27 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ Some Biological Characteristics of Ten Fish Species in a Tropical Man-made Lake, Taabo Reservoir, West Africa. N’guessan Gustave ALIKO a *, N’guessan Emmanuel ASSEMIAN b , Koffi Charles BOUSSOU c , Koffi Félix KONAN d a,b,c,d Laboratory of Environment and Aquatic Biology, Environment Departement, Jean Lorougnon Guédé University, BP 150 Daloa, Côte d’Ivoire. a Email: gustavealiko@yahoo.fr b Email: assemanuel@yahoo.fr c Email: bkofficharles@live.fr d Email: konanfelix@yahoo.fr Abstract We studied some biological aspects of ten fish species from the Taabo Lake in Ivory Coast (West Africa). These fishes were the most targeted by the artisanal fisheries with about 77% of the captures. Fish populations were sampled during five surveys. Fish size, sex ratio, length-weight relationships and condition factor were analyzed. Standard lengths indicated a wide range for the ten species and the highest sizes recorded in this study were lower than those observed in other aquatic systems of West Africa. Sex ratios differed between species and showed significant variation from the expected ratio of 1:1 for seven species. Males were more abundant in four species (Distichodus rostratus, Tilapia zillii, Pellonula leonensis and Schilbe mandibularis) whereas females predominated in three species (Chrysichthys nigrodigitatus, Synodontis schall and S. punctifer). The values of the parameter “b” in length-weight relationships ranged from 2.702 to 3.267 and the ten fish species presented an allometric growth. The condition factor showed a wide range for all the studied species. Keywords: Size; Sex ratio; Length-weight relationships; Condition factor; Freshwater fishes; Taabo Reservoir; West Africa ------------------------------------------------------------------------ * Corresponding author. http://asrjetsjournal.org/ mailto:assemanuel@yahoo.fr American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 28 1. Introduction Taabo Lake was built on Bandama River (one of the four main rivers of Ivory Coast) in 1979. Fishing activities are developed in this lake since its inception. Catches had dropped significantly from 433 tons in 1979 to 70 tons in 2006. The known fish fauna of this lake is 50 species but fisheries based on 10 major species which account for about 77% of the total catch. These species were: Chrysichthys nigrodigitatus, Distichodus rostratus, Oreochromis niloticus, Tilapia zillii, Labeo coubie, Pellonula leonensis, Heterotis niloticus, Synodontis schall, Synodontis punctifer and Schilbe mandibularis [1]. Fishing affects fish communities through changes in total biomass, species composition and size structures [2, 3, 4, 5]. The main direct effect of fisheries is obviously to reduce the abundance of target species. It affects mainly the larger species that become rare in the catches and landings [6, 7]. High fishing mortalities among large fish can have negative consequences on the recovery of stocks. The response of species to the effects of fishing depends on their biological characteristics [8]. Moreover, it is well known that habitat change induced by the creation of the lake can affect the biology and ecology of fish species [9]. The aim of the present work is to provide informations dealing with some biological characteristics of the ten species mentioned above in Taabo Reservoir. These characteristics include: size range, sex ratio, length-weight relationships and condition factor. The results of the study permit to fill a gap on these species biology and can also be used as inputs for fisheries management in this reservoir. 2. Material and methods 2.1. Study area The study was carried out on Taabo hydroelectric reservoir (06°20’N-06°40’N and 5°W-5°30’W). Taabo Lake has a main channel total length of 16 km and an area of 69 km2 at the average operating level of the dam (124 m). At this level, average water volume is around 625 106 m3. It encompasses a catchments area about 58700 km2 and a mean annual flow of about 128.7 m3 s-1 [10]. Macrophytes were very abundant in this artificial lake (Figure 1). 2.2. Data collection Fish populations were sampled during five surveys: July, October and December in 2006, and March and June in 2007. Data were collected through experimental fishing using gill nets which were usually set during the afternoon at about 16:00 h and lifted the following morning at about 07:00 h. Fish specimens were identified according to [11]. Five sampling stations were retained in Taabo Reservoir: one site in the riverine zone, another site in the transitional zone and three sites in the lacustrine zone. More sampling sites were retained in the lacustrine zone because of its relative extensive area. For all individuals caught, the standard length (L) was measured to the nearest millimeter and the total weight (W) was recorded to the nearest gram. Sexes of fishes were separated by examining the gonads. