The meristic and morphometric characteristics of Gymnarchus niloticus are described and linear equations relating various parts of the body to the head length or total length are given. The age of G. niloticus in Lake Chad (Nigeria) was determined from growth marks on the opercular bones. The mean lengths for age, and mean weights for age obtained for the first five years of life are given. The assymptotic length and the von Betarlanffy growth parameters for the males and females combined are give

Sagua, V.O.

English

Aquatic Commons

179
STODIES on THE IMO CZ If OF Gymnarchus nlloricus ill LA{ E CO:EI AGE DETEEUA-
TIOR1 AND R0111fTH; WiERISTIC AND IVIORPHOMETRIC C;W7ACTERS
By
V. O. Sages
Lake 7:esaerch, testite to
Maidu9uri, Nimie
ABSTRACT
The rneristic and morphometric chal'acteristics ol Gymnarchus fuloticus are described
and linear regression equations relating various parts of the body to Lne head iength or total
length are given.
The age of Gymnarchus niloticus in Lake Chad was determined from growth marks on the
opercular bones. The mean lengthi for age, and mean weights for age obtained for the first five
years of life are given. The assympto tic length and the von Betarlanffy growth p.arameters for
the males and females combined were as follows;
Loo = 211.32cm; K = 0.1215; = 2.05224.
INTRODUCTION
Gymnarchus nilotkus, "dan sarki" in hausa, meaning "prince", is an electrical fish emitting
continuous weak eiectricat signals generated from an electric organ located in its rat - like tail
(Lissman, 1951). It has been shown that Gymnarchus niloticu.s has the ability to locate objects
in the water by means of the electric signals it emits (Lissman and Machin, 1958). This fish
is of bilogical interest because of its peculiar shape and habits; but it is also economically impor-
tant in Lake Chad for it is a very much relished edible fish species which during the drought
era (1972 to 1984) became the dominant piscivore in the lake.
The body of Gymnarchus is elongated, round in section and tapers to end in its rat-like
black tail. A long rayed dorsal fin extends from just behind the head to a short distance from the
tip of the tail. No pelvic, anal or caudal fins are present. The mouth is large and bears large
and sharp incisiform teeth in both jaws. Its colour-is blue-grey becorning progressively darker
towards the tail. The diet of the adults is alsmost exclusively fish (Segue, 1983); but the juveni-
les also feed upon insect larvae and nymphs.
Gymparchus is common though not abundant in River Nile, River Niger in the riverine
areas around the Niger Delta and in some eaSt African lakes such as Lake Turkana.- It is however,
absent from Lake Victoria.
The breeding habits of Gymnarchus were discussed by Budgett (1900) and the origin of
its electric tissues determined by Dahlgren (1914). Information on the general biology and
growth patterns of Gymnarchus is however scanty. The scientist at ORSTOM Laboratory in
Lake Chad made only cursory observations on its length-weight relationships (Durrand and
Loubens, 1967). Paucity of biological knowledge of this economically important fish could
be due to its relative scarcity where it is found.
The study ot the detailed biology of Gymnarchus niloticus in Lake Chad was initiated
in August 1982 and this p r is part of the results of the study.
180
MATERIALS AND METHODS
Freshly caught specimens of Gymnarchus niloticus obtained from hook and line or gill net
fishing at Baga and the nearby fishing islands in Lake Chad were used for this study. The under-
listed detailed body measurements were made on a standard one-metre measuring board also with
the aid of a 3 metre plastic measuring tape graduated in mm. The weight of the whole fish was
taken' with a top loading salter balance to the nearest gramme. The opercular bones were dissec-
ted and cleaned in fresh water from all flesh, and skin. The length of the opercular bones was
measured in an entero-posterior plane from its insertion to the free posterior border. The meris-
tic characters recorded included the numbers of gill-rakers; dorsal fin rays; lateral line scales;
numbers of teeth in the upper and lower jaws; and numbers of preanal and postanal vertebrae.
There was difficulty in enumerating the vertebrae in the tail,
Body Mrrurement
Total length: The length of the fish from the tip of the snout to the posterior extremity
of the tail.
Head length: The distance from the tip of the snout to the posterior border of the oper-
culurri.
Efe ['Looter: The distance from the anterior to the posterior margin of the eye measured
across the ptipil.
Snouth-Dorsal fin: The direct distance from the snout to the anterior insertion of the
dorsal fin.
Length of Dorsal fin: The direct distance from the anterior insertion to the posterioi
insertion Of the doilal fin measured at its base.
Length of tail: The distance from beginning to the posterior end of the tail.
