Journal of Environmental Protection, 2011, 2, 982-995
doi:10.4236/jep.2011.27113 Published Online September 2011 (
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish
Community Structure by Effluent from Wood
Processing Factory
Benedict Obeten Offem*, Irom Bassey, Gabriel Ujong Ikpi
Department of Fisheries and Aquatic Sciences, Cross River University of Technology, Cross River State, Nigeria.
Email: *
Received June 1st, 2011; revised July 6th, 2011; accepted August 5th, 2011.
In order to plan a management programme for ensuring maximum production of fish in Cross River, impacted down-
stream changes in the fish community structure by effluents from wood processing industry, six years after establish-
ment, was examined. Monthly samples were collected between January and December each year from 2000 to 2006 in
three reaches (Upriver: I, Mid-river: II and Downriver: III) along the length of Cross River. Representatives of the fish
families Osteoglossidae (i.e. Heterotis niloticus), Cichlida e (Tilapia melonopleura) and Characidae (Bryocinus nurse),
Clupeidae (Cynothrissa sp), Mormyridae (Mormyrus deliciosus), Clariidae (Clarias gariepinus), Bagridae (Bagrus
bayad) and Cyprinidae (Barbus occidentalis) were found to have declin ed in their impo rta n ce co mpa red to p re-indu stry
period. On the other hand, Bagridae (Chrysichthys nigrodigitatus), Cichlidaae (Orechromis niloticus), Claridae
(Clarias anguillaris) and Mochokidae (Synodontis clarias) have currently emerged as most important. Estimated value
of growth co efficient (b) of the length-weight relationship changed from isometry (b approx. = 3) to negative allometry
(b 3), condition factor values decreased from range between 0.53 and 1.30 to range between 0.22 and 0.62. Main
feeding groups of fish ; planktivores, carnivores and insectivores declined in numbers while omnivores and detritivores
increased, resulting in dominance of benthic and semi-pelagic omnivores. Values of fecundity distribution varied from
56,012 ± 5234 eggs, mode 12,500 and median 58,345 to m ean val u e 23,122 ± 232 eggs, mode 2500 and median 20,349,
egg size from mean valu e; 1.82 ± 0.07 mm, mode 2.2, and median; 1.8 to va lues of 0.8 ± 0.04 mm, mode; 1.3 and me-
dian 1.1 and Gonadosoma tic inde x from 20.5 ± 3.2, mode 19.1 ± 2.2 and median 21.4 to values o f 12.4 ± 2.3, mod e 4.5
and median 9.5 respectively. Three species found to have appeared in the river were Tilapia monody, Chrysichthys
maurus and Synodontis violaceus. The appearance of these species and disappearance of 36 others indicates the re-
structuring of the fish community of the Cross River by effluents from the wood processing industry.
Keywords: Fish Community, Fish Composition and Abundance, Diet Changes, Len gth-Weight Relationship,
Reproductive Biology
1. Introduction
In Africa, a large proportion of both rural and urban
populations live in vicinity of inland or coastal waters.
Examples are Cairo on River Nile, Khartoun at Conflu-
ence of blue and white nile, Kampala (Lake Victoria),
Kinshasa and Brazaville (River Zarie/Congo), Banjul
(River Gambia), Niamey and Bamako (River Niger) and
some national capitals located along the coast e.g Abid-
jan, Dakar, Rabat, Da Es Salam and Luanda [1]. Settle-
ments close to natural waters offer man’s greatest hopes
for livelihood and material supplies. Egypt’s Delta Lakes
supply 50% of annual fish consumption [2]. Large basins
in Africa: Niger, Benue, Sokoto, Ouema, Shire, Barotse,
Kafu flats, Massili & Okarango have at least 100 species
each [3]. However, Potential annual yield of small sys-
tems is about 2 million metric tons [4]. In some coastal
cities: Zaire, Ethiopia, Keyna, Madagascar and Tazania
freshwater fish are more important than marine fish and
may contribute up to 90% of the total landings [4,5]. Ni-
gerian inland water bodies are primarily used for fishing
and the fisheries, is private sector driven and operates
mainly in remote rural areas [6]. It contributes 86% of
domestic fish production [7]. It is source of employment
and provides income and nutrition for about 3 millions
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory983
rural dwellers that depend on fish for livelihood. Nigeria
could be self-sufficient in fish production and major ex-
porter of fish if these water bodies are properly managed.
