Seasonal Variation in The Biology of Chrysichthys Auratus (Geoffroy Saint- Hillaire, 1809) in Ogun State Estuary, Nigeria

This study examined the seasonal variation in the biology of Chrysichthys auratus (Geoffroy Saint-Hillaire, 1809) in Ogun State estuary, Nigeria. Fish were collected in the wet season (May-June, 2018) and dry season (NovemberDecember, 2018) from commercial fishermen at the estuary using the bamboo trap. The fish length, weight and other biological parameters were measured and estimated using standard procedures. A total of 432 samples with length ranging between of 10 to 25.9 cm and corresponding weight ranging between 10g to 91g were examined during the two seasons. The wet and dry season showed the peak frequency at size class 14 – 14.9cm and 19 – 19.9cm respectively. The study revealed a the sex ratio (M:F) of 1:0.7 and of 1:0.47 in the wet season and in the dry season respectively. The length-weight relationship showed a negative allometric growth in the wet dry and combined seasons with a “b” value less than 3. Size at maturity were 19.5 cm, 17.30 cm and 18.5 cm for males, females and combined sex respectively in the wet season while in the dry season, all the males were immature and the females attained maturity at a length estimated to be 17.00 cm. The study showed a positive relationship between fecundity and total length of C. auratus in the wet (r2 = 0.41) and dry (r2 = 0.45) while a positive relationship was also observed between ovary weight and fecundity in the dry season (r2 = 0.88) and wet season (r2 = 0.83). Seasonal assessment of biology and distribution of C. auratus is recommended to provide requisite information for sustainable management of the species. Introduction Chrysichthys species are among other species that have been reclassified to the family Claroteidae (Paugy et al., 2003; Olaosebikan and Raji, 2004) and Chrysichthys auratus commonly known as Long fin catfish belongs to this family. C. auratus is abundant in many African waters playing a pivotal role in the ecology of fisheries particularly in Nigeria, considering the significant trophic level in occupies in aquatic ecosystem (Abdul and Adekoya, 2016). According to Kuton and Akinsanya (2016), Chrysichthys species tolerates both fresh water and brackish water conditions and they thrive abundantly along the coastal waters with littoral mangrove belts where roots and tangles of plants exist (Ezenwa, 1978). Not only is it considered as a valuable fish species amongst the indigenous and ubiquitous freshwater fish of the coastal waters of Nigeria (Akinsanya et al., 2007), it is sought after for its flavour and chemical composition (Uneke, 2018) and this have called for the considerations of its culture that have also resulted in several studies on its biology. The goal of sustainable fisheries in meeting the growing fish demand of many populace worldwide have driven studies on the biology of fisheries resources, their exploitation and the potential yield of water bodies in terms of fish production (Abdul and Omoniyi, 2011). Extensive studies have been done on diet composition and trophic ecology of fish species from different water bodies especially reservoirs, lakes and lagoons. However, fish population and structure are altered by reproduction, recruitment, growth, mortality, environmental factors and season (Adeyemi et al., 2009). These factors have also made the pattern of fish population unpredictable affecting its assemblage and exploitation. Despite the array of information (Abdul, 2015; Abohweyere and Falaye, 2008; Akombo et West African Journal of Applied Ecology, vol. 27(1), 2019: 95 107 al., 2011, 2014; Victor et al., 2013; Nwadiaro and Okorie, 2013) about fish species biology, populations dynamics and fish community structure of fish species in Nigeria, the need to monitor the changes in the biology of C. auratus will sustainably guide its exploration, sustainability and established results will also guide the development of fisheries policy options, allowing to evaluate and foresee the possible effects of seasons on the biology of aquatic resources. Thus this study, which aimed to provide information seasonal variation in the biology of Chrysichthys auratus (Geoffroy Saint – Hillaire, 1809) in Ogun State coastal estuary, Nigeria. Materials and Methods Description of the Study Area The study was carried out in Ogun State estuary in Ogun Waterside Local Government Area. The Local Government is one of the twenty local government areas of Ogun State, Nigeria (Abdul, 2009). Ogun State Estuary is situated between 004015’E – 004030’E and 06020’N – 06045’N and bounded in the east by Lekki lagoon. It covers an area of 26 km2 (Ssentengo, 1983) and it is one of the several 96 West African Journal of Applied Ecology, vol. 27(1), 2019 estuaries that empties into the Atlantic ocean through Lagos harbour. Abdul et al. (2003) reported that the lack of direct access to the Atlantic Ocean makes it essentially fresh water. The four major rivers that inundate the estuary are River Ogun, River Oshun, River Mosafejo and River Oni. Sampling duration Samples of C. auratus were collected seasonally from commercial fishermen at the estuary using the bamboo trap locally called ‘Apaye’. The trap is usually staked in a slant position in the water, as the fish seeks refuge in the trap. During the wet season (May-June, 2018), 209 samples were collected and during the dry season (November – December, 2018) 144 samples were collected. Collection and identification of fish samples The fish samples of various sizes were randomly collected immediately after capture. They were removed from the traps and kept on ice to maintain freshness. The samples were then taken to the laboratory, identified to the species level using the field guide (Olaosebikan and Raji, 2004) and sorted into sexes. The total length (TL), (a measurement Figure 1: Map of Ogun State estuary (Source: Abdul et al., 2015) Adekoya et al: Seasonal variation in the Biology of Chrysichthys auratus 97 from the tip of the snout to the extended tip of the caudal fin) was measured with a meter rule on a measuring board to the nearest 0.1 cm, while the total weight (TW) of fish was also recorded using a sensitive scale (Model: EK5350) to the nearest 0.1 g. Sex ratio The sex of the samples captured over each sampling period was determined. This was done by visual observation of the external features and examination of the various gonads. Females were recognized by the distended aperture and males were recognized by the presence of testes. The number of identifiable males and females of C. auratus for each sampling period were recorded and used to determine the sex ratio from the naturally hypothesized ratio of 1:1. Sex ratio = no of males / no of females (Akintade et. al., 2016). Growth pattern The length – weight relationship was determined from the formula; W = aLb (Pauly, 1984) Where W= Body weight in grams L= Total length of fish in centimetres a = Intercepts b= Growth exponent or regression coefficient ‘a’ and ‘b’ are constants and their values were estimated from the log transformed relationship of length-weight below. ‘b’ was used to determine the growth pattern of the fish at 95% confidence limit. Log W= log a + b log L Condition Factor The condition factor (K) was estimated using the formula below; K = 100 * W ⁄ L (Pauly, 1984) Where; K= Condition factor W= Weight of fish (g) L= Length of fish (cm) b = Growth exponent from LWR Fecundity Estimation and Oocyte/Egg Diameter Ripe eggs were removed from gravid fish, weighed and preserved in Gilson’s fluid. The preserved ovaries were washed several times in distilled water to get rid of the preservative, then eggs were separated from the ovaries, placed on filter paper to remove excess water. The number of eggs was estimated using gravimetric method, the diameters of ten oocytes from each ovary were measured, and the mean oocytes diameter was determined. Thus, fecundity is calculated as Log F = log a + bLogTL Where; F = fecundity TL = total length a and b are constant and exponent respectively. Maturity Stages Determining the maturity stages of C. auratus was done through the use of hand lens. Slight pressure was put on the abdominal area of the samples to see if milt or eggs were released and afterwards dissection took place by cutting away the abdominal wall with a scissors to expose the gonads, which were removed, weighed and stored in Gilson’s fluid for preservation. Stages of gonads were classified as described by Nikolsky (1963) and Bucholtz et al. (2008).