Available online at http://ajol.info/index.php/ijbcs
Int. J. Biol. Chem. Sci. 9(1): 246-258, February 2015
ISSN 1997-342X (Online), ISSN 1991-8631 (Print)
Original Paper http://indexmedicus.afro.who.int
Traditional exploitation of edible freshwater oyster Etheria elliptica (Lamarck, 1807) in Pendjari River (Benin-West Africa): assessment of income, human pressure and options for management
Giraud David AKELE 1* , Simon AHOUANSOU MONTCHO 1, Antoine CHIKOU 1, Guy Apollinaire MENSAH 2 and Philippe Adédjobi LALEYE 1
1 Laboratoire d’Hydrobiologie et d’Aquaculture, Faculté des Sciences Agronomiques, Université d’Abomey-Calavi, 01 BP 526 Cotonou, Bénin. 2 Institut National des Recherches Agricoles du Bénin (INRAB), CRA-Agonkanmey ; 01 BP. 884 Cotonou, Bénin. *Corresponding author, E-mail:[email protected]
ABSTRACT
Freshwater oyster Etheria elliptica (Mollusca: Bivalvia) was harvested in Pendjari River as important source of protein and income. The river crosses the Pendjari Biosphere Reserve, a protected area in Northern Benin. The freshwater oyster is harvested outside and within the protected area with increasing human pressure. The current study aims at assessing fishery production, human impact on wild stocks and suggests sustainable management options. Individual semi-structured interviews and monitoring of oyster collection were used to collect data on harvesters’ socio-economic characteristics, biomass production and seasonal income in 2008 and 2009. Mean shell size and weight as well as size frequency distribution were analyzed in relation to harvesting pressure. Oyster harvesting is the main traditional activity of Berba women in dry season, at low water level period. The oyster collectors used artisanal tools such as chisel-like metal, stone-hammer, rope, metal pan and pot. Biomass production, oysters mean size and weight decreased from protected sites to unprotected zones revealing negative anthropogenic pressure on oyster natural stocks. Size frequency distribution displayed an important proportion of smaller individuals in open access sites whilst larger size oysters were more encountered in protected zones. Therefore, exploitation appeared likely as a potential threat for oyster bed. Moreover, the oyster collectors not only exploited individuals without observing a minimum limit size, but they also targeted each year the same sites regardless rotation planning. Finally, oyster`s shell, a main substrate for larval settlement and bed restoration, were left on riverbanks after exploitation, reducing stocks reconstitution potential. Appropriate measures were suggested to Reserve staff for sustainable management. © 2015 International Formulae Group. All rights reserved.
Keywords: Freshwater mollusk, harvesting activities, human pressure, fisheries management.
INTRODUCTION bivalves has critically increased in the last Worldwide, mollusks bivalves group three decades as highlighted by export trends include many edible species of great since the seventies (124 300 tonnes in 1976 commercial interest such as mussels and and 459 300 tonnes in 2005) (Lovatelli et al., oysters (Gosling, 2003).The world market for 2008). Such increase was mainly due to
© 2015 International Formulae Group. All rights reserved. DOI : http://dx.doi.org/10.4314/ijbcs.v9i1.22 G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 mussels with 257 300 tonnes in 2005 small fisheries were assumed to be facing (Lovatelli et al., 2008). However, production unsustainability and overexploitation (poor increase was mainly provided by mussel and growing local population; unregulated aquaculture since natural stocks have faced exploitation, high demands for limited overexploitation. Moreover, freshwater resources) (Narvarte et al., 2007), substantial mussels (Mollusca: Bivalvia: Unionoida) were information on human pressure on freshwater stressed to be threatened worldwide mainly oyster are still lacking. The abundant literature due to overfishing (Anthony and Downing, available on species mainly focused on 2001; Strayer, 2006). systematic and spatial distribution (Graf and In the tropics, oysters are found in Cummings, 2006; Van Bocxlaer and Van rivers and coastal areas and supported many Damme, 2007). Information