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 29 Figure 1: Map of Taabo Reservoir. 2.3. Data analysis For each species, the size range and the average of standard length were determined. The highest standard length observed in this study was compared to the highest standard length recorded in other hydrosystems of West Africa. Sex ratio of each fish population was determined as number of females for one male. We used the Chi- square test to assess if the sex ratio (male:female) differed significantly from the expected 1:1. The length-weight relationship was established for each species following [12] formula: W = a*L b (1) Parameters “a” and “b” were obtained by linearization of the previous equation through logarithmic transformation as following: logW = loga+ b*logL (2) Where: W = the weight of fish in gram (g), L = the standard length of fish in centimeter (cm), a = exponent describing the rate of change of weight with length (= the intercept of the regression line on the Y axis), b = the slope of the regression line also designed as allometric coefficient. The linear regression was computed using the least square method. The parameter “b” of the length-weight relationship was compared with the theoretical value 3 using the Student’s ‘t’ test [13] to assess whether these species exhibit isometric (b=3) or allometric (b≠3) growth. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 30 The condition factor (CF) of fishes was estimated from the relationship: CF = 100*W/L b (3) Where: W = weight of the fish in g, L = the standard length of the fish in cm and “b” = the value obtained from the length-weight equation relationship. 3. Results 3.1. Fish size Average, minimum and maximum of standard length of the ten fish species sampled in Taabo Reservoir are given in table 1. Differences between minimum and maximum showed a wide size range of captured fishes. The highest standard lengths recorded in this study were lower than those observed in other hydrosystems of West Africa. Table 1: Size range (standard length) in ten fish species from Taabo Reservoir; N: sample size, SD: standard deviation, (*): references from [11] Standard length characteristics in Taabo Lake Highest size recorded in West Africa* Fish species N Min (mm) Max (mm) Average ± SD (mm) (Standard length in mm) Chrysichthys nigrodigitatus 537 50 385 145.46 ± 38.02 473 Distichodus rostratus 396 47 530 158.16 ± 72.88 625 Tilapia zillii 183 39 197 98.61 ± 25.42 210 Oreochromis niloticus 125 43 308 128.52 ± 64.92 395 Labeocoubie 180 91 430 193.19 ± 62.29 750 Pellonula leonensis 500 23 56 36.68 ± 5.26 86.9 Synodontis schall 144 88 170 134.94 ± 14.78 370 Synodontis punctifer 143 83 202 133.47 ± 24.10 192 Heterotis niloticus 199 89 675 169.47 ± 86.05 980 Schilbe mandibularis 222 60 270 132.70 ± 33.07 300 3.2. Sex ratio in fish populations Sex ratio differed among fish species. For seven of the ten species, the sex ratio showed significant variation from the expected ratio of 1:1 (Chi-square test, p < 0.05). Males were more abundant in four species: Distichodus rostratus, Tilapia zillii, Pellonula leonensis and Schilbe mandibularis. Whereas females predominated in three species: Chrysichthys nigrodigitatus, Synodontis schall and S. punctifer (Table 2). American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 31 Table 2: Sex ratio in populations of ten fish species from Taabo Reservoir; χ 2 : chi-square value, s: significant difference at p = 0.05, ns: no significant difference at p = 0.05 Fish species Males Females Sex ratio (male:female) χ 2 Difference with the ratio (1:1) Chrysichthys nigrodigitatus 62 152 1:2.44 37.85 s Distichodus rostratus 78 22 1:0.28 31.36 s Oreochromis niloticus 26 21 1:0.81 0.53 ns Tilapia zillii 46 15 1:0.32 15.75 s Labeo coubie 23 17 1:0.74 0.90 ns Pellonula leonensis 113 77 1:0.68 6.82 s Heterotis niloticus 5 7 1:1.41 0.33 ns Synodontis schall 15 30 1:2.00 5.00 s Synodontis punctifer 25 48 1:1.92 7.25 s Schilbe mandibularis 29 7 1:0.24 13.44 s 3.3. Length-weight relationships (LWR) The values of the exponent “b” of LWR ranged from 2.702 (Chrysichthys nigrodigitatus) to 3.267 (Pellonula