However, the fish yields of most inland waters are
generally on the decline [8], which has been attributed to
causes ranging from environmental degradation of the
water bodies due to anthropogenic inputs from neighbouring
communities and industries [9,10]. Impacted changes in
water quality are reflected in the biotic community struc-
ture with the vulnerable dying, while the most sensitive
species act as indicators of pollution [11-14] listed major
industries responsible for water pollution in Nigeria to
include petroleum, mining, wood, pulp, pharmaceuticals,
textiles, plastics, iron and steel, brewing, distillery, fer-
mentation, paints, beverages, food and agriculture. It was
reported [15] fertilizer effluents from industrial city of
Kano polluted Jakara reservoir. High levels of toxic
heavy metals including copper, zinc, chromium, iron and
manganese were detected [16,17] in fish from Warwade
Reservoir also in Kano. An increased in the pollution
levels of the Cross River estuary by petroleum product
spillage has been reported [18] which is a problem to the
fishery industry.
To achieve management goals, it is necessary to know
the changing pattern of fish populations in water bodies.
This paper assesses the state of the fish community of the
Cross River, before and after the establishment of wood
processing industry at the shoreline of the upper portion
of the river, in relation to species composition, food habit
groups, length-weight relationship, fecundity and condi-
tion factor. It is expected that the information provided
from the study will contribute to the formulation of
management interventions for optimum utilization and
sustainable socio-economic development of the Cross
2. Materials and Methods
2.1. Study Site
The study site is the Cross River, a floodplain river lo-
cated at the South Eastern part of Nigeria (Figure 1) on
Latitude 4°25´ - 7°00´ N, Longitude 7°15´ - 9°30´ E. It is
bounded in the South by the Atlantic Ocean, East by the
Republic of Cameroun, the Nigerian states of Benue in
the North, Ebonyi and Abia in the West and Akwa Ibom;
South West. Climate of the study area is defined by dry
season and wet season. The wet season (April-October)
is characterised by high precipitation (3050 mm ± 230
mm), while the dry season (November-March) is marked
by low precipitation (300 mm ± 23 mm). Mean annual
temperature ranged from 15.5 ± 7.6 (wet season) to
32.6 ± 5.4 (dry season). For the purpose of this
study three sampling sites were selected along the length
of the river, with one site randomly selected in each of
the following reaches; upriver, middle river and down-
river. The effluent point of a wood processing industry
was located at the shoreline of the upper portion of the
river which was covered by savanna grassland and 3 km
from the river source with rocky, gravel and sandy sub-
stratum. The middle river was 100 km from river source
with rocky substratum and shoreline sparsely shaded by
forest and savanna grassland. Downriver had a muddy
substratum and opens up into the Cross River estuary,
with shoreline thickly shaded with rainforest.
2.2. Ichthyofaunal Sampling
The ichthyofuana of the river was sampled at the same
time of physico-chemical sampling in all the reaches using
variety of fishing gears which included; gill net (22 - 76
mm stretched mesh size), seine net (10mm stretched mesh
size) and cast net (10 mm stretched mesh). On each occa-
sion sampling was between 09.00 and 12.00 am. Genus
and species identifications was carried out for the Cypri-
nids [20]; Bagrids [21], Clariidae [22]; Clupeidae and
Mugilidae [23]. Species abundance of each reach was pre-
sented as a numerical contribution by each species. This
was determined by calculating the percentage each species
represented of the total catch for each reach based on the
number of species.
2.3. Length-Weight Relationship And Condition
Fish weights were measured to nearest 0.1 g and total
length (TL) to nearest 1mm. Length- weight relationship
(LWR) was estimated from the equation; W = aLb [24]
and was logarithmically transformed into logW = loga +
blogL. W = weight of fish in grams, L = total length of
fish in centimeters, a = constant of proportionality and b
= allometry coefficient. The parameters a and b are esti-
mated by method of least squares regression [25] using
the log trans -formed data. The condition factor was de-
termined using the expression [26] as K = W·100/L3. K =
Condition factor, W = Total body weight, L = Total
2.4. Gut Content Analysis.
To determine changes in food habit groups of fishes
samples were transported to the laboratory under ice to
minimize post mortem changes. Each specimen was
measured for total length (cm) and weight (g) with date,
time and location [27]. Fish samples were preserved in
deep freezers at –10. The fish were later dissected, gut-
ted and preserved in 4% formalin for subsequent analysis.