on bivalve small artisanal fisheries (Haupt et al., 2010; exploitation remains limited to harvesting Van Damme, 2011; Ikpi and Offem, 2012). activities description and harvesters socio- Wild oysters are commonly harvested by local economic traits (Ampofo-Yeboah, 2000; Ikpi residents especially women as main traditional and Offem, 2012). However, understanding activity. Oyster exploitation provides protein how human pressure and harvesting technique and improves income of local communities affect stocks is of great importance not only to (Bogan, 2008; Haupt et al., 2010; Ikpi and settle sustainable management measures but Offem, 2012). However, small artisanal also for sensitization and environmental fisheries, especially in Africa, were education activities targeting residents’ considerably unregulated and were declining dwellers (Hartill et al., 2005). In fine, this also due to overexploitation (de Bruyn et al., 2009; contributes to secure their livelihood. Haupt et al., 2010). For instance, in South The freshwater oyster E. elliptica was Africa, de Bruyn et al. (2009) reported steady harvested by residents in Pendjari River for decrease in biomass of oyster Striostrea livelihoods. The river crosses the Pendjari margaritacea between 1935 and 1955 due to Biosphere Reserve (PBR), a protected area lack of sound regulations. Wild stocks decline (Benin-West Africa) on two-third of its course led to adverse socio-economic consequences (Agbossou and Okounde, 2001). In open for poor people, especially for women, owing access sites, heavy and unregulated to significant decrease of both low-cost exploitation is leading to stocks collapse; protein and income (Narvarte et al., 2007; Ikpi therefore, exploitation pressure is increasing and Offem, 2012). on oysterbed within the Reserve where Etheria elliptica (Lamarck, 1807) collection was under control (Kiansi, 2011). (Mollusca: Bivalvia: Etheriidae) is the only Indeed, within the context of ongoing co- freshwater oyster occurring in many lakes and management between local population and the rivers through Africa (Graf and Cummings, Reserve managers, access was given to 2006; Van Damme, 2011). The oyster was surrounding people to collect some reserve mostly harvested by growing indigenous resources such as medicinal plants, straw and populations along artisanal fisheries in Africa halieutic resources, as they contribute to anti- (Van Damme, 2011; Ikpi and Offem, 2012). poaching activities and biodiversity In West Africa, the traditional exploitation of conservation. Overall, oysters appeared to be freshwater oyster has been described in many the main resource requested by poor and small fisheries mostly in Ghana (Ampofo- growing local residents after fish species Yeboah, 2000) and in Nigeria (Ikpi and (Kiansi, 2011). This leads to increasing Offem, 2012). Besides, species oysterbed pressure on the oyster stocks within the decline was pointed out in Burkina Faso reserve. Therefore, an establishment of (West Africa) resulting from excessive sustainable management strategy for oyster gathering by riverine population of Volta wild stocks is one of the priorities of the PBR Basin (CNRST, 1992). Although African managers. This work aims at analyzing 1) the 247
G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 socio-economic characteristics of harvesters Porga7, Pouri, Setchendinga) were recorded. (sex, age, ethnic group, profession, income of Distributed in five harvesting groups, they exploitation), 2) the impact of harvesting were active in different sections of the river: pressure on natural stocks and the open access (Borne 0-5) and protected zones implication for co-management success and (Borne 18-28 and Borne 30-40), during 2008 the sustainable use of this resource. and 2009 harvesting seasons. In Tampanga, 60 oyster collectors exclusively active in open MATERIALS AND METHODS access site were registered. As the latter were Study area and species far from the Pendjari Biosphere Reserve, their The study was carried out along the collection activities remained limited to Pendjari River in Northern Benin (West Tampanga accessible sites (Figure 1). Finally, Africa) (10°24 N, 1°21E). The river crosses 150 active harvesters were