Length-Weight Relationship and Condition Factor of Two Species of Tilapia and One Species of Mormyrops from a Tropical Dam in a Southwestern State, Nigeria

Introduction

Fish are aquatic organisms consist of gill bearing cranium but lack limbs with digits, they are cold-blooded animals. They are of great importance for the Nigeria aquaculture industry and its production is gradually increasing. Fish like other hunting activities has been a major source of food for human race and has put an end to the unsavoury outbreak of anaemia, kwashiorkor etc. Cichlidae genera include Oreochromis, Sarotherodon and Tilapia. They add values to the life of fish farmers and commercial fishery of inland waters of most countries of the world. They are tolerance to a variety of habitats and omnivore nature of their feeding habit confers them an Journal of Ecology & Natural Resources

important element of fish farming which demands mode of their life and ecological requirements [1]. Mormyrops species are well distributed in swamps, lakes and rivers of most Nigerian fresh water bodies. Mormyridae include Hyperopisus, Mormyrus, Petrocephalus, Mormyrops, Marcusenius and Gnathonemus. The family Mormyridae is highly variable in shape of their head. Their common characteristics include upward pointing of pectoral fin, narrow gill openings and their eyes are small and weak and covered by a thin layer of skin. This species has always been consumed for their oily and tasty flesh according to Achionye-Nzeh, Babatunde and Aminu, Nzeh and Lawal [2, 3, 4]. FAO reported that Nigeria is one of the largest importers of fish in the developing world, importing about 600,000 metric tonnes annually [5]. To solve this short-fall in fish supply, Nigeria must be fully involved in proper aquacultural management. Scientifically sound management of fish resources relies on the basic knowledge on the biology of the species, including information on population structure, such information influences the development of the management strategies and strategies for conserving biodiversity [6]. Morphological characters such as morphometrics and meristics have been commonly used to identify stocks of fish [7, 8]. Virtually, nothing is known about the morphological population structure of Cichlidae and Mormyridae species from Itapaji dam, Ekiti State, Nigeria. It is vitally important to obtain detailed knowledge on the population structure of these commercially exploited species and to apply knowledge to the management of the fisheries in the dam. This research work was therefore designed to compare the length-weight relationship and condition factor of Oreochromis niloticus, Tilapia zillii and Mormyrops anguiloides collected from Itapaji Dam, Ekiti State, Nigeria.

Results

A total number of one hundred and fifty (150) Tilapia species consisted of one hundred and thirteen of Tilapia zillii (65 females and 48males) and twenty-four of Oreochromis niloticus (11 females and 13 males) and Mormyrops anguiloides (11 females and 2 males) were collected for this work. Table1 shows the ranges and mean values of the total length and body weight of famales and males of the three species of fish collected from Itapaji water reservoir, Ekiti State Nigeria. Among the males, Mormyrops anguiloides had highest mean length with 30.05cm while the T. zillii, and O. niloticus had 16.59 and 16.68 respectively. The results on length among females followed the same format of the results on males. Both males and females of M. anguiloides outweighed others. The males had the mean weight of 172.0g while the female had 145.12g. The males and females of O. niloticus followed with 84.7 and 85.11g respectively.

Table 1: The ranges and mean values of the total length and body weight of the three fish species collected from Itapaji Dam, Ekiti State, Nigeria. Table2 shows the condition factor and length-weight relationship of the males and females of the three species of fish sampled from Itapaji dam. The mean condition factor ranged from 0.75 in the females of M. anguiloides to 1.75 in the females of T. zillii. The length- weight relationship of the females and males shows that the ‘b’ value ranged from -1.16 in the males of O. niloticus to 10.12 in the males of M. anguiloides. The correlation coefficient “r” ranged from 0.09 in the males of O. niloticus to 0.64 in the females of M. anguiloides.

