Hecht, T.

Kaunda, E.

Aquatic Commons

/4 (/UU-FISh Ich;iiciI Repo,i Lcsue /Vo: / "br 2002 
Fishing in the deeper waters of Lake Malawi and the Precautionary 
Princip'e: A case study of Bombe, Batizyclarias nyasensis (Siluroidei) 
E. Kaunda' and T. Hecht2 
Dept of Aquaculture & Fisheries Science, University of Malawi. Bunda College. Box 2 1 9 Li1ongw'b rvlalawi. 
Email : ekaunda'amalawi . net 
Dept of Ichthyology & Fisheries Science, Rhodes University. P/Bag 94. Graharnstown. South Africa 
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Introduction 
The lake Malawi genus Bathyclaricis be- 
longs to the family Clariidae that falls in the 
group of the specialised catfishes (Reagan 
1907. Alexander 1975). Jackson (1959) cre- 
ated the genus Bathyc/arias. endemic to Lake 
Malawi. However. Greenwood (1961) syno- 
mised the Lake Tanganvika DinOtopterus and 
Bce/h pc/arias where Bath ye/ar/as became the 
junior synonym. Recently. using morphologi- 
cal. osteological and zoogeographical evi- 
dence. Anseaume & Teugels (1999) rein- 
stated Bathyciaria.r as the valid generic name 
of the species found in Lake Malawi. 
While the species of Bathyciarias and other 
catfishes are the target of artisanal fisheries in 
some localised areas and may contribute to 
about 20 per cent of the catch by weight, 
(Banda et a/. 1995). biological and ecological 
studies on Bath cc/arias have never been 
made to foster effective management options 
of the species. Scant' information and spo- 
radic observations on the biology of Bathy- 
c/ar/as were made by Lowe (1952). Jackson 
(1961) and Thompson et aI.(1996). This lack 
of information led to omission of the Bathyciarias 
from the trophic model that was constructed for 
the Lake Malawi pelagic food web (Allison et. al. 
1995). In cognisance that life hist?ry traits such 
as age, growth and reproductive biofogv influence 
diet and food habits and/or vice ver'a (Wootton 
1990). and that a fill dietary analysis is necessary 
to elucidate ecological role of a species 
(Nagelkerke 1997), this study was conducted to 
investigate life history traits. diet and food habits 
of B. nyasensis. The information obtained would 
assist in determining the trophic role of B. 
nyasensis. 
Materials and Methods 
Samples of B. nyasensis (Bathyciarias species 
with a n.Igose head and a light crescent on the 
caudal fin) were collected month or bimonthly 
from the South-East Ann of Lake Malawi be- 
tween December 1996 to November 1998 using a 
17 in long research vessel Ndunduma. powered 
by a single caterpillar engine of 386 hp. The Gui- 
lopur bottom trawl net with a headrope of 23 in. a 
vertical opening of 4 m and a codend mesh size 
of 38 mm was used to obtain the material. The 
2 
Aqua-Fish Technical Report Issue No: 1 Nov 2002 
trawling speed was 3-3.5 knots and the trawis 
were made at depths of 8 — 122 m. 
Out of 776 fish that were dissected, 343 
with identifiable stomach contents were ana- 
lysed. Identification of prey items was made 
to the lowest taxon possible. Prey items were 
placed in a single petri dish and sorted into 
different taxonomic categories. The relative 
volume of each prey category was estimated 
visually. Prey items in each category were 
then counted. Where small prey items (e.g., 
zoopla.nkton) were numerous, they were 
placed in a known volume of water and thor- 
oughly mixed by swirling the water, of which 
10 % was removed for sorting and enumerat- 
ing. At most this dilution was done twice. 
Stomach contents were analysed for the 
whole sample, fish size and in relation to wa- 
ter depth using numerical, percent of occur- 
rence and volumetric methods according to 
Hynes (1950), Hyslop (1980). Tudorancea, 
Fernando, & Paggi (1988) and Cortés (1997). 
Index of Relative Importance (IRI) was com- 
puted according to Palomares, Garces, Sia III 
& Vega (1997): 
IRI= %Vx%F 
Where % V = percent volume 
%F = percent frequency of occurrence 
Isotopic composition of different size 
groups of fish were used to determine ontoge- 
netic diet changes of B. nyasensis. Carbon isotope 
ratios (6'3C) of fish ranging from 37 to 73 cm to- 
tal length (TL) and principal prey items (fish and 
zooplankton) were determined. 
Parameters relating to filtering ability were de- 
termined by taking measurements on the first gill 