recorded among the Pendjari Biosphere Reserve for 200 km riverine communities. Respondents were out of 380 km of its course and is a natural chosen for this survey based on their level of border between Burkina Faso and Benin involvement in and dependency on oyster (Agbossou and Okounde, 2001). The Pendjari harvesting (active every year), harvesting sites River has a tropical hydrological regime with location (open access and protected zones), a low water season between December and harvesting activity importance in dry season June and a flood season from September to or time allocated (main activity in dry season) October. The Pendjari River is located in and agreement to take part in the research. Sudanian climatic zone with one rainy season Individual semi-structured interviews were (April/May to October) and one dry season used to collect socio-demographic data (sex, (November to March). Mean rainfall reached age, sociolinguistic group and occupation) of annually 1000 mm with 60% falling between harvesters along the river. Overall, 120 July and September Daily temperature varies harvesters out of 150 were involved in data between 11 °C (at night in December) up to collection. 40 °C (at noon in April) in the area (Sinsin et Oyster fishery production and income al., 2002).The riverbed is characterized by estimation different types of sediments serving as To assess harvesting output, oyster substrates for sessile mollusks bivalves such exploitation was monitored during 2008 and as freshwater oyster E. elliptica . 2009 harvesting seasons (December to July) The Pendjari Biosphere Reserve covers in different stations. Oyster fishery production an area of 4666.4 km 2 and is composed of was analyzed by oyster biomass estimation. Pendjari National Park (2660.4 km 2) which Biomass (kg) per collector and per site was represents the core integrally protected area estimated after weighting individual where human activities are prohibited, the production in the field both in open access and Konkombri hunting zone (251 km 2) and the protected zones. Biomass measuring tool was hunting zone of Pendjari (1750 km 2); the latter a metal pan containing 30 kg of living oysters. hosts oyster harvesting activities. Total production per collector was obtained by multiplying the total numbers of pans Sampling and data collection collected by each harvester at the end of the Socio-demographic traits of oyster harvesters season by 30 kg. Prior to data collection, the number of Living oysters were processed, sun- harvesting groups actively collecting oyster dried and sold in market with a small unit within the Reserve was recorded from reserve bowl containing about 1.5 kg of dried oysters managers. Similar data were collected from and costing 2 500 FCFA. The collector key persons about harvesters operating in income varies depending on duration of a open access land. Overall, 90 harvesters living harvesting trip (one or two weeks) and sites. in surrounding villages (Daga, Porga 6, The mean income per collector was estimated 248
G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 per different sites considering the market price collectors at Tampanga and 20 collectors at of dried oysters sold with smaller unit bowl. Borne 0-5) and within the protected area (51 The seasonal income of the collector was collectors at Borne 18-28 and 14 collectors at estimated taking into account the number of Borne 30-40). Oyster exploitation is trips per season. dominated by women (93% of interviewees). Assessing human impact Men mostly helped women to locate oysters Oyster exploitation took place in the stock in profound parts of the river. Age of river both outside and within the protected harvesters ranged from 17 to 85 years with an area. In the current study, we monitored two average attaining 50 years. Most oyster open access sites (Tampanga and Borne 0-5) collectors in open access zone were younger and two protected sites (Borne 18-28 and (mean age: 43 years). In opposite, old Borne 30-40) (Figure 1). Human impact was harvesters (mean age: 53 years and 75% of expressed through three harvesting pressure them aged over 40 years) were mostly criteria summarized in Table 1. Three human encountered in the