leonensis). None of the studied fishes presented an isometric growth (i.e. b ≠ 3). For seven species, “b” was significantly lower than 3.0 (Student test, p < 0.05), indicating a negative allometric growth. These seven species were: Chrysichthys nigrodigitatus, Tilapia zillii, Labeo coubie, Synodontis schall, Synodontis punctifer, Heterotis niloticus and Schilbe mandibularis. In contrast, the three other species presented a positive allometric growth with an exponent “b” significantly higher than 3 (Table 3). Table 3: Parameters of the length-weight relationships for ten fish species from Taabo Reservoir; N: sample size, b: the exponent of LWR, a: the intercept of the regression line on the Y axis, R 2 : determination coefficient, A+: positive allometric growth, A-: negative allometric growth. Fish species N b a R 2 t value of the Student test Growth Chrysichthys nigrodigitatus 537 2.702 0.258 0.946 60.955 A- Distichodus rostratus 396 3.071 0.173 0.983 6.634 A+ Tilapia zillii 183 2.972 0.271 0.968 3.755 A- Oreochromis niloticus 125 3.115 0.219 0.990 6.080 A+ Labeo coubie 180 2.966 0.211 0.980 3.593 A- Pellonula leonensis 500 3.267 0.139 0.917 155.217 A+ Synodontis schall 144 2.734 0.259 0.819 56.990 A- Synodontis punctifer 143 2.867 0.221 0.930 18.109 A- Heterotis niloticus 199 2.930 0.170 0.985 10.666 A- Schilbe mandibularis 222 2.785 0.179 0.943 120.611 A- American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 32 3.4. Fish condition factor (CF) Elementary statistics on fish CF are given in table 4. The highest average of CF was recorded with Tilapia zillii (CF = 5.01 ± 0.70) while the lowest was obtained with Pellonula leonensis (CF = 1.08 ± 0.16). For both species, CF indicated a wide range. Table 4: Condition factor in ten fish species from Taabo Reservoir; N: sample size, SD: standard deviation. Fish species N Min Max Average ± SD Chrysichthys nigrodigitatus 537 1.72 8.52 4.48 ± 0.77 Distichodus rostratus 396 0.43 5.92 1.78 ± 0.32 Tilapia zillii 183 3.43 7.99 5.01 ± 0.70 Oreochromis niloticus 125 1.83 5.75 3.07 ± 0.53 Labeo coubie 180 1.64 5.08 2.80 ± 0.35 Pellonula leonensis 500 0.69 2.37 1.08 ± 0.16 Synodontis schall 144 3.16 6.67 4.52 ± 0.66 Synodontis punctifer 143 1.71 4.40 3.13 ± 0.44 Heterotis niloticus 199 0.93 2.29 1.69 ± 0.20 Schilbe mandibularis 222 1.17 4.02 1.93 ± 0.36 4. Discussions Our results revealed wide fish sizes for the studied species in Taabo Lake; and the highest standard lengths recorded were lower than those observed in other hydrosystems of West Africa. This situation could be due to overfishing or unfavorable environmental conditions in the studied lake [3, 9]. Different sex ratios were recorded in this study. According to [14], the sex ratio in fish populations can vary annually, by metabolic changes under the influence of hormones, leading to unbalanced ratios. Besides, different mortality, growth, spatial segregation, and/or behavior for males and females during the life cycle could determine a change in the sex ratio and the predominance of one sex (reviewed by [15]). Therefore, the different proportions of sexes in the sampled populations could be the result of diverse factors. Indeed, uneven distribution of males and females could be due to difference between catchability of sexes [16, 17]. Knowledge on length-weight relationships of fish helps in predicting potential yield and determination of size at capture for obtaining optimum yield, as these parameters are directly related to weight of the fish. The relationship is also useful in differentiating populations as variations occur in populations of different localities [12, 18]. According to [19] the values of the exponent ‘b’ usually remain between 2.5 and 4.0 and in majority of the cases the value was not equal to ‘3’. Our results coincide with the interval of 2.5 – 4.0 (‘b’ ranged from 2.702 to 3.267 for the studied species). The values of condition factor indicated a wide variation. In fish, the condition factor reflects, through its variations, information on the physiological state of the fish in relation to its welfare. From a nutritional point of view, higher values are due to accumulation of fat and gonadal development [12]. According to [20], the range 2.9-4.8 is the recommended condition factor as suitable for matured fresh water fish. Ours results showed that the averages of CF range from 1.08 to 5.01. