|Impact of Market Hunting on Mammal Species in
JOHN B. FA,* JAVIER JUSTE,t JAIME PEREZ DEL VAL,$ AND JAVIER CASTROVIEJOt
jersey Wildlife Preservation Trust, Les Augrès Manor, Trinity, Jersey JE3 5BF, Channel Islands, email email@example.com fEstación Biológica de Doflana, C.S.I.C., Pabelion del Peru, Aparlado Postal 1056, 41080 Sevilla, Spain
*Cooperación Espafiola, Malabo, Guinea Ecuatorial
Absfract The Impact of commercial bunting on forest mammals was studied in two regions on Bioko and Rio Muni in Equatorial Guinea, west Africa. Harvests were assessed from carcass counts in the main markets in the areas. A total of 10,812 carcasses of 13 species were recorded in Bioko, and 6160 carcasses of3O species were recorded in Rio Muni. Biomass of harvested mammals was 111,879.63 kg in Bioko and 64447.87kg in Rio Muni. For the 12 prey species selected for study in Bioko, harvests totaled 7.15 animals/km2 or 6293 kg’ km2. Harvests for the 17 prey species in Rio Muni were 3.22 anlmal.s/km2 or 2406 kg/km2. We used a model developed by Robinson and Redford (1991) to estimate potential harvests based on animal production rates. Total production was 147.90 animals/km2 and 139.12 animals/km2 in Bioko and Rio Mun4 respectively. Po tential harvest figures varied considerably by species. Comparison of actual and potential harvests showed that five primate species (Cercopithecus erythrotis Cercopithecus nictitans Cercopithecus pogonias, Cercopith ecus preussi, and Mandriflus leucophaeus) and one ungulate (Cephalophus ogilbyl) in Bloko were being bunted unsustainably. Only two of the 17 species (Cercopithecus nictitans and Cephalophus dorsalis) in Rio Muni were being bunted unsustainably. Percent deviation of actual from potential harvests averaged 498 times greater than sustainable harvest in Bloko and 1.03 times greater in Rio Muni. For the two sites together figures ranged from dose to 28 times greater than potential to 0.08% of the potential harvest. Although bunt ing methods and the commercialization potential of species may affect their presence in markets, these fig ures show that Bioko animals are heavily exploite4 some of them unsustainably. This poses severe risks for the conservation of the island’s unique fauna that must be addressed immediately.
Impacto de Ia caza comercial sobre las especies tie mamiferos en Guinea Ecuatorial
Resunien: El impacto de la caza comercial de mamiferos de Ia selvafue estudiado en dos regiones en Bioko y Rio Muni, en Guinea Ecuatorial, Africa Occidental Las cosechasfueron estimadas apartir del conteo de an imales muertos en losprincipales mercados del area. Un total de 10,812 animales muertos de 13 especiesfu eron documentados en Biokoy 6160 animales muertos de 30 especiesfueron documentados en Rio Muni. La biomasa de los mamiferos recolectadosfue de 111,879 kg en Biokoy 66,447.87 kg en Rio Muni. Para las 12 especles depresas seleccionadaspara su estudlo en Bioko, la recollección totalizó 7.15 animales/km2 o 62.93 kg/km2. La recolecciOn para his 17 espedes depresa en Rio Munifue de 3.22 animales/km2 o 2406 kg/km2. Utilizamos un modelo desarrollado por Robinson y Redford (199!) para estimar las cosechas potenciales basadas en las tasas deproducdtin animal Laproducción total fue de 147.90 animales/km2 y 13912 an imales/km2 en Bioko y Rio Muni respectivamente. Las cifras sabre Ia cosechapotenclal varariaron consi
ranbtl e entre las distintas espedes. Las comparaciones de las recolecciones reaies y las potenciales mos
traron que cinco especies de primates (Cercopithecus erythrotis, Cercopithecus nictitans, Cercopithecus pogo nias, Cercopithecus preussi y Mandriflus leucophaeus) y un ungutado (Cephalophus ogilbyi) en Bioko estaban siendo cazadas enforma no-sostenible. Solamente 2 do las 17 especies (Cercopithecus nictitans y Cephalophus dorsalis) en Rio Munl estaban slendo cazadas enforma no-sostenible. La desviaciOn delporcentaje do recole cdon real con respecto alpotencial,fue enpromedio 498 veces mayoral do la recolecciOn sostenida en Bioko y 1.03 veces mayor en Rio Muni. Para los dos sitiosfuntos, las cifras oscilaron entre 28 veces mayor que la co
sechapotenclal a un 0.08% tie ía cosechapotencial. Si bien los métodos tie cazay elpotendal tie comercializa dOn puede afectar sit presencia en los mercados, las dfras actuates muesfran que los animales en Bioko es tan severamente explotados, alguno tie los cuales enfonna no-sostenible. Esto plantea severos riesgospara hi conservaclOn tie ía singular fauna de hi ida, por to que esteprobtema debe ser tratado enforma inmediata.
Fa et aL
Wild animals are an important source of protein in many tropical forest countries in Africa (Ajayi 1971, 1983; Adeola & Decker 1987). For exploited species, it is im portant that the rate of harvest does not exceed that of production because over-exploitation leads to depletion. Harvest should be a replaceable form of mortality and should substitute for some of the natural, annual mortal ity rather than increasing total mortality of a population (Caughley 1977).