arch. 'The measurements included the length of 
the gill rakers, two on the lower arm and one on 
the upper arm of the first gill arch. The filter.ing 
area was estimated by calculating the two trape- 
zoidal areas formed by the gill rakers where the 
lengths of the gill rakers were the heights of the 
trapezoids. 
Results and Discussion 
Distinct changes in relative importance (% IRI) 
of prey types were noted as fish grew (Fig. I). 
The relative importance of zooplankton increased 
from about 10 % in small fish (20 - 29.9 cm TL). 
to 91.9% in large fish (size F, 70- 86.5 cm TL). 
"1'ish remains" decreased in importance from 
29.1 % in small fish (20 - 29.9 cm TL) to 2.6 % 
in large fish (70 - 86.5 cm, TL). The relative im- 
portance of insects decreased from 16,7 % (20- 
29.9 cm, TL) to less than I % in large fish (70- 
86.5 cm TL). The importance of all prey items in 
the small size category (20-29.9 cm TL) may 
have been masked by the high portion of 
'amorphous, digested food" (47.7. %). However, 
the amount of this amorphous material decreased 
n 343 
Fig. 1. Relative irnpottance (%IRJ) of piincipal prey items of 
Bathvciarias nyasensi.c of different sizes. 
3 
Amorphous material 
20.29.9 30.39.9 40.49.9 50-59.9 60.69.9 70+ 
Size groups (TL, cm) 
Aqua—Fir/i Tech akal Report issue No: I Nov 2002 
-17.5 
-18 - 
l8.5 - 
-195- 
-23 
-205 i 
-21 
-21.5 
0 10 20 30 40 50 60 70 80 
Total length, cm 
Fig. 2. & 3C values in Bathyclarias nyasensis from Lake Malawi. 
The fitted 2 order polynomial is provided in the text. 
350 
300 
øJj 
250— 
200 — 
150 -L 
- 50 
C-) 
Sze of fis8 TL, om) 
Fig. 3. Change in filtering 
nyasensis at different sizes. 
with fish size (5.5 % in size, 70-86.5 cm TL) 
Results on 613C values also showed clear 
ontogenetic diet changes. A second order 
polynomial equation fitted the data (Fig. 2), 
and the equation was of the form: 
= 
- 33.188 + 0.4997L - 0.0045L2 (r2 
0.598. ii = 12, p0.02) 
where: L = total length of fish in cm. 
From Fig. 2, it is apparent that the '3C val- 
ues changed from "light" or "more negative" 
in small fish (approximately —21 %o) to 
"heavy" or "less iiegative" in medium size 
fish (-19.5 %o) and to "light" or more nega- 
tive in large fish (-20,7 %) suggesting that 
fish changed diet with change in size. The 
ö'3C of approximately —21.0 %o, is indicative 
of diet shift to zooplankton assuming a i -2 %o 
enrichment factor from pry item (613C value 
of zooplankton --23 %) to, consumer (DeNiro 
area of Bathyclarias 
& Epistein 1978; Fry & Sherry 1984). 
Results obtained on filtering area showed a dis- 
tinct change as B. nyasensis reach 50 — 60 cm IL 
fish size (Fig. 3). 
From the foregoing results on gilt content and 
carbon isotope analyses, arid filtering area, onto- 
genetic diet changes of B. nyasensis are clearly 
evident. In addition, an analysis of diet of B. 
nyasensis from different water depths suggests an 
ontogenetic, habitat diet change phenomenon. 
From Fig. 4. fish that were caught from the 
deeper waters (> 40 m. depths [N and DE), con- 
sumed higher proportion of zooplankton than fish 
from shallower waters (<40 m, depth SH). Con- 
versel.y, fish from the shallower waters (<40m 
deptl SH), had higher proportions of fish than 
fish from deeper waters (> 40 in, depths IN and 
DE. 
By being zooplanktivorous in deeper waters, 
4 
20 30 40 50 60 70 80 SO 100 
Fig. 4. % IRI of prey categories of small (SF, 20-40cm TL) and large (LF, 
40 
— 86 cm, IL) Bath vciarias nyasensis caught from different water depths. 
S shallow (10- 40 in). I = intermediate (41- 60 m) and D = deep water 
(61- 86 in). Note that only preiten with high % IRI are shown, 
and considering the zooplankton dynamics in 
Lake Malawi (Allison et al. 1995b), it is ap- 
parent that B. nyasensis feeds on the pelagic 
food web of the lake, although the species 
was omitted from the pelagic food web that 
was constructed by Allison ef a!. (1995a). 
From an in situ 24-hr die! study (Kaunda 
2001), food consumption per biomass (QIB) 
ratio of 3.16 was estimated. This was similar 
to the estimate of 3.31 0.61 bf (Ngatunga 
& Allison 1996) which was obtained using an 
empirical model of Jarre et al. (1991). Using 
the QIB value of 3 it was estimated that B. 
nyasensis increases overall zooplankton con- 
sumption in the pelagic zone by 11 per cent 