The age of the fish was determined by examining the opercular bone through incident
light in the fresh state. Light passing through the bone reveals alternating bands of opaque and
transluscent growth markings and the distance from the anterior insertion of the operculum
to each fransluscent marking was recorded. The growth band on the edge of th3 operculum
bOne was noted as well as the date the fish was caught. OM an annual period tne incidence
of occurrence of transluscent and opaque growth markings vvas determined. The relationships
between the length of the operculum bone and the total length of each fish was determined
from a linear regression equation.
Scales were also collected from each fish and examined for growth markings.
RESULTS
Age and Orwoth Rate of laymnarchus niloticus
The scales of G. niloticus were found unsuitable for age determination. There were no
clear growth checks on the 'tiny scales. The opercular bones were therefore, used. Figure 1
shows the relationship between the length of the operculum and total length of G. nil oicus.
The linear regression equation relating the two is LP = 0.0242G + 0.0272, (r = 0.947).
(LP = Length of the operculum in cm and L - Total length of the fish in cm). The highly
positive correlation coefficient and the fact that th., intercept of the line on the y axis is nearly
sero are worthy of note. The operculum grows proportionally with the length of the fish and
is thus Vied object for use in direct back calculation for age-length data.
18
The translucent, narrow bands on the operculum wgre formed from Decernber to January
when the lake's water level WM highest and the water temperatures were at their lowest (Figure
2).
The mark represents a period of reduced biological activity and reduced growth. The opa-
que and broader growth band un the opercular bones were found during the remaining months
of the year. They represent periods of increased growth and increased biological activity.
A fish which has an opaque band plus one transluscent growth mark was classed as one
year old, vvhile those having an additional opaque Land, not a second narrow transluscent band
vvas classed one year plus, i.e. aged betvveen one and two years old.
In finding the mean value of the length for each year class, fiches of une yearand one year
plus were grouped togethr as one year olds etc.
By measuring the distances of the transluscent bands from the point of insertion of the
operculum and using the regression equation relating the length of operculum to the total length
of the fish, back calculations oi the length of the fish at each year mark un the operculum were
made.
Table 1 shows the comparison between age-length age-weight data for Gymnarchus niloticus
cus obtained by direct observation and from back calculations, which reveals a very close' fit in
the figures.
Table 1 Mean age-length, aJe-weight data for Gymnarchus niloticus from direct
observations and back calculations
(Miles and Females combined)
68.94 83.88 100.12
85.50 99,25
2083 3528
2500 3850
114.17
114.42
125.61
124.00
129.04
Age (years)
Mean length (cm)
(Back calculation (N)
Mean length (cm)
(Direc, observation (N)
Wan weight (gm)
(cunputed)
Mean Weight (gm)
(Direct observation)
7512
Table 2 shows thE age-length, age-vveight data for male and female G. niloticur for data
directly observed. At each age the males are slightly heavier and longer than the females, The
males therefore, have faster giowth rate than the females during the first five years.
The Walform growth transformation plot of Lr +1 (Ordinate) against Lt (abscissa) gave
a linear relationship represented by the equation:
Lt + 1 = 0.886 Lt + 24.09 (r = 0.9951).
for the combined female and male data from which the asymptotic length (Loo) was found
ro be 211.32cm. The von Bertalanffy growth parameters were obtained from plotting Ln (Loo
Lt) (ordinate) against t (abscissa)
137.31
2
140.00
3
9038
10420
Age (Year)
Females O
Mean length (cm)
Annual increment (cm)
Mean weight (gm)
Number observed (N)
alto 3 nly
Mean length (cm)
Annual increment (cm)
Mean weight (gin)
Annual increment (gm)
Number observed
182
Tab 9 - Mean ClIC-1061:01 and mean caeInght for females and and males
G. ilotions horn Lake Chad (direct observation)
121.50
17.07
6230
2060
2
the resultant slope (K) being 0.1215 and to 2.0524.
By repeating the calculatins using the data for the males only the following results were
obtai ned:
Lt + 1 = 34.9009 + 0.7711 Lt (r = 0.9554)
L 00 = 152.4723 cm
K 0.2599
to = 1.2298.
Insufficient data made calculation of the von Bertalanffy growth parameters for the females
alone unreliable as there were insuffient points.
Figure 3 shows the graphs relating tO the calculations for the growth parameters given above.
Meristic Characters of G. niloticus
The values obtained for the meristic characters investigated are shovvn in Table 3.