protected sites. Berba tribe pressure degree were defined as following: women dominated (94%) oyster exploitation Tampanga (high human pressure), Borne 0-5 with few Mina (3%) and Kakamba (3%) (medium human pressure) and Borne 18-28 people. Oyster E. elliptica was locally called and Borne 30-40 (low human pressure). “Kourou” by Berba people, Adja” by Mina Values of parameters such as mean and “Teounkoro” by Kakamba. Harvesters shell size and weight of oyster and size practiced mainly agriculture in rainy season distribution frequency were estimated and whereas exploitation is the major occupation compared among sites in relation to human in dry season. pressure degree . Shell size (height or anterior posterior length) was measured with a caliper Artisanal oyster harvesting and actual to 0.1 mm. Shell sizes were measured both for management strategies living individuals and relict shells from recent Overall, oyster was harvested yearly in collection. Total weight of living oysters was dry season by native population mainly Berba measured to 0.1 g in different stations. women. In open access land, exploitation started at low level of the water period Data analysis (November/December) and ended at the onset Frequency and percentages were used of rainy season in June/July. In contrast, to analyze socio-demographic backgrounds of exploitation within protected area started later, respondents among sites. The Kruskall-Wallis after the game hunting activities to avoid non parametric test (H) was used to compare, hunting accident and ended at the onset of among all sampling sites, oyster biomass, rainy season. Thus, the duration of income, oysters mean size and weight as data exploitation decreased from open access sites were not normally distributed (Dagnelie, (6 months at Tampanga) to protected zone 1984). Shell size frequency distributions were (about two months from May/June to July) compared among sites using G-test (G). with identical collection method. Oyster harvesting technique is traditional and RESULTS artisanal. In shallow zones, women wade Socio demographic traits of oyster through the river and locate oyster clumps collectors with their feet. As sessile organism, oyster The socio-demographic traits of the lived attached to hard substrates (rocks, dead oyster harvesters in the study area included woods) or in colonies in the river bottom. their age, gender, ethnic group and Thus, artisanal tools such as chisel-like metal occupation. Overall, interviews were (locally called «Koutourou») and a conducted with 120 collectors of oyster E. stone/hammer (called «wangui») were used elliptica both in open access sites (35 respectively to pry oysters and break large 249
G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 oyster colonies. Oysters were stocked in metal pan attached with a rope to women chest. Shell size, total weight and size frequency Afterwards, the harvested oysters were distribution stocked in heaps on riverbanks and were Overall, 674 shells were measured. boiled in metal pot to remove easily the meat Shell size ranged between 7.2 mm and 154 from shell. Fuel wood was cut from gallery mm with mean size of 60.6 ± 19.4 mm. forests surrounding the river. Oyster meat was Significant differences were found in mean later sun dried and conserved with ash. size among the 4 sites (H = 158.6342, Overall, oyster harvesting suffered of N = 674, p < 0.01). Mean size increased from lack of rotation system in all stations. open access sites (49.5 ± 14.7 mm Interviews followed by observations of (N = 176) and 57.34 ± 16 mm (N = 254) licenses issued for the exploitation at respectively at Tampanga and at Borne 0-5) to protected sites revealed that collection protected zones (70.73 ± 17 mm (N = 138) occurred each year mainly in the same sites. and 73.93 ± 22.87 mm (N = 106) respectively Moreover, there was neither indicated at Borne 18-28 and in Borne 30-40). minimum size, nor limited landing biomass Overall, 491 living oysters were for harvesters. Furthermore, empty shells were weighted on exploitation sites. Individual left on river banks and in mounds, albeit they weight ranged from 4 g to 240 g with 64.5 ± could be returned back to river to serve as 42.8 g mean weight. Significant