synodontis (0.71%) and Sciaenochromis (0.57%) contributed more than 0.5% each (Fig. 4). The least were the genera Barbus, Corematodus, Docimodus and Labeo with 0.013%, 0.011%, 0.011 and 0.002%, respectively. The 1998 survey recorded Pallidochromis (0.86%), Chilotilapia (0.84%), Engraulicypris (0.79%), Ctenopharynx (0.74%), Nimbochromis (0.65%) and Hemitaeniochromis (0.62%) having contributions of more than 0.5% each. The genera Labeo, Lichnochromis and Clarias with 0.01%, 0.002% and 0.002%, respectively were the least.
From the 149 fish species recorded in the recent survey, the SEA dominated with 119 species followed by central with 113 species. The trend was similar to the one reported in 1998 survey. The third highest number of fish species (92) was also registered in SWA while Nkhata Bay and Karonga on the other hand recorded 62 and 46 species, respectively. In terms of number of sampled specimens recorded in the recent survey, SEA with 14,890 was again dominant followed by SWA and central with 10,075 and 8,713 individuals, respectively. Nkhata Bay and Karonga were the last two with 3,305 and 1,104, respectively. A further assessment of the 38,037 spacemen indicated that Otopharynx argyrosoma (2,541), Copadichromis virginalis (2,051), Lethrinops oliveri (2,011), Aulonocara minutus (1,675) and Placidochromis longimanus (1,607) had a numerical advantage from the rest.
The 1998 survey reported a total of 49,223 specimens which comprised 2,170 in Karonga, 5,882 in Nkhata Bay, 10,244 in the central, 13,987 in the SWA and 16,940 in the SEA. At regional level, the highest number of fish species was recorded in the SEA with 125 which was closely followed by the central part with 120. The third highest was SWA with 110 species whereas Nkhata Bay and Karonga were the last two with 70 and 43 species, respectively. A numerical assessment of the 158 fish species indicated that Otopharynx argyrosoma (3,899), Bagrus meridionalis (2,282), Copadichromis virginalis (2,136), Diplotaxodon limnothrissa (1,984) and Synodontis njassae (1,812) were among the top five fish species.
Size Structure and selectivity of selected fish species
The top most abundant fish species were subjected to length-based analyses and Table 2 provides a summary of the parameters used. The top five dominant fish species were subjected to further length-based analyses to determine their size structure. The fishes were analysed at two levels; using the gamma distribution function and logistic model for gear selectivity. Table 2 shows the solver fitted parameters that this study used to plot length frequency graphs and selection ogive.
Table 2
Solver fitted parameters for Gamma distribution and Logistic Models for the top 5 fish species
| | | Gamma Model Parameters | Logistic Model | Literature | Management Implication |
| Fish Species | Sample (n) | Phi | Sigma | Modal Length (mm) | L50 | ∑ | Length-at-Maturity (Lm) |
2020 | Otopharynx argyrosoma | 2,541 | 0.81 | 105.30 | 85 | 81 | 0.14 | 92 | |
Copadichromis virginalis | 2051 | 0.95 | 115.38 | 109 | 97 | 0.08 | 80 | Mature |
Lethrinops ‘sp oliveri’ | 2011 | 0.80 | 105.44 | 83 | 81 | 0.13 | 120 | Immature |
Aulonocara minutus | 1675 | 0.72 | 100.40 | 72 | 67 | 0.20 | 42 | Mature |
Placidochromis longimanus | 1607 | 0.83 | 93.45 | 77 | 74 | 0.20 | Unknown | NA |
1998 | Otopharynx argyrosoma | 3,899 | 0.70 | 118.59 | 82 | 85 | 0.14 | 92 | |
Bagrus meridionalis | 2,282 | 2.95 | 101.97 | 298 | 290 | 0.014 | 330 | Immature |
Copadichromis virginalis | 2,136 | 1.02 | 118.46 | 120 | 114 | 0.08 | 80 | Mature |
Diplotaxodon limnothrissa | 1,984 | 0.98 | 93.40 | 90 | 132 | 0.04 | | |
2.06 | 78.65 | 160 | | | 150 | Mature |
Synodontis njassae | 1,812 | 1.1 | 118.54 | 130 | 122 | 0.11 | 120 | Mature |
Figure 5 and Table 2 show modelled size structure for O. argyrosoma, C. virginalis, L. ‘sp oliveri’, A. minutus and P. longimanus sampled in the recently survey. The study registered the smallest O. argyrosoma of 51mm and the largest being 121mm with a mean total length of 83.3 ± 0.25mm. The model predicted a modal length of 85mm for the fish. The smallest C. virginalis measured 40mm and the largest was 160mm with a mean total length of 97 ± 1.02mm. The model estimated a value of 108mm as the modal length for the fish. The 2020 survey recorded sizes of L. ‘sp oliveri’ that ranged from 32mm to 155mm with an average total length of 83.9 ± 0.35mm. The predicted modal length was estimated at 83mm. Length for A. minutus ranged from 40mm to 80mm with a mean total length of 67.7 ± 1.08mm. The sampled fish had a length distribution estimate of 72mm as modal length. The sizes of P. longimanus ranged from 42mm to 135mm with an average total length of 75.6 ± 0.85mm. The predicted modal length of P. longimanus was 77mm.