140.5
10380
10380
2
141.00
10500
1
Number
Eurnii
Meristic Character Means, mudes, frequencies,
of cecurrenc
Table 3 Some ineristic cheroterisiks uf G. Fliiialicus fruru Lake Chad
Dorsal fin rays 43
Number uf pro-anal vertebrae 5
Post-anal vertebrae
Pre-anal latarali line scales
Number of gill rakers on lower 51 5-9; mean, 6.75 mode, 7. Frequency 5 (f2);
limb of 1st gill arch. 6 (f18), 7 (f24); 8(5); 9 (f11).
48 24-28.24 (f6 26(f21) 28(f21)
48 14 constant in all specimens
Morphometric Characters of G. nilotcus
Relationships between lengths of parts of the body and the head length or total length
vvere as follows:
Body part as percentage of total length (TO
Teeth in lower jaw
Teeth in upper jaw
Body Part
(ED) Eye diameter
(LP) Length of pectoral fin
(MW) Mouth width
183
Range 168 to 215. 73% vvere from 201 to 210.
Mean: 206 Std. 8.05
47 constant in all specimens.
5 51-87. Mean number to the end of dorsal fin
is 70 vertebrae.
18 Range: .82 to 100: Mean number 88,
Body pan as percentage of hued length (HI)
Arithmetic means, standard deviation
3.09% HL Sid = 0.97, N = 46
26,29% HL Std 2.89, N = 42
31.65% HL Std = 2.26, N =15
Linear Regression Equations Relating Lengths of Body Parts tu the
Total Lengths (TI) or Head Lengths (HL)
Length of Pectoral-fin (Lp) and Head Length (HP) Lo - 0.9413 + 0.1919 HL
(r= 0.9196) N = 58
Diameter of eye (ED) and Head Length (HL) ED = 0.3537 + 0.0734 FIL (r = 0.2147)
N = 58.
Length of dorsal fin (LDF) and Total length (TL) LDF = 2.2247 + 0.7647 TL
(r = 0.99220) N 56.
Snout to Anus Length (LSA) and Total L3ngth (TL) LSA = 27832 + 0.5048
TL (r= 0.9877) N = 57.
Body Part Arithrnetic means, standard deviation
Head length 17.00% TL Std = 1.09, N 47
Tail length 9.55% TL Std = 1.66, N = 44
Dorsal fin length 74.09% TL Std = 2.50, N = 44
Snout to Anus length 47.46% TL Std = 2.37, N - 45
184
Length of operculum Lop and Total length (TL) Lop 0.0269 + 0.0242 TL (r 0
0.9467) N = 60.
Head Length (HL) and Total Length (IL) HL = 1.2565 + 0.1834 TL (r 0.9807)
N . 68.
Weight (W) and Total Length (TL) relationship. Log W = 2.978 Log TL 2.41
(1-2 0.9862) N . 58;
It is note-worthy that the positive correlation coefficients obtained for the linear regre-
ssion lines given above are highly significant for the number of degrees of freedom et 1% level.,
However, the correlation coefficient (r 0.2417) obtained for the linear regression of the eye
diameter and head length was not significant. The eyes may have a subordinate fun.-:tion in vi-
sion in G. nil otcus which is able to navigate and locate objects in its environment by use of its
electric organ discharge activity.
DISCUSSION
Knowledge of the age composition of a fish population is essential to good resource mana-
gement. Correct age information is necessary for longevity prediction, establishing groveth rate
records, the age at maturity and determining the assymptotic length and weight of fiihes. Con-
cerning Gymnarchus niloticus, there is no previous scientific vvork known to the-author on its
age and growth rate in Lake Chad or elsewhere. This study therefore, provide's the first attemut
at determining the age of this highly prized fish which is also being cultivated in water reservoirs
in the southern part of Nigeria. Due to the insufficiency of specimens, it has not been possible.
to cross check the age data by the length-frequency technique.
The use of bony parts in the determination of the age of Nigerian fishes has, however,
been done by Bayagbona (1968) using the Otoliths of croakers; Fagade (1974) using opercular
bones of Tilapia tnelanotheron; Ezenwa and Ikusemiju (1981) using the first dorsal spine of
Chrisichthys nigrodigitatus and Willoughby (1979) using veterbae for Lake Kaioji's Synodontic
species. In Lake Chad, Hopson (1972) used scales of Lates niloticus, on which he found "winter
rings" formed during the coldest months of January and February to determine age.