differences in substrates for larval settlement. Indeed, apart mean weight were found among sites from a minor use as supplemental food for (H=168.472, N=491, p < 0.01). Mean weights ostrich reared in Dassari (30 km away from were lower in accessible sites (36.4 ± 20.2 g Porga), oyster shells were useless for local and 59.2 ± 36.6 g respectively at Tampanga population. and at Borne 0-5). Protected sites recorded greater mean weight values with 79.9 ± 40.9 g Biomass production and income and 81.9 ± 49.4 g respectively at Borne 18-28 Mean biomass for all sites and seasons and at Borne 30-40 (Figure 3). accounted for 247.6 ± 130.4 kg/collector Both oyster size and weight showed an (Mean ± SD). Biomass per collector was increase with the decrease of harvesting significantly different among sites pressure degree. (H = 62.677, N = 140, p < 0.01). Lower Size class distribution (Figure 4) of biomasses were recorded in open access sites harvested E. elliptica individuals was drawn with 118.9 ± 78.1 kg/collector and both in open access sites (Tampanga and 199.3 ± 90.5 kg/collector respectively at Borne 0-5) and protected sites (Borne 18-28 Tampanga and at Borne 0-5 whereas higher and core area). Significant differences in size values were estimated at protected sites (310.5 class distribution were found among the four ± 122.5 kg/collector and 320.3 ± 72.3 sites (G = 86.099, df = 3, p < 0.01). Shell class kg/collector respectively at Borne 18-28 and 41-80 mm held the highest proportion of at Borne 30-40) (Figure 2). harvested specimens for all stations. Larger Overall, mean seasonal revenue per size classes (81-120 mm and 121-160 mm) collector was estimated to 23 615 ± 14 335 were more abundant in protected sites with FCFA covering a trip (5 to 15 days). The 24.2% and 38.7% respectively at Borne 18-28 Kruskal-Wallis test (H = 71.044, N = 140, and in core area. Larger specimens accounted p < 0.01) showed significant difference among for 9.2% and 11.2% respectively in Tampanga sites. Although, a limited harvesting period, and Borne 0-5. Conversely, Tampanga in seasonal revenues per collector in protected unprotected zone recorded the highest zone were almost two-fold ones in open proportion (24.1%) of small oysters (0-40 mm access zones (Table 2). shell class) among sites.
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Figure 1: Oyster collection sites distributed along the Pendjari River located inside and outside the Pendjari Biosphere Reserve (Benin-West Africa).
450
400
350 b c 300
250 a 200 Biomass per collector(kg) Biomass per 150 a
100
50
0 Tampanga Borne 0-5 Borne 18-28 Borne 30-40 Open access sites Protected sites Harvesting sites
Figure 2: Oyster biomass (Mean ± Standard Deviation) per collector and per site during 2008 and 2009 harvesting seasons. (Kruskal-Wallis test, p < 0.01; Sites with different superscripts differ significantly).
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140 140
Shell size (mm) 120 120
Weight (g) Weight(g)
Shellsize (mm) 100 100
80 80
60 60
40 40
20 20
0 0 Tampanga Borne 0-5 Borne 18-28 Borne 30-40 Sites
Figure 3: Shell size (Mean ± SD) and oyster weight (Mean ± SD) of sampled oysters during 2008 and 2008 harvested seasons.
80.0 Tampanga (N=176) 70.0 Borne 0-5 (N=254) Borne 18-28 (N=138) 60.0 Borne 30-40 (N=106) Frequency (%) Frequency 50.0
40.0
30.0
20.0
10.0
0.0 0-40 41-80 81-120 121-160
Size class (mm)
Figure 4: Size frequency distributions of oyster E. elliptica collected in the Pendjari River both from open access sites (Tampanga) and protected sites located within the Pendjari Biosphere Reserve.
Table 1: Characteristics of sampling sites in relation to human pressure degree on wild oysters.
Sampling Existence of Number of Annual harvesting Harvesting sites regulation harvesters duration (month) pressure level Tampanga No regulation Unlimited 6 – 7 High Borne 0-5 No regulation Close riverine only 5 Medium Borne 18-28 Closed period Limited 2 Low License required Borne 30-40 Closed period Limited 2 Low License required
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Table 2: Mean income per collector (per trip) and per season during 2008 -2009 exploitation period in open access and protected sites (1dollar US = 507 FCFA).