Table 2 and Fig. 6 show the selection ogive of the five fish species. The length at which O. argyrosoma had 50% chance of being retained by the 38mm cod-end meshes was 81mm. The values of L50 for C. virginalis, L. ‘sp oliveri’, A. minutus and P. longimanus were estimated as 97mm, 81mm, 67mm and 74mm, respectively.
The modelled size structures for Otopharynx argyrosoma, Bagrus meridionalis, Copadichromis virginalis, Diplotaxodon limnothrissa and Synodontis njassae for the former survey are shown in Fig. 7. The survey registered the smallest O. argyrosoma of 35mm and the largest being 155mm. The modelled length frequency distribution clearly indicated that the study sampled most of the fish species with a total length ranging from 75mm to 115mm having a mean total length of 88 ± 1.01mm. The model predicted a modal length value of 82mm for the fish. According to this study that recorded a total of 2,282 individual B. meridionalis, the smallest fish measured 50mm and the largest was 980mm. The modal length of the fish species was estimated at 298mm with a mean total length of 306.7 ± 2.11mm. The smallest C. virginalis measured 45mm and the largest was 195mm having a mean total length of 114.6 ± 0.89mm. The sampled fish had a modal length estimate of 120mm. The study that registered a total of 1,984 D. limnothrissa specimens reported the smallest fish of 30mm and the largest being 195mm. The modelled length frequency distribution showed that the sampled fish belonged to two length peaks; the first one was at 90mm while the second one was at 165mm. Because of this, the model generated two separate parameters with the first modal length at 90.2mm followed by another one at 160.2mm. The sizes of S. njassae ranged from 40mm to 205mm with an average total length of 123.3 ± 1.11mm. The predicted modal length from the model was 129.8mm.
Figure 8 and Table 2 show the L50 values for O. argyrosoma, B. meridionalis, C. virginalis, D. limnothrissa and S. njassae. The length at which O. argyrosoma had 50% chance of being retained by the 38mm cod-end meshes was 85mm. The value for L50 for B. meridionalis was estimated at 290mm while for C. virginalis, D. limnothrissa and S. njassae were 114mm, 131.8mm and 122mm, respectively.
Fish species diversity
An assessment of the fish diversity between the two surveys was conducted and the results indicated diversity indices of 3.84 and 4.06 for 2020 and 1998 surveys, respectively. Building on the estimated indices in both surveys, the effective number of species (ENS) was estimated as 47 for the 2020 survey and 58 for the 1998 survey. Besides reporting differences in the total number of fish species, the two surveys further differed on a number of fish species in that some species were appearing either in the 1998 or the 2020 survey.
The 2020 biomass assessment survey reported fish species amounting to 36 that were not there in the 2020 survey whereas the 1998 survey registered a total of 43 fish species that were not recorded in the recent survey (Table 3). Within the fish species that were only available in the recent survey, a total of 16 genera originating from Cichlidae and Mormyridae families were recorded. Out of the 43 fish species that were exclusively available in the 1998 survey, a total of 20 fish genera formed from 3 families namely Cichlidae, Clariidae and Cyprinidae were recorded. The study recorded unusually significant quantities of Mormyrids during the 2022 biomass assessment survey. A total of 18 specimens of the rare Mormyrops anguilloides (Linnaeus, 1758) measuring 1,110mm to 1,215mm with an average weight of 12.4kg were caught around Luweya River mouth in Nkhata Bay.
