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Growth, Mortalities and Exploitation Rates of Alestes Baremoze (Joannis, 1835), Brycinus Nurse (Rüppell, 1832) and Schilbe Inte

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Growth, Mortalities and Exploitation Rates of Alestes Baremoze (Joannis, 1835), Brycinus Nurse (Rüppell, 1832) and Schilbe Inte ISSN 2277-0836; Volume 2, Issue 1, pp. 1-10; January, 2013. Journal of Agriculture and Biodiversity Research ©2013 Online Research Journals Full Length Research Available Online at http://www.onlineresearchjournals.org/JABR Growth, Mortalities and Exploitation Rates of Alestes baremoze (Joannis, 1835), Brycinus nurse (Rüppell, 1832) and Schilbe intermedius (Rüppell 1832) from the lower reaches of the White Volta River (Yapei), Ghana *Seth Mensah Abobi1 and Werner Ekau2 *1Department of Fisheries and Aquatic Resources Management, University for Development Studies. P.O. Box TL 1882 Tamale, Ghana. 2Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstr. 6 D-28359 Bremen, Germany. Downloaded 22 December, 2012 Accepted 30 January, 2013 A six-month study was conducted from October 2011 to March 2012 to assess the biology and state of exploitation of three major species namely Alestes baremoze, Brycinus nurse and Schilbe intermedius from the lower reaches of the White Volta River (Yapei), Ghana. Routines in FAO-ICLARM Stock Assessment Tools II (FISAT II) were used to determine the growth and mortality parameters and exploitation rates (E) from the length-frequency data generated from the landing surveys. Alestes -1 baremoze had asymptotic length (TL∞) of 46.73cm and growth rate (K) of 0.94 year . Brycinus nurse had -1 -1 TL∞=28.38 cm, K= 1.10 year whereas Schilbe intermedius had TL∞=27.93 cm, K=0.95 year . Brycinus nurse which was the third most abundant species among landings composition was being exploited at a rate of 0.77, above the optimal exploitation rate (E0.5). Alestes baremoze which was the most abundant species had exploitation rate of 0.30 whilst Schilbe intermedius had the exploitation rate (E) of 0.36. Both species populations in the lower reaches of the White Volta River are thus under exploited. Keywords: White Volta River, biology, exploitation, length-frequency, asymptotic length, species and population. INTRODUCTION The Lake Volta and its tributary Volta Rivers which form land, and therefore may not be able to act as natural the most important inland fishery area in Ghana have in filters for nutrients and silt, and now do not provide the last four decades undergone great changes in its breeding grounds for many fish species [1]. ecology, limno-chemistry and socio-economy. Increased As a result of these problems, the Lake Volta Research pressure on land along the banks has led to high rates of and Development Project (VLR&DP) was undertaken deforestation. This has resulted in increased soil erosion under the Food and Agricultural Organization (FAO) and leading to the transportation of high loads of silt and United Nations Development Programme during the first nutrients through rivers into the lake, thereby contributing decade of the lake’s existence [2-3]. These studies came to its eutrophication. Furthermore, wetlands bordering the to an end in 1978. Since then, systematic data collection lake are being converted to agricultural land or grazing from the Lake Volta and Volta Rivers natural resources has been lacking. There have been calls for renewed studies to facilitate their management due to declining catches. Growth, mortality and exploitation rates are of *Corresponding Author's Email: [email protected]. great importance to fisheries management and modeling. 2 J Agric Biodivers Res Figure1. Map of Ghana with an insert of the detailed study area showing Yapei, and the three landing sites; Pataplapei, Porturto, and Aglassipei. This study was conducted in response to the calls for These practices are targeted at more catches for more renewed studies to provide relevant data for sustainable income. Fish can only be harvested at the maximum management of the Volta fisheries. sustainable yield (MSY) when all the biological Alestes baremoze, Brycinus nurse and Schilbe parameters are known. intermedius are very abundant from October to March in The study was therefore conducted to provide relevant the Yapei stretch of the White Volta River. During this information on the growth parameters, the mortalities and period, these fishes constitute a significant part of the fish exploitation rates of Alestes Baremoze, Brycinus Nurse diet of the local inhabitants. In this way, they provide a and Schilbe Intermedius from the lower reaches of the source of animal protein. Subsequently, the importance of White Volta River (Yapei), Ghana. the Yapei fishery cannot be underestimated. Yapei is 52km from the Northern regional capital Tamale. Tamale is the third largest city in Ghana with a population of MATERIALS AND METHODS 537,986 [4]. Yapei is easily accessible as it is on the Accra-Kumasi-Tamale trunk road. Yapei is one of the nine Study Area main sources of smoked