Each stomach contents were emptied into a Petri-dish and
observed under a binocular microscope. Individual
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory
Figure 1. Map of Cross River State showing study area.
food items were identified to the lowest taxonomic level
and the entire content analysed using frequency of occur-
rence [26].
2.5. Fecundity, Gonadosomatic Index, Egg Size
Analysis of fecundity was limited to the peak spawning
period (May-July) and only ripe female fish (494) were
used for the estimation. Ovaries were excised from body
cavity of each fish and preserved in Gilson fluid [28].
Only the largest eggs (0.5 - 3.0 mm) in each sample were
used for fecundity estimation. Fecundity was calculated
by multiplying the total weight of eggs by the number of
eggs per gram weight [29].
Gonad cycle was determined from gonado - somatic
index (IG) expressed according to De Vlaming et al. [30]
IBody weight
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory985
IG was used to follow seasonal changes in the gonads.
Egg diameter was measured from samples collected
from different parts of the ovary (anterior, middle and
posterior parts) using ocular micrometer mounted on a
binocular microscope Imevbore [31].
2.6. Data treatment and analysis
The mean and standard deviation of each of the phys-
ico-chemical parameters were calculated. Analysis of
variance (ANOVA) was used to test for statistical differ-
ences between the means of the physical and chemical
parameters of the sampling years. Presence – absence data
(quantitative scale issue) was used as a measure of com-
munity composition. Based on the information obtained,
fish species were qualitatively categorized as follows:
Disappeared: Species not sampled in all subsequent
studies after first appearance.
Declined: Species that are either absent or present in
relatively low numbers or observed in commercial
catches but not encountered in later years.
Appeared: Species not present in all pre-impoundment
and post-impoundment studies except after establishment
of wood processing industry.
Valuable: Species present in the river and contributing
at least 10% to sampled fish in terms of weight and
Permanent: Species present in all pre-impoundment
and post-impoundment studies conducted on the river.
3. Results
3.1. Fish Species Composition and Status
Table 1 presents all fish species identified in both
pre-industry (2000) and post-industry (2001-2006) estab-
lishment studies undertaken on the Cross river. Important
freshwater species categorised as ‘disappeared’ include
Sarotherodon melanopleura, H.fasciatus, (Cichlidae);
Mormyrus rume, Mormyrus tapirus, M. anguilloides, P.
bovei, G. cyprinoides, G. senegalensis, H. occidentalis, M.
isidori (Mormyridae); Hydrocyanus vittatus, Bricynus
macrolepidotus, Bricynus chaperi (Characidae); Polyp-
terus senegalensis, Polypterus endlicheri, Cala-
moichthys calabaricus (Polypteridae) (Table 2). while
three species namely Synodontis violaceus (Mochokidae);
Chrysichthys maurus (Bagridae) and Tilapia monody
(Cichlidae) appeared in the Cross River for the first time.
These species are grouped as ‘appeared’ in this study.
Table 1. List of species identified in Cross River during the 13 years of study.