(Kaunda 2001). Predation of zooplankton in 
the pelagic zone of Lake Malawi may there- 
fore have been underestimated. 
This new information raises questions to 
the implications of fishing in the pelagic zone 
of Lake Malawi with promotion of 'efficiept 
fishing gears to deeper waters and where lar- 
ger species are usually selectively' overfished 
in a multispecies fishery (Welcomme 1999) 
and suffer from a "fishing down" syndrome 
(Pauly ci a!. 1998). The question that imme- 
diately comes up is would overfishing of B. 
nyasensis lead to cascading effects to the lake 
ecosystem? While cascading effects of plantivo- 
roris fish in natural systems are equivocal, their 
effects on the size distribution of zooplankton is 
unequivocal (Currie ci a!. 1999). 
The presence of B. nctsenvis in the offshore 
areas could affect the cornmunit\ ::rucic of the 
lower trophic levels in two ways. Firstly, B. 
nyaxens/.c may provide the right zooplankton size 
to other zooplanktivorcs such as the lake fly C 
eduJis which has been identified as one of the 
most important zooplankton grazers in the lake 
(Turner 1982). The presence of planlctivorous fish: 
leads to dominance of small size zooplankton 
(Currie et a!. 1999). and the gape of C. ethilis lirn— 
its the size of zooplankton that can be ingested 
(Irvine 1995), Hence, it can be expected that the 
absence or a reduced abundance of B nyasensis 
would lead to dominance of large zooplankton 
which can riot be ingested by C. er/u/is, resulting 
in starvation-induced mortality of the latter. In 
turn, a reduced biomass of C. er/u/is would affect 
Diplotaxodon "elongate", as this species relies 
almost entirely on C. er/u/is (Allison ci a!. 1995). 
Secondly, B. rlyasensis may reduce competition 
for dilute resource of the herbivorous zooplank- 
ton, Tropothaptornus cunningtoni. it is known 
that the carnivorous zooplankton Mesoqyc!ops 
aequatorialis aequatorialis, larval Engraulicypris 
5 
Aqua-Fish Technical Report Issue No: 1 Nov 2002 
100 
90 
80 
70 
60 
— 50 0 
40 
30 
20 
10 
0 
LI' I liD 
Aqua-Fish Technical Report Issue No: I ATov 2002 
sarciella and C. edulis feed on T. cunninginnii. B. 
nyasensis feeds on both MI. a. aequaroriaii.s and 
T. cunningioni. While the circumstances under 
which B. nyasenvl.v take one type of zooplankton 
and not the other are not known. it can be as- 
sumed that by eliminating M a. aequatorialis, B. 
nyasensis reduces competition between M a. 
aequalorialis. E arc/ella larvae and C. cc/u/is 
for T. cunningtoni. The absence of B. nyasensis 
and therefore increase in abundance of Iv! a. 
aequatorialis may therefore, lead to stiff compe- 
tition for T. cunningtoni which may culminate in 
low production of C. cc/u/is and E. sardella lar- 
vae with possible effects to the upper level of the 
food web. 
While these predictions remain speculative, it 
may be possible that in the past such ecological 
consequences have not been noticed because the 
offshore region has not been intensively fished 
and the pelagic ecosystem was hence considered 
to be stable (Allison ci al. 1995). The current in- 
terest to expand demersa! trawling in the off- 
shore region (Allison ci al. 1995) that may lead 
to elimination of B. nyasensis may thus lead to 
undesirable ecological consequences to the 
'stable" pelagic ecosystem of Lake Malawi. 
These predictions should serve as a warning that 
exploitation of the offshore fisheries should pro- 
ceed on the basis of the precautionary principle 
which is defined as when an activity raises 
threats of harm to human health or the environ- 
ment. precautionary measures should be taken 
even if some cause and effect relationships are 
not fully established scientifically' 
(Raffensperger & Tickner 2002). 
Acknowledgements 
The Icelandic International Development 
Agency (ICEIDA) principally ftmded the study. 
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Fishing in the deeper waters of Lake Malawi and the precautionary principle: a case study of Bombe,  Bathyclarias nyasensis (Siluroidei)

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Fishing in the deeper waters of Lake Malawi and the precautionary principle: a case study of Bombe, Bathyclarias nyasensis (Siluroidei)

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