fish to the Tamale Central Market [5]. The Tamale Central Market is the largest This study focuses on the lower reaches of the White market center in the metropolis and majority of the Volta River around Yapei. Yapei together with Bupei inhabitants buy their food stuffs from this market. It thus (where the Black Volta passes) form the main commercial makes Yapei an important supplier of fish especially fishing areas in the stratum VIII of the Lake Volta. Yapei is smoked fish. Fishermen at the Yapei stretch of the White located within the Central Gonja District of the Northern Volta River practice unregulated and unselective Region of Ghana. It lies within latitudes harvesting of fish, with fishermen harvesting all sizes of 9° 10′ 0″ N, 1° 10′ 0″ W (Figure 1). According to Abban et al. fish without regard to the sustenance of the fishery. [6] there are four main seasons that can affect fishing in the Abobi and Ekau 3 Volta basin namely; dry season, January –March (lowest mean surface habitat temperature of 28.8oC measured water level); pre flood season, April-June (water level rising); during the period of the study and as described below: flood season, July-September (highest water level); and post flood season, October-December (water level drawdown). Log10 L∞ M = - 0.00066-0.279 Log10L∞ + 0.6543 Log10 K + The study was done during post flood and dry seasons 0.463 Log10 T. and along 8.84 km stretch of the lower reaches of the White Volta River at Yapei. Three non-overlapping The fishing mortality rate (F) was calculated from the landing sites namely Pataplapei, Porturto and Aglassipei equation; F= Z– M and the rate of exploitation (E) from E were selected (Figure 1) to provide a representative = F/Z [13]. The exploitation rate assesses if a stock is overview of the fisheries in the Yapei stretch of the lower over fished or not, on the assumption that optimal value E reaches of White Volta River. Porturto is the biggest and (E opt) is equal to 0.5. The use of E or 0.5 as optimal most important landing site along the stretch. It is central value for the exploitation rate is based on the assumption and it is right in the Yapei Township and for the purpose that the sustainable yield is optimized when F= M [14]. of this study it is considered as midstream. Porturto serves as the main landing point not only for Setting the Initial Values in Elefan I Routine its fishers but also for other fishers and fish traders from neighboring landing sites and villages. Pataplapei is The length-frequency data was carefully scrutinized using upstream, 4.8 km from Porturto whilst the landing site steps outlined in FISAT II (User’s guide). Under the Aglassipei is downstream, 4.04km from Porturto (Figure ELEFAN I routine, growth curve that "best" fits a set of 1). Pataplapei lies within latitudes N 09° 08. 555', W 001° length-frequency data, using the value of Rn as a criterion 12.166'; Porturto within N 09.08.427', W 001° 09.547' whilst was used. Two of the options provided by identifying the Aglassipei lies within N 09° 06.724', W 001° 07.710'. “best” growth curves of the species. Fish Sampling and Measurement FISAT II namely K-scan and response surface analysis were mainly used for Data Points Selection Procedure in the Fish sampling and landings survey were done once in Length-Converted Catch Curve Routine of Fisat Ii month in each of the three landing sites and it spanned over the study period (October 2011-March 2012). The The growth parameters (K and L∞) as obtained from the fish sampling and landing surveys were conducted on the ELEFAN I routine were inputted in the first tab of length- days of visit between 6:00 am and 12:00pm GMT. The converted catch curve routine, the data points were then samples were taken from the local fishermen’s catches computed by clicking the “Compute Data Points” which and examined for their species composition using keys subsequently plotted results of the length-converted provided by [7-8]. The total lengths of samples of catch curve and displayed in the second tab. The default selected species for detail study were recorded to the selected points presented under the second tab of the nearest 0.1 cm using a fish measuring and grouped into length-converted catch curve routine were changed by 1cm total length (TL) class interval and their mid-lengths clicking on the “Reset Selections” command button. The were used for length frequencies distributions samples. final selection of the data points was achieved after visually inspecting the catch curve presented under the second tab Data Analysis of the routine and identifying better sequence of points. In some cases, a series of trials of changing the Growth and Mortality selected default points and reselecting other points and comparing the goodness of the statistical results of the The analysis of the population parameter estimates was regression analysis which were presented in the third tab of done using the routines in the FAO-ICLARM Stock the routine were very helpful in achieving the final selection.
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