Family/species 1997 2000 2003 2006 2010
Oreochromis niloticus *** *** * * **
Tilapia galilaeus ** ** * * **
Tilapia mariae * * * * *
Tilapia zilli ** ** * * *
Tilapia monody * *
Sarotherodon galilaeus ** ** * * *
Sarotherodon melanopleura * * *
Sarotherodon melanotheron ** * * * *
Hemichromis fasciatus * * *
Hemichromis bimaculatus * * * *
Protopterus annectens ** ** *
Polypterus senegalus * *
Polypterus endlicheri * *
Calamoichthys calabaricus ** **
Denticeps clupeoides ** * *
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Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory
Distichodus rostratus * * * * *
Cynothrissa sp ** ** * * *
Pellonula vorax * **
Heterotis niloticus ** ** * * **
Mormyrus rume ** ** * **
Mormyrus deliciosus ** ** * * *
Mormyrus tapirus * * * *
Mormyrops anguilloides * *
Petrocephalus bovei ** ** **
Petrocephalus. Ansorgii ** ** * * *
Gnathonemus cyprinoides * * *
Gnathonemus senegalensis *
Hyperopisus occidentalis * * *
Marcusenius isidori * *
Marcusenius psittacus *
Hepsetus odoe * ** *
Hydrocyanus vittatus * * *
Bricynus nurse ** ** * * *
Brycinus chaperi ** *
Bricynus macrolepidotus * ** * *
Clarias anguillaris *** *** ** ** **
Clarias cameronensis ** ** ** ** **
Clarias gariepinus ** ** ** ** **
Clarias pachynema * *
Clarias aboinensis * * * * *
Heterobrachus longifilis ** ** * * *
Heterobranchus bidorsalis * * * * *
Bagrus docmak * * * *
Bagrus bayad ** ** * * *
Chrysichthys auratus *** *** * * **
Chrysichthys nigrodigitatus *** *** * * **
Chrysichthys walker * * * * *
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Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory987
Chrysichthys furcatus * * * * *
Chrysichthys maurus * * *
Chrysichthys filamentosus ** ** *
Synodontis membranaceus *** ** *
Synodontis omias * * *
Synodontis rabbianus * * * *
Synodontis nigrita * * * * *
Synodontis schall *** *** **
Synodontis obesus ** ** * * *
Synodontis courteti * * *
Synodontis eupterus * * *
Synodontis gambiensis *
Synodontis ocellifer * *
Synodontis velifer * *
Synodontis sorex * * *
Synodontis violaceus * * *
Malapterus electricus * * * * *
Barbus occidentalis ** ** * * *
Barilius senegalensis * * *
Barbus macrops *
Barilius loati * *
Labeo coubie * * * * *
Labeo senegalensis * * * * *
Labeo parvus * * * * *
Eutropius niloticus ** ** * * *
Eutropius micropogon ** ** * * *
Schilbe mystus ** * * * *
Schilbe intermedius * * *
Parachanna obscura * * * * *
Lates niloticus * * * *
Phago loricatus * *
*: Scarce, **: Common, ***: Abundant.
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Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory
Copyright © 2011 SciRes. JEP
Table 2. Qualitative description of current status of fish species identified in Cross River during period of study.
Disappeared Declined Appeared Permanent Valuable
S. melanopleura
P. annectens
P. senegalensis
P. endlicheri
C. calabaricus
D. clupeoides
P. vorax
M. rume
M. tapirus
M. anguilloides
P. bovei
G. cyprinoides
G. senegalensis
H. occidentalis
M. isidori
H. odoe
H. vittatus
B. chaperi
B. macrolepidotus
Clarias pachynema
C. Filamentosus
S. omias
S. rabbianus
S. schall
S. courteti
S. eupterus
S. ocellifer
S. velifer
S. sorex
B. senegalensis
B. macrops
B. loati
S. intermedius
P. loricatus
T. galilaeus
T. melanotheron
Cynothrissa sp
H. niloticus
M. deliciosus
P. Ansorgii
B. nurse
C. anguillaris
H. longifilis
B. bayad
L. niloticus
B. occidentalis
E. niloticus
E. micropogon
S. mystus
S. violaceus
C. maurus
T. monody
P. obscura
L. coubie
L. senegalensis
L. parvus
M. electricus
S. nigrita
C. furcatus
C. walker
C. aboinensis
C. gariepinus
C. cameronensis
H. bidorsalis
D. rostratus
T. mariae
T. zilli
S. galileus
H. bimaculatus
O. niloticus
C. anguillaris
C. nigrodigitatus
C. auratus
Species grouped as ‘declined’ include Tilapia galilaeus,
Tilapia melanotheron (Cichlidae); Cynothrissa sp (Clu-
peidae); H. niloticus (Osteoglossidae), Mormyrus delici-
osus, Petrocephalus ansorgii (Mormyridae); Bricynus
nurse (Characidae); Heterobranchus longifilis (Clariidae)
Bagrus bayad, Synodontis obesus (Mochokidae); Barbus
occidentalis (Cyprinidae); Eutropius niloticus, Eutropius
macropogon (schilbeidae). 5 fish species which have
been grouped as ‘valuable’ in the Cross River are Oreo-
chromis niloticus (Cichlidae), Clarias anguillaris (Clarii-
dae), C. auratus, C. nigrodigitatus (Bagridae); and Syno-
dontis membranaceus (Mochokidae). Chysichthys furca-
tus, Chrysichthys walker (Bagridae); Clarias aboinensis,
Clarias gariepinuss (Clariidae); Tilapia mariae (Cichlidae)
were grouped as permanent.