Harvesting sites Mean income/collector/ Seasonal income per trip (FCFA) (*) collector (FCFA) (*) Open access Tampanga 14 186 a 44 000 a sites Borne 0-5 12 063 a 41 200 a Protected sites Borne 18-28 32 000 b 76 500 b Borne 30-40 32 500 b 77 000 b (*) Significant difference (p < 0.01) Values with common superscript do not differ significantly
DISCUSSION (Vodouhè et al., 2009). Then, low implication Oyster harvesting and its socio-economic of men in exploitation could be seen as a importance factor limiting pressure on oyster beds. Worldwide, oysters serve as Age of collectors could likely influence supplemental food source (Gosling, 2003). the pressure on oysterbeds. In Senegal, The freshwater oyster E. elliptica was mangrove oyster harvesting appeared to be gathered as important source of food and difficult owing to the time spent away income for poor people livelihood through household. Therefore, exploitation is devoted oyster shellfisheries in Africa (Van Damme, to old people, especially to women without 2011; Ikpi and Offem, 2012). domestic and marital duties (Cormier-Salem, Traditional oyster harvesting in the 1987). Similar trend was recorded in our study Pendjari River constitutes a cornerstone in especially, concerning exploitation activities socioeconomic life of local populations. within protected area. In this zone, long time Exploitation provides low-cost protein and spent in forest (at least two weeks), facing generates useful income especially to women. extreme daily temperature variations from 11 Moreover, importance of oyster as main °C at night to 40 °C during the day (Sinsin et animal protein source was emphasized since al., 2002) and stress linked to presence of wild most of fish production of river was exported animals (lion, elephant, buffalos etc.) limited to Burkina Faso where it generates more younger women participation. These women revenue to fishermen. Along the Pendjari were more active in open access zone. In River, characteristics of oyster harvesters are opposite, oysters were collected both by in accordance with past surveys. Thus, young and old women in White Volta (Ghana) harvesters are mainly farmers in rainy season (Ampofo-Yeboah, 2000). Furthermore, as and exploitation occurred in dry season as traditional activity, oyster harvesting was reported in past studies in Ghana (West generally limited to certain ethnic groups Africa) (Ampofo-Yeboah, 2000). In Africa, (Bay, 2000; Ampofo-Yeboah, 2000). In White previous studies stated that exploitation was Volta (northern Ghana), E. elliptica was traditional women activity through many collected by seven tribes (Dagombas, Gonjas, small fisheries while men were occupied by Konkombas, Mos, Dagartis, Ewe and Hausa) fish capture (Ampofo-Yeboah, 2000; whereas Berba ethnic group people mostly Glaubrecht, 2010). In our study area, men dominated exploitation in the Pendjari river. were involved in fish capture, hunting In our study area, Berba is most the dominant activities; Non timber forests Products ethnic group populating the villages collection. Some men were recruited as forests surrounding the Pendjari River. Thus, guard by the Pendjari Biosphere Reserve staff dominance of Berba women on oyster 253
G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 harvesting could be seen as an advantage for survey sites were chosen depending on human setting appropriate strategies for oysters’ pressure degree on oyster stocks from open management. Indeed, it seems likely easier to access zone to protected zone. Human convince limited number of ethnic groups for pressure impact was characterized with mean conservation goals (Vodouhè et al., 2009). shell size, mean oyster weight and oyster size Oyster exploitation contributes greatly frequency distribution and significant to household income of collectors. In the variations were proved among exploitation study area, price of sun-dried oysters varied sites. Overall, previous studies assumed that between 1 600 FCFA/kg and 2 000 FCFA/kg oyster mean shell size decline was an over year. Oyster price is about 4 times more indicator of stocks decline and overfishing expensive than the cost of dried freshwater (Rius et al., 2006; Haupt et al., 2010). For oyster E. elliptica in Ghana (270 - 450 instance, Ajana (1980) reported size range FCFA/kg) (Ampofo-Yeboah, 2000). In decrease of mangrove oyster Crassostrea contrast, mangrove oyster was sold 2 000 gasar from 60-70 mm to 45-55 mm in 1977 FCFA/kg in Senegal (Bay, 2000) and is due to over exploitation in coastal fisheries in almost similar to our results. Estimated Nigeria. Our study underlined a decrease of revenue per collector was twice higher in mean size and weight of oysters from protected zone (32 000 – 32 500 FCFA) after protected zones to open access zones. This