Discussion

Among the three species of fish sampled for this work, Mormyrops anguiloides had the least sample size. The possible reason for the low number of the species may be due to over exploitation by the local fish farmers thereby makes the species a threatened one. The results further showed that the Tilapia zillii was a dominant species in the reservoir which was followed closely by O. niloticus during the collection periods. The result of length- weight regression analysis in this work showed that both the males and females fish exhibited allometric growth. The values of “b” showed that both males and females of T. zillii and .O.niloticus exhibited negative allometric growth while the female and male of M. anguilodes exhibited positive allometric growth pattern. This means that T. zillii and O. niloticus become thinner or slender with increase in their length as it was reported by King, and Riedel, et al. while M. anguiloides becomes stocky or robust as its length increases [11, 12]. The results obtained in this work were similar to what was documented on fishes of some Nigerian inland waterbodies. Notable among these Authors are Sangu et al. and Offem, et al. [13, 14]. They reported 35.9% and 24% of their samples respectively exhibited negative allometrics. Dan-Kishiya (2013) reported that all the five fish species collected from a water supply reservoir in Abuja had negative allometric growth while Oso and Iwalaye reported three out of their four collections from Ero dam in Ekiti State, Nigeria had negative allometric [15, 16] . The number of sample of Mormyrops species collected for this work was very low, in contrast to the result of Offem et al. in which the species was one of the dominant species collected from the flood plain river of Calabar, Nigeria[14]. The possible Journal of Ecology & Natural Resources

reason for the number available may be due to over exploitation of the samples by the fish farmers. The b values reported for T. zillii and O. niloticus males and females in this research work had were 1.81, 0.91 and -1.16, 1.14 respectively. The values were similar to 1.4 and 2.3 reported on Wasai river, Kano samples by Imam et al. but in variance to the range of 4.73 to 15.59 recorded for T.zillii, Sarotherodon melanotheron, T. guinesis,T. mariae and O. niloticus from Badagry creek, Lagos by Akintade, et al. [17, 18]. The ‘b’ values of females and males of M. anguiloides 8.20 and 10.12 showed that they exhibited positive allometry growth. The results were similar to 6.82 reported for Bathygobius soporator by Akintade et al. 18]. The reasons for differences in the fish growth patterns, may be due to availability of food, the season of the collection, poor environmental conditions, competition along the food chains. The effective management of fishery resources requires considerable knowledge of population parameters such as length-weight relationship. The relationship is essentially important in fishery biology for it allows estimation of average weight of fish from a given length grou, assess the well-being of individuals and to determine the possible differences between separate unit stocks of the same species [19, 20]. This relationship is also important in fisheries management for comparative growth studies [21]. The length- weight relationship provides valuable information on the habitat where the fish lives, and also in aquatic ecosystems modelling [22, 23]. Condition factor (K) is described as a morphometric index needed to evaluate physiological status of fish based on the fact that those individuals of a given length which have a higher mass are in a better condition. The relationship of length –weight can be used in the estimation of condition factor (K) of fish species. The condition factor obtained in this study ranged from 0.75 in the female M. anguiloides to 2.25 in the female O. niloticus which were varied from what some Authors reported, for example Oso and Iwalaye, reported 0.99455 to 4.3457 for four fish species from Ero dam, Ekiti State, Ajani reported range of 0.45 to 2.25 for five tropical fish of a coastal lake in Lagos Nigeria, Nwadiaro and Okorie reported the condition factor ranged from 0.49 to 1.48 in Oguta lake, Kumolu-Johnson and Ndimele reported the K value of the range of 0.91 to 8.46 for twenty-one fish species from Ologe lagoon, Lagos, while Abowei and Hart also reported K value of 1.10 for Cyloglossus senegalensis in Nikoro river. Ajani and Wale reported that “K” value less than 1 implied that the fish are not in good condition within their habitat while values greater than 1 implied that the fish are in good condition of physiological state within their habitat [24, 25, 26, 27, 28]. The “K” values obtained in this study were greater than 1except in the M. anguiloides on which 0.75 was reported. Based on our findings, it could be said that all the fish samples are in good condition of well-being except the female of M. anguiloides in Itapaji dam, Nigeria. It was observed that the ‘K’-values obtained in this work were lower than the range of 2.9 to 4.8 recommended as suitable for matured freshwater fish by Bagenal and Tesch [29]. The variation in values may be attributed to variations in variations in weight and stage of maturity. Other factors which might have contributed to the variations may include stress, season, availability of feeds, mutagens from human interference, and other water quality parameters.

Conclusion

The results obtained on the T. zillii and O, niloticus showed that they exhibited negative allometric growth while the M. anguiloides exhibited positive allometric growth. Also, the K values obtained revealed that the fish samples were in good condition of well-being except the females of M. anguiloides. Results also showed that the population of M, anguiloides is being threatened. Various factors may have contributed to variations recorded among the fish species hence; there may be need for proper examination of the water quality parameters and other factors affecting the Itapaji dam to establish the suitability of the reservoir for fish breeding.