3.2. Changes in the Length-Weight Relationship
and Condition Factor
Before establishment of wood processing industry dis-
tribution of values of allometry coefficient (b) ranged
between 1.2 and 3.65 and mean b-value for all the species
was 2.64 ± 0.32 and the mode; 3.5 (Figure 2). These val-
ues diminished considerably after six years to a range
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory989
from 1.2 - 2.2, mean value of 1.82 ± 0.23 and mode of 2.0.
The distribution of b values after six years deviate sig-
nificantly from the cube value (b = 3, P > 0.05). The con-
dition factor of the fish sampled varied from 0.53 to 1.30
with mean value of 0.772 ± 0.12, mode; 1.20 and median;
0.72 to a range between 0.22 and 0.62 with mean 0.45 ±
0.06, mode; 0.55 and median 0.40 (Figure 3).
3.3. Changes in Food Habit Groups of Fishes
Analysis of frequency of occurrence of food objects in
the different fish families showed that intake of phyto-
plankton, fish fry, insects, crustaceans and mollusk de-
clined six years after the establishment of the industry
(Table 3). Ingestion of detritus, seeds, fish parts and
macrophytes increased considerably.
3.4. Distribution of Major Food Items in the
Study Area
Distribution of four selected fish species and their prey
indicated that these fish were predominantly located in a
few specific areas six years after the establishment of the
industry (Table 4). Nymphaea and spirogyra are the
main diet of O. niloticus in the pre-industry era (2000)
and it was found that these food items were distributed in
reaches II and I respectively with a percentage occur-
rence of between 0.1% - 10.0% for spirogyra and 0.1% -
20.0% for nymphaea. Meanwhile, for H. niloticus, Reach
I and II had the highest abundance of spirogyra and
nymphaea respectively with the percent abundance of 0.1
- 20.0% for spyrogyra and 0.1% - 10.0% for Nymphaea.
Pellonula sp and detritus were the main diet of C. garie-
pinus and the abundance of this diet was higher in Reach
I and III respectively. C. nigrodigitatus consumed mainly
Macrobrachium sp and Nertina sp distributed predomi-
nantly in Reach II and III respectively. However, nine
years after, major food items in the diet of all the main
fish species were detritus and macrophytes particularly in
Reach I.
Figure 2. Changes in the frequency of the mean growth coefficient of the fish community.
Figure 3. Changes in the frequency of the mean condition factor of the fish community during six years.
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory
Table 3. Changes in the food habits in the different fish families.