The problem usually encountered in determining the ages of tropical fist as by usg of
growth marks or checks formed seasonally on skeletal parts,,arises from little eivironrnental
changes, especially of temperatura This is particularly true of low latitudes. But the further
one goes (north or south) from the equator or the higher one goes in latitude, the greater the
seasonal temperature &drips. Lake Chad located in a semi-arid area between latitudes 120
30'N and 140 30'N experiences well marked seasonal changes in water temperature with a drop
of about 10oC betvveen the warmest months (May/June 28oC) and the coldest months, (Decem-
ber/January, 17.08oC). The cold period also coincides vvith high water level. This seasonal
drop in temperature has also been associated vvith reduction in feeding intensity (Sagua, 1983)
and hence results in a reduced growth rate reflected on the operculurn as transluscent band. The
validity of the transluscent growth band for age determination arises from its formation only
once in a year. Fagade (1974) has shown that other seasonal changes such as salinity could also
cause formation of growth marks on operculum of Tilapia mcalnotheron in Lagos Lagoon.
186
The biggest specimen of G. niloticus seen during this study in Lake Chad INE15 a Female
of 157 cm IL.weighing 14.25kg. Durand and Loubens (1969) found, as their largest sj...ecinien
in Lake Chad, a fish of 151 cm TL, weighing 15.50 kg but its sex vvas not stated.
The observed largest size is 74.4% of the calculated assymptotic length (211 cm) for the
combined data for females and males. This vaiue ^opears a good estimate since the high 'fishing
pressure in Lake Chad makes largor and older fisheL LO be rare.
The value of Loo (152 cm) obtained for the males would, therefore, appear to be an
under estimate. This is, however, the best information that is obtainable from the observed
data.
A certain amount of variability in the meristic'and morpnometric characters will be ONPEZ-
ted in G. niloticus over its wide geographic distribution rango in Africa in which tha speci
is endemic. The results in this paper for Lake Chad, ,should therefore, form basis tor useful
comparison vvith those of vvorkers in other parts of the contMent and the water bodies in Nigeria.
- 186
REFERENCES
RAYAGBONA, E. 0. (1986). Aga determination and Bertalanffy growt parameters of Pseudotolithus typus
and P. senegalensis using the burnt otolith technique. I. W. Afr. Sci. Ass. 13: 147-158.
BUDGET, J. S. (1900). The breeding habits of Protopterus, Gymnarchus and some other West African Fishes.
Science 11: 1015.
DAHLGREN, U. (1914). Origin of the electr'c tissues of Gymnarchus niloticus. Pubis. Carnegie Instn 182:
159-194.
DURAND, J. R. and LOUBENS, G. (1969). Courbes longuer-poids de 46 especes de poissons du bassin Tcha-
dien. Fortlamy. ORSTOM. fey. 1969.
EZENWA, 13-.1.0. and IKUSEMIJU, K. (1981). Age and growth determinations in the catfish, C'hrysichthys
nigrodigitants (Lacepede) by use of the dorsal spine. J. Fish Biol. 19: 345-351
FAGADE, S. 0. (1974). Age determination in Tilapia melanotheron (Rupeli) in the Lagos I-agoon, Nigeria.
In: The Ageing of Fish (TB. Ragenal, Ed.). Proceedings of Symposium. 234pp. Ole Working: Unwin.
HOPSON, A. J. (1972). A study of the Nile perch (Lates niloticus (L) Pisces; Centropomidae) in the Lake
Chad. 93pp. Overseas Research Publications No. 19. Foreign and Commonwealth Office, Overseas Develop-
ment Administration.
LISSMANN, H. W. (1951), Con tinuous electrical signals from tne tail cf a fish, Gymnarchus nlloticus. Nature.
Lond. 167: (4240): 201-202.
LISSMANN, H. W. and KE .WACHIN (1958). The mechanism of object location in Gymnarehus ntioticus and
similar fish.
LISSMANN, H. W. and K-. E. MACHIN (1958). The mechanism of object location in Gymnatchus niloticus
and similar fish. J. Exp. BioL 35: (2): 451-486.
SAGUA, V. O. (1983). Preliminary report on the biology of G. niloticus In Lake Chad. Lake Chad Research
Institute Annual Report 1983. 61-65.
WILLOUGHBY, N. G. (1979). Some aspects of the ecology of Syndontis (Pisces Siluroidei) in Lake Kainji
during its early years. Proceedings of the International Conference on Kainji Lake and River Basins Develop.
ment in Africa, Ibadan, 11-17M December, 1977. Publ. and Printed by Kainji Lake Research Institute. 376
386.
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Studies on the biology of Gymnarchus niloticus in Lake Chad: Age determination and growth; meristic and morphometric characters

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Studies on the biology of Gymnarchus niloticus in Lake Chad: Age determination and growth; meristic and morphometric characters

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