a collecting trip of two weeks. This indicates indicates the negative impact of unregulated the importance of protected zone for exploitation on bivalves stocks. This finding is harvesters and pointed out stocks decrease in supported by Haupt et al. (2010) in South open access sites. Seasonal revenue per Africa who reported that size of oyster collector varied between 44 000 FCFA and specimens of protected zones was higher than 77 000 FCFA depending on number of trips ones of unregulated zones in relation to and exploitation zone. Higher seasonal human pressure degree. Likewise, Rius et al. incomes were obtained in protected zones. (2006) reported similar trend for mussels Our findings were in accordance with the ones which size was smaller in open access zones of Bay (2000) who reported 50 000 than in protected sites. In the Pendjari River, FCFA/collector for a season of five months oyster shells reaching a maximum shell height while Cormier-Salem (1987) observed higher of 126 mm was observed in completely annual income (109 000 FCFA/collector) in protected core zone where clandestine Senegal. Although most part of production exploitation occurred. Consequently, the served as domestic consumption, revenue lower mean size (60.6 mm) of harvested from oyster sale appeared to be greatly oysters could be likely attributed to important for poor population with low annual overfishing. income (72 000 FCFA), especially for women Although the variations among sites, (Projet Alafia, 2009). size frequency distribution indicated higher proportion of smaller oyster individuals in Human pressure impact open access sites, it was hypothesized that in Throughout the world, unregulated absence of selection, indiscriminate harvesting harvesting of bivalves was known to affect occurred. Therefore, size frequency revealed adversely wild stocks (Haupt et al., 2010; the natural population structure (Hartill et al., Gosling, 2003). In Burkina Faso (West 2005). Consequently, the dominance of Africa), decline due to overexploitation of smaller individuals in open access sites mollusk species including E. elliptica was evidenced existing threats on oyster beds reported (CNRST, 1992). In the current study, (Rius et al., 2006). Indeed, owing to annual
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G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 heavy exploitation, harvesters did not enable In absence of minimum limit size, juveniles to grow enough before exploitation exploitation targets all sizes especially occurred. Likewise, collection of smaller immature oysters and affects natural oysters in the Pendjari River was fostered by population to self-recruit. This likely leads to lack of defined minimum size such as the first overexploitation (Hartill et al., 2005; Strayer, maturity size (Hartill et al., 2005; Rius et al., 2006). Past studies have demonstrated that 2006; Haupt et al., 2010). Our outcomes size limit was established as useful tool for brought evidence of human pressure on oyster harvest regulations (Strayer, 2006) mainly in beds in the Pendjari River. Consequently, cases where management data were not appropriate management strategies were available (Hartill et al., 2005). The freshwater required to overcome threats on freshwater E. oyster E. elliptica reached maturity size in elliptica in the river. range of 57-65 mm in the Pendjari River (personal communication). Therefore, a Current management strategies of oyster minimum limit size of 65 mm should be fishery fostered in harvesting stations especially in In the study area, management open access zone where immature individuals strategies remain limited to closed season in were mostly gathered. Furthermore, past protected zone mainly due to wild stocks experiences stated that minimum size was protection and hunting activities. Field easy to implement as rationale was easily monitoring of oyster exploitation and accepted by collectors (Hartill et al., 2005; harvesting pressure assessment pointed out Carranza et al., 2009). three (3) main types of threats on wild stocks: Rotational strategies lead to production 1) exhaustive harvesting with negative increase since the stocks were left with fallow implication for oyster beds, 2) lack of years for recovery whereas absence of rotation minimum size and 3) absence of rotational caused stock decline (de Bruyn et al., 2009). system. Rotating fishing areas have been demonstrated Exhaustive harvesting occurred when to be a sustainable tool to manage sessile and harvesters collected all available oysters sedentary invertebrates stocks (de Bruyn et stocks encountered in sites before they moved al., 2009). In the Pendjari River, collection to untapped streams (Rius et al., 2006). Such occurred each year in the same sites because