Feeding habits during pre and post establishment of wood industry
Fish families 1997 2000 2003 2006 2010
Cichlidae Phytoplankton 43% Phytoplankton 15%
Macrophytes 28%
Phytoplankton 5%
Macrophytes 34%
Macrophytes 33%
Detritus 21%
Phytoplankton 29%
Macrophytes 8%
Cyprinidae Phytoplankton 24% Phytoplankton 10%
Seeds 34%
Seeds 37%
Detritus 40%
Seeds 23%
Detritus 44%
Phytoplankton 17%
Detritus 32%
Bagridae Mollusk 33%
Crustacea 25%
Fish parts 38%
Mollusk 15%
Crustacean 8%
Fish parts 40%
Detritus 32%
Fish parts 34%
Detritus 44%
Molluscs 10%
Fish fry 39%
Crustacean 18%
Clariidae Fish fry 34% Fish fry 10%
Fishparts 29%
Fish parts 39%
Macrophytes 21%
Detritus 23%
Fish parts 31%
Macrophytes 33%
Worms 10%
Fish fry 28%
Detritus 18%
Schilbeidae Detritus 41% Detritus 51% Detritus 45% Detritus 40% Detritus 34%
Insects 24%
Fish fry 12%
Mormyridae Detritus 22% Detritus 32% Detritus 44% Detritus 39%
Seeds 23%
Detritus 24%
Worms 18%
Clupeidae Insects 20% Insects 6%
Worms 21%
Detritus 23%
Detritus 34% Insects 28%
Fish fry 20%
Osteoglossidae Insects 15%
Macrophytes 13%
Insects 8%
Macrophytes 24%
Seeds 40%
Macrophytes 21%
Seeds 32%
Macrophytes 28%
Seeds 12%
Insects 27%
Macrophytes 23%
Distichodontidae Insects 12% 1nsects 5%
Detritus 24%
Detritus 32%
Detritus 42% Detritus 12%
Insects 18%
Mokochidae Insects 10% Insects 3%
Detritus 23%
Detritus 25% Detritus 31% Insects 25%
Table 4. Distribution of dominant preys with reaches (in parenthesis). Or: Oreochromis niloticus, Cl: Clarias gariepinus, Ch:
Chrysichthys nigrodigitatus, He: Heterotis niloticus, La: Labeo coubie, Br: Bricynus sp.
Fish species Or Cl Ch He La Br
No of fish 220 190 201 118 136 343
Empty stomach 0 0 12 171
Food items
0.1 - 20.0(III)
0.1 - 10.0(II)
Dipteran adult
0.1 - 20.0 0.1 - 20.0(I)
0.1 - 20.0(I)
0.1 - 10.0(III)
Nertina sp
0.1 - 20.0(III)
Fish (prey)
Pellonula sp
0.1 - 20.0(I)
0.1 - 20.0(I)
Nymphaea 0.1 - 20.0(II)
0.1 - 20.0(I)
0.0 - 20.0(I)
0.1 - 20.0(I)
0.1 - 10.0(II)
Spirogyra 0.1 - 10.0(I)
0.1 - 10.0(II)
Detritus 0.1 - 20.0(I)
0.1 - 10.0(I)
0.1 - 20.0(I - III)
0.1 - 20.0(I)
3.5. Fecundity, Egg Size and Gonadosomatic
Distribution of fecundity values varied from 56012 ±
5234 eggs, mode 12500 and median 58345 in 2000 to
mean value 23122 ± 232 eggs, mode 2500 and median
20349 in 2006 (Figure 4), gonadosomatic index from 20.5
± 3.2 eggs, mode 19.1 ± 2.2 and median 21.4 to values of
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory991
12.4 ± 2.3, mode 4.5 and median 9.5 (Figure 5) and egg
size from mean value; 1.82 mm ± 0.07 mm, mode 2.2, and
median; 1.8 to values of 0.8 mm ± 0.04 mm, mode; 1.3
and median 1.1 (Figure 6) and respectively.
Figure 4. Changes in the frequency distr i bution of fec undity of the fish species during the six years.
Figure 5. Changes in the frequency distribution of the gonadosomatic index of fish species during the six years.
Egg size (mm)
Figure 6. Changes in the frequency distribution of egg sizes of fish species during the six years.
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory
4. Discussion
4.1. Changes in Fish Species Composition and
The list of fishes shown in this study is the current fish
assemblage in the Cross River as a result of restructuring
of the communities that previously occupied the Cross
River and its floodplains by effluent from an associated
wood industry and the recovery effect after closing down
the industry by law. Disappearance of some components
of the original fish community and alterations in the
abundance of some species has resulted in the current
fish assemblage. It has been reported [32] that wood fac-
tory waste are toxic when they cover the bottoms of wa-
ter bodies, they decrease the amount of pH and dissolved
oxygen. The lower pH range could be as a result of high
acid content of organic effluents from the wood process-
ing factory located upriver. This could lead to the disin-
tegration of the river ecosystem resulting in the decline
or disappearance of those species whose life cycles have
been disrupted by these changes. The fish community of
the Cross River appears to have been gradually trans-
formed from those adapted to unpolluted river conditions
to those adapted to polluted conditions. It was shown that
the disintegration of river ecosystem has led to a reduc-
tion in the number of fish species in the world’s water-
sheds [33].