Table 3
Fish species that appeared in either the 2020 or 1998 survey
No | Fish Species | 2020 | 1998 | Fish Species | 2020 | 1998 |
1 | Aulonocara brevinidus | - | + | Lethrinops micrentodon | - | + |
2 | Aulonocara brevirostris | - | + | Lethrinops microdon | - | + |
3 | Aulonocara macrochir | - | + | Lethrinops sp blue - orange | + | - |
4 | Aulonocara maylandi | + | - | Lethrinops sp long | - | + |
5 | Aulonocara sp blue-nose | + | - | Lethrinops sp matumbae | - | + |
6 | Aulonocara sp deep | - | + | Lethrinops stridei | - | + |
7 | Aulonocara sp gold | + | - | Lichnochromis acuticeps | - | + |
8 | Aulonocara sp long | - | + | Metriaclima aurora | - | + |
9 | Aulonocara sp maleri | + | - | Metriaclima zebra | - | + |
10 | Aulonocara sp rostratum deep | - | + | Mormyrops anguilloides | + | - |
11 | Aulonocara sp yellow | - | + | Mylochromis ericotaenia | - | + |
12 | Barbus litamba | - | + | Mylochromis formosus | + | + |
13 | Bathyclarias atribranchus | - | + | Mylochromis lateristriga | + | - |
14 | Buccochromis sp eucinostomus | + | - | Mylochromis melanotaenia | - | + |
15 | Champsochromis spilorhynchus | - | + | Mylochromis plagiotaenia | + | - |
16 | Copadichromis borleyi | + | - | Mylochromis sp cf balteatus | - | + |
17 | Copadichromis chrysonotus | + | - | Mylochromis sp silver torpedo | + | - |
18 | Copadichromis cyaneus | - | + | Otopharynx inornatus | - | + |
19 | Copadichromis pleurostigmoides | - | + | Oreochromis karongae | - | + |
20 | Copadichromis prostoma | - | + | Otopharynx argyrosoma | + | - |
21 | Coptodon rendalli | + | - | Otopharynx heterodon | - | + |
22 | Dimidiochromis dimidiatus | - | + | Otopharynx selenurus | + | - |
23 | Dimidiochromis strigatus | - | + | Otopharynx tetraspirus | + | - |
24 | Diplotaxodon argenteus | + | + | Otopharynx tetrastigma | + | - |
25 | Diplotaxodon macrops | + | - | Placidochromis acuticeps | - | + |
26 | Diplotaxodon sp big - eye | - | + | Placidochromis hennydaviesae | + | - |
27 | Docimodus evelynae | + | - | Placidochromis milomo | + | - |
28 | Exochromis annagens | + | - | Protomelas lobochilus | - | + |
29 | Hemitaeniochromis sp urotaenia deep | - | + | Protomelas taeniolatus | + | - |
30 | Hemitaeniochromis sp urotaenia shallow | - | + | Scienochromis ahli | + | - |
31 | Labeo mesops | - | + | Scienochromis benthicola | + | - |
32 | Lethrinops furcifer | - | + | Stigmatochromis sp spilostichus | - | + |
33 | Lethrinops leptodon | - | + | Stigmatochromis sp woodi shallow | - | + |
34 | Lethrinops machrochir | + | - | Stigmatochromis spilorhynchus | - | + |
35 | Lethrinops macracanthus | + | - | Tramitichromis intermedius | + | - |
36 | Lethrinops macrochir | + | - | Tramitichromis variabilis | + | - |
37 | Lethrinops macrophthalmus | + | - | Trematocranus intermedius | + | - |
38 | Lethrinops macrorhynchus | + | - | Tyrannochromis macrostoma | - | + |
39 | Lethrinops marginatus | + | - | | | |
Total | 36 | 43 |
Key: Available species is denoted by a positive symbol (+) while absence is denoted by a negative symbol (-)