harvesting technique was assumed to decimate river banks «belong» to different communities oysters’ populations and caused stocks decline living along the river. The staff of Pendjari (Anthony and Downing, 2001; Rius et al., Biosphere Reserve should establish rotational 2006). In the study area, Berba women mainly period among harvesting sites in accordance exert high pressure. Exhaustive harvesting with harvesters and their collectivities as land- leads to negative effects on natural population owner. Oyster-based research should recovery because almost all colonies are investigate species growth performance to removed. Indeed, presence of colonies or establish a rotational period, which in turn conspecific living oysters was required to could ensure stocks conservation and increase trigger larvae settlement due to pheromone oyster production for dwellers welfare. secretion (Tamburri et al., 2008). Consequently, management strategies should Implication for sustainable use strategies indicate a remaining oyster density per site to Despite the lack of management guarantee oyster stock reconstitution strategies underlined above, closed harvesting especially in protected zone. season (December-April) in protected zones as well as the obligate license remained useful
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G. D. AKELE et al. / Int. J. Biol. Chem. Sci. 9(1): 246-258, 2015 management strategies (Strayer, 2006; Haupt necessary to collect scientific species-based et al., 2010). However, current management data for stocks conservation. strategies should be enforced. In addition, community-based management approach of ACKNOWLEDGMENTS exploited oyster populations is considered as We wish to thank BIOTA West Africa one of the most promising ways to link program for funding this research work. We sustainability and economic growth (Carranza also thank Pendjari oysters’ collectors for et al., 2009). Furthermore, empty oyster shells their participation to the research work. Many should be returned back to the river after thanks to our field guide, Dominique exploitation. It was demonstrated that Sambieni for production data collection. We conspecific shells were preferred substrates thank all anonymous reviewers for their for larval settlement and stocks recovery valuable contribution. (Tamburri et al., 2008). Currently, empty shells were almost useless for local people and REFERENCES were left on riverbanks. In the Pendjari River, Agbossou E, Okounde J. 2001. Réalisation oyster specimens reach sexual maturity at des études hydrologiques et 65 mm (personal communication). Therefore, d'aménagement de la Réserve de a minimum harvesting size should be enforced Biosphère du complexe de la Pendjari. mainly in open access sites where estimated Rapport d’étude, Tome 1 et tome 2. mean sizes remained notably lower (49.5 mm CENAGREF, Cotonou, Bénin, p. 42. at Tampanga and 57.34 mm at Borne 0-5). Ajana AM. 1980. Fishery of the mangrove Conversely, oysters collected in protected oyster, Crassostrea gasar , Adanson zones were mostly larger than recommended (1757) in the Lagos area, Nigeria. limit size. In this respect, implementation Aquaculture , 21 : 129-137. efforts of minimum limit size should target Ampofo-Yeboah A. 2000. Aspects of the open access zones. fishery, ecology and biology of the freshwater oyster ( Etheria sp . Lamarck, Conclusion 1807) in northern Ghana. Master of This study exhibited the income Philosophy degree in Zoology provided by freshwater oyster exploitation and University of Cape Coast, p. 42. revealed effect of human harvesting pressure Anthony J, Downing J. 2001. Exploitation on oyster wild stocks in the Pendjari River. trajectory of a declining fauna: a Mean shell size and weight of oysters as well century of freshwater mussel fisheries as size frequency distribution displayed the in North America. Can. J. Fish. Aquat. effect of human pressure degree on wild Sci. , 58 (10): 2071-2090. oysters stocks. Likewise, the traditional Bay T. 2000. Contribution à l’évaluation du harvesting technique has potential threats for stock d’huîtres de palétuviers wild stocks. Sound sustainable strategies (Crassostrea gasar Adanson) dans le should be implemented to guarantee oyster Parc National du Delta du Saloum stocks restoration and conservation. As the (Sénégal). Gembloux : Travail de fin main proportion of harvesters collect oysters d’études. Faculté Universitaire des inside and outside the Reserve, regulation Sciences Agronomiques de Gembloux, could be also implemented in accessible p. 75. zones. Community-based strategies should be Bogan AE. 2008. Global diversity of useful for oyster beds sustainable freshwater bivalves (Mollusca: management. Future research works were
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