Species which were predominant one year before es-
tablishment of wood processing industry appear to have
been affected by changes in the physical and chemical
conditions of the water following effluents and this had
led to species either disappearing or becoming reduced
considerably in terms of numbers. The once abundant but
now scarce fish species give indication of changes in fish
populations and relative abundance in Cross River. This
observation could result from the fact that these species
experienced the greatest natural mortality during the
early years of establishing the wood processing industry
because most of them were at top level of the food chain
resulting in a faster decline in their abundance.
In the different years fish species sampled were 72
(1997), 70 (2000), 46 (2003), 36 (2006) and 62 (2010)
indicating that the fish species composition of the Cross
River was undergoing changes. This observation could
be attributed to the disappearance of fish species from the
river due to disintegration of the river ecosystem and
alteration of living conditions after establishment of in-
dustry. Fish species whose life cycles have been particu-
larly disrupted by the establishment of industry are un-
able to adapt to new conditions and may disappear from
the community [34]. The disappearance of fish species
from the Cross River within the short time still raises
conservation concerns, especially when as many as thirty
six even species were involved.
4.2. Changes in Relative Abundance of Fish
Many fish species adapt to environmental changes in
water bodies to varying degrees and continue to exist at
changed abundance [35]. Whereas some species have
maintained their populations at about the same level
when compared with the pre-industry period, others have
declined while others have increased in their numbers as
a result of favourable environmental changes. Changes in
relative abundance are underpinned by alteration of the
existing ecological and biophysical processes after estab-
lishment of industry such as the obvious reduction in the
population of invertebrates and the invasion of aquatic
weeds upriver close to effluent point. The industrial ef-
fluents created new conditions that were favourable to
herbivorous fish species and detritus species which re-
sponded with increase in numbers of individuals and
total biomass [35-38]. Some of these fish species include
Labeo coubie, Clarias aboinensis, Clarias cameronensis,
Tilapia mariae, Distodontus rostratus, Heterobranchus
bidorsalis and Malapterurus electricus. The conditions
created in the Cross river by the industrial effluents ap-
peared to have been unfavourable to some fish species
such as T melonotheron, H niloticus, Cynotrissa sp, M
deliciosus, P ansorgii, B. occidentalis and B. Bayad,
Lates niloticus, Labeo spp and Bagrus spp. resulting in
their decline. The effluents could have contaminated
food supply and their spawning grounds. On the other
hand, Cichlidae (Oreochromis niloticus) Bagridae
(mainly (Chrysichthys nigrodigitatus and C. auratus),
Clariidae (Clarias anguillaris) and Mochokidae (S.
Schall and S. membranaceus) have currently emerged as
most important. Notwithstanding the general decline in
relative abundance of Bagrids, however, Chrysichthys
nigrodigitatus has continued to remain most abundant
species in terms of number even after the period of exis-
tence of the wood processing industry. The reason for the
extreme proliferation of this species is not clear despite a
common phenomenon in tropical water bodies that no
one species dominates the fishery for a long time hence it
is expected that over time other species apart from Chry-
sichthys nigrodigitatus are suppose to dominate the Cross
River fish community. In terms of numbers, Cichlids
have declined slightly in relative abundance compared
with pre-industry figures. Of the Cichlids in the Cross
River, Tilapia melanotheron was not doing well in rela-
tive abundance compared with T. mariae over the period
of existence of the industry, a situation which is similar
to what pertained in the Weija Reservoir after 28 years of
impoundment [39].
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Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory993
4.3. Changes in the Length-Weight Relationship
and Condition Factor
Seventy six percent of the species presented in this study
exhibited a trend of isometric growth (b = 3) depicting
dimensional equality before the establishment of wood
processing industry [24]. This trend contrast greatly from
the species during the existence of the industry where the
pattern indicated an acute negative allometry. This may
be attributed to the change in ecological parameters at the
freshwater environment after establishment of the indus-
try in which these species have carved their ideal niche.
The riverine environment is characterized mainly by high
oxygen content, low salinity, high nutrient content and
higher productivity in contrast to the contaminated
post-industry condition [40]. To counter the scarcity of
nutritional resources at post-industry establishment so-
matic growth is less important and energy is diverted to
reproductive processes [41]. The general trend of nega-
tive allometry exhibited by some most fish species after
establishment of industry compared to the isometry ob-
served in most of the species in the study area before
existence of industry may be regarded as floodplain ad-
aptations to survive in the polluted river.