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 33 5. Conclusion The ten fish species studied in the Taabo Reservoir had a wide range of standard lengths and also wide range of condition factor. For seven species, the sex ratios showed significant variation from the expected ratio of 1:1. The values of the exponent “b” of LWR ranged from 2.702 to 3.267 and the ten fish species presented an allometric growth. These results may be useful for future studies. Acknowledgments This work was supported by the International Foundation for Science (IFS). The authors are grateful to professional fishers operating on Taabo Lake for their help and cooperation. References [1] N. G. Aliko. “Peuplement ichtyologique du lac de barrage de Taboo (bassin du Bandama, Côte d’Ivoire) : diversité taxonomique, structure des populations et production exploitée.’’ PhD Thesis, Nangui Abrogoua University, Abidjan, Côte d’Ivoire, 2012. [2] D. Pauly, V. Christensen, J. Dalsgaard, R. Froese and F.Jr. Torres. “Fishing down marine food webs”. Science, vol. 279(5352), pp. 860-863, 1998. [3] D. Pauly, M. L. Palomares, R. Froese, P. Sa-a, M. Vakily, D. Preikshot and S. Wallace. “Fishing down Canadian aquatic food webs”. Canadian Journal of Fisheries and Aquatic Sciences, vol.58, pp.51-62, 2001. [4] G. Bianchi, H. Gislason, K. Graham, L. Hill, X. Jin, K. Koranteng, S. Manickchand-Heileman, I. Payà, K. Sainsbury, F. Sanchez and K. Zwanenburg. “Impact of fishing on size composition and diversity of demersal fish communities”. ICES Journal of Marine Science, vol.57, pp. 558-571, 2000. [5] C. Pet-Soede, W. L. T. Van Densen, J. S. Pet and A. M. Machiels. “Impact of Indonesian coral reef fisheries on fish community structure and the resultant catch composition.” Fisheries Research, vol.51, pp. 35- 51, 2001. [6] J. D. Stevens, R. Bonfil, N. K. Dulvy and P. A. Walker. “The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems.” ICES Journal of Marine Science, vol. 57, pp. 476-494, 2000. [7] D. Pauly, V. Christensen, S. Guénette, T. J. Pitcher, U. R. Sumaila, C. J. Walters, R. Watson and D. Zeller. “Towards sustainability in world fisheries”. Nature, vol. 418, pp. 689-695, 2002. [8] G. A. Begg and G. Marteinsdottir. “Spatial partitioning of relative fishing mortality and spawning stock biomass of Icelandic cod.” Fisheries Research, vol. 59, pp. 343-362, 2003. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2015) Volume 14, No 3, pp 27-34 34 [9] C. Lévêque and D. Paugy. ‘’Impacts des activités humaines.’’ in Les poissons des eaux continentales africaines : diversité, écologie, utilisation par l'homme. C. Lévêque and D. Paugy (eds).. Ed. Paris, IRD, 1999, pp. 365-383. [10] K. L. Kouassi, Y. A. N’go, T. Gnagne, B. Kamagaté, N. H. Mélèdje and I. Savane. “Improvement of the Assessment Method of the Main Terms of the Hydrological Balance of the Hydroelectric Dam Lakes of Côte d’Ivoire in a High Hydropluviometric Fluctuations Context: Case of Taabo Lake”. European Journal of Scientific Research, vol.19 (1), pp. 71-84, 2007. [11] D. Paugy, C. Lévêque and G. G. Teugels. Poissons d'eaux douces et saumâtres de l'Afrique de l'Ouest. Tomes 1 et 2. Paris-Turvuren: IRDMNHN-MRAC, 2003, 457 + 815p, . [12] E. D. Le Cren. “Length-weight relationships and seasonal cycle in gonad weight and condition in the perch (Perca fluviatilis).” Journal of Animal Ecology, vol. 20, pp. 201-219, 1951. [13] J. H. Zar. Biostatistical Analysis. New Jersey : Prentice–Hall, Englewood Cliffs, 1999, pp. 1-662. [14] G. V. Nikolsky. The ecology of fishes (translated by L. Birkett). London : AcademicPress, 1963, pp. 1- 352. [15] A. E. A. Vazzoler. Biologia da reprodução de peixes Teleósteos: teoria e prática. Maringá : EDUEM- SBI, 1996, pp. 1-169. [16] D. Sudarshan, T. E. Sivaprakasam, M. E. John, A. K. Bhargava, S. M. Patil and V. V. Naik. “Chartered fishing vessels operations in the EEZ.” Fishery Survey of India Publication, vol. 1, pp. 1- 29, 1991. [17] M. E. John and D. Sudarshan.. “Fishery biology of yellowfin tuna occurring in oceanic fishing in Indian Seas” in Tuna Research in India, Fishery Survey of India, D.Sudarshan and M.E.John (Eds.). Bombay, pp. 39- 61, 1993. [18] S. L. Chonder. “Length-weight relationship of mature female Labeo gonius (Hamilton) from the Keetham Reservoir.” Journal of the Inland Fisheries Society of India, vol. 4, pp. 216-217, 1972. [19] W. R. Martin. The mechanism of environmental control of body form in fishes. University of Toronto: Ontario Fisheries Research Laboratory 70, 1949, pp. 1-172. [20] T. B. Bagenal and F.W. Tesch. Methods of Assessment of Fish Production in Fresh Waters. IBP Handbook N o 3, 3 rd ed. London: Oxford Blackwell Scientific Publication, 1978, pp. 101-136.