The fact that 65% of the entire 78 species examined
had condition factor above mean and that the overall
mean condition factor did not significantly deviate from
the value of 1.0 showed that the majority of the fish in
the populations of Cross River inland wetlands were in
good condition before the establishment of wood proc-
essing industry, thus justifying the dimensional equality
of their growth pattern. The high condition factor of the
fish species in the river pre-industry era is an indication
of abundant food. Low condition factor showed during
existence of industry indicates that food resources had
4.4. Changes in Food Habit Groups of Fishes
Though herbivores and detritivores were the least impor-
tant in terms of weight and numbers in pre industry es-
tablishment studies undertaken in the Cross River, they
have currently increased significantly in numbers and
weight becoming the most important in terms of numbers.
From reports of studies undertaken after establishment of
industry, benthic carnivores have declined in their im-
portance probably due to contamination by effluents that
have reduced food resources and spawning grounds for
benthic and pelagic invertebrates and fish preys. Further
downstream in Reaches II and III, however, benthic om-
nivores have continued in their importance and have
maintained their dominance in the Cross River. The om-
nivores together (i.e. both semi pelagic and benthic om-
nivores) have, however, become most important in the
Cross River after the existence of the industry.
Importance of herbivores after six years of the exis-
tence of the industry has increased considerably com-
pared with the period before the establishment of indus-
try, possibly due to declining food sources.
The composition of piscivores after six years of estab-
lishment of wood industry declined compared with one
year pre industry existence. The general decline in the
importance of piscivores in the Cross River during the
period of existence of industry could be attributed partly
to the disappearance of some piscivorous fishes as well
as their restricted distribution due to decline of fish preys
and their preference for unpolluted river conditions
which have reduced over the period of existence of the
industry. Clarias gariepinus has maintained its impor-
tance as a “fish-eating fish” in the Cross River during the
period of existence of wood industry whereas
Hemichromis fasciatus has disappeared and Clarias an-
guillaris has declined in abundance. The wider food
spectrum exhibited by C. anguillaris , C. nigrodigitatus
and O. niloticus revealed trophic flexibility [42]. The
ecological advantage of this is that it enables a fish to
switch from one category of food to another in response
to fluctuation in their abundance. Another advantage is
the ability of the species to utilize many different food
objects effectively and this probably accounts for their
higher abundance in the study area during pre-industry
period [43,44].
The fact that detritus dominated the gut contents of the
freshwater species in the study area during the existence
of the industry implied that most of the fish species in the
Cross River inland wetlands are detritivores during this
4.5. Fecundity, Gonadosomatic Index and Egg
Disparity was noted between fecundity of fish in this
study area before establishment of industry and that of
fish populations six years after the existence of the in-
dustry. The higher values of fecundity among popula-
tions before establishment of industry can be attributed to
the greater abundance of food in the river pre-industry
period. Pre-industry fish species of the Cross River were
therefore more superior than post-industry and can be
better broodstocks in the farms. Such related differences
in fecundity had been observed [45-48] and were attrib-
uted to environmental factors such as differential abun-
dance of food and water quality.
Fecundity may be reduced if individual fish mature at
a smaller size, and individuals in poorer condition may
experience increased mortality during environmentally
stressful period [49]. Small egg size implies that survival
Copyright © 2011 SciRes. JEP
Downstream Changes on a Tropical Fish Community Structure by Effluent from Wood Processing Factory
rate of the progeny will be low [29]. Therefore, the larger
egg sizes of fish species during pre-industry period im-
plied that pre-industry Cross River provided more suit-
able habitat for the species.
The low GSI, fecundity and variation in the egg sizes
observed for species during post-industry period may be
attributed to the sudden change in the environmental
factors due to effluents discharged from the wood proc-
essing industry. Similar observations have been reported
[50]. A change in the environmental factors results in
significant changes in the egg size [51]. The implication
of these results is that the reproductive potential of fish
species in Cross River is greatly affected by the envi-
ronmental factors and human activities.
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