Robinson and Redford (1991) developed a simple model to provide estimates of potential (sustainable) harvest rates for different neotropical forest mammals. They focused on species traditionally important to sub sistence hunters and calculated maximum production for a species (in numbers of animals/kin2) as the number produced yearly under optimal conditions. They used measures of population density and the intrinsic rate of natural increase to estimate potential harvest rates for different species. This provides a figure for the optimum sustainable harvest when production is at a maximum and harvesting has minimal effects on the natural popu lation. The optimum sustainable population is the num ber of animals of a species that results in maximum pro ductivity but not exceeding the carrying capacity of the habitat. Assessing the impact of hunting on wildlife pop ulations is thus possible when figures generated by the model are compared to actual harvest data. These com parisons are useful in situations in which detailed life- history parameters for accurate estimates of the effect of hunting on population structure are not available.
In countries where commercial hunting of game for
human consumption is important, data from markets can provide short- and long-term insight into the impact of hunting on bushmen species. In Africa, counts of numbers of mammals, birds, and reptiles entering mar kets can provide understanding of seasonal and longitu dinal dynamics of wildlife use and exploitation (Colyn et al. 1987; Kalivesse 1991) and even the biology of species (Gevaerts 1992). We examined the impact of harvests for some mammal species by relating calculated poten tial harvests with actual take levels. We examined only those species that comprised a minimum of 1.5% of the total weight of game taken and for which data on den sity and life-history parameters were available.
Equatorial Guinea consists of a territory on the African mainland, Rio Muni, (26017 km5 and five islands. Rio Muni is bordered by Cameroon to the north, Gabon to the east and south, and the Gulf of Guinea to the west. The two most important
islands are Bioko (formerly Fernando P00, 2017 2) and Annobon (formerly Pa galü, 17 km2). Intact tropical rainforest is found a few ki lometers inland from the coast and still covers most of the country, 59% of Rio Mimi and 28% of Bioko. Al though the amount of primary rainforest in Bioko is low, because most forest was cut to plant cacao, most of the island is covered in well-conserved, tall secondary forest (Fa l992a).
Hunting in Equatorial Guinea Is practiced openly and in tensively by professional hunters, and meat is brought into markets throughout the year. Unlike other West Af rican countries, there are no closed hunting seasons.
From October 1990 through October 1991, harvest in formation was collected from two market sites (Mun doasi and Central) in Bats, Rio Muni, and from the prin cipal market (Mercado Central) in Malabo, Bioko Island. The latter market is divided into separate sections (Luba and Riaba).
Vertebrate carcasses were counted by the authors and by trained, local observers hmiliar with all entry points of bushineat to the markets and the species concerned. Reliability was checked regularly. Species were re corded by their common names to avoid confusion in nomenclature. Sampling was conducted on 424 market days, 212 at each locality. Game was brought in daily by intennediaries between hunters and market-stand pro prietors. We visited the markets daily between 0630 hours and 1200 hours because all meat arrives to be sold between 0700 and 1100 every morning. Only fresh car casses were counted, although some smoked meat is brought in. Numbers of carcasses recorded represent minimum extraction because some game is consumed in villages or sold before it reaches the market (Colell et al.
1995). Age and sex information was not recorded. No
menclature follows Haltenorth and Diller (1987).
Prey species bio
mass was calculated by multiplying the number of animals by the individual species weights. Be cause as some of the carcasses for sale are young ani mals, this method tends to overestimate the total weight of meat sold.
Hunting areas that supply the market sites were desig nated “reservoir areas.” Market species restricted to nv erine or swamp forests (collared mangabey, Cercocebus torquatus; De Brazza’s monkey, Cercopitbecus negiec
nlte rate of increase is the exponential of the intrinsic rate of Increase (e,...) and is the increase in the popula tion size from time t to time t ± 1. Variables such as den sity and intrinsic rate of increase in mammals have been shown to be predictably related to their body mass and trophic level occupied (Peters 1983; Robinson & Red ford 1986). Maximum finite rate of increase was calcu lated using age at first birth (a), age at last reproduction (w), and birth rate of female offspring (b) from Cole’s (1954) equation.
rm(w + 1)
ins; water chevrotain, Hymoschus aquaticus; grey-
checked rnangabey, Lophocebus aibigena; otter, Lutra maculicolils; talapoin, Miopitbecus talapoin; Bates’ dwarf antelope, Neotragus batesi; sitatunga, Tragela phus spekel) were not included in these analyses be cause of difficulties in measuring the size of these areas. The size of tern firme areas was determined, conserva tively, from interviews with hunters,in both localities.
In Bioko, hunters sending meat to the Luba market
section use the Malabo/Luba districts, especially the western slopes of Pico Basilé and the northern slopes of the Gran Caldera de Luba. The island’s eastern districts of Baney and Riaba and areas stretching into the south eastern highlands and coast serve the Riaba market sec tion. In Rio Muni, meat primarily from the LitoraL district (S2,000 km2) enters both markets. Because Bioko pri mates are restricted to certain parts of the island, distri bution data gathered by Butynski and Koster (1995) were used to calculate reservoir areas for these species.
Production for each species was determined using in formation on population density at carrying capacity, the maximum rate of population increase, and the den sity that produces the maximum sustained yield. We used data on observed densities of species, as opposed to predicted densities (see Robinson & Redford 1991). Avenge densities (number of animals/kin2) were taken from an extensive survey of the relevant literature. No density information was obtained for Dendrobyrax dot- sails, Crossarchus spp., Man/s gigantea, and Thriono mys swinderianus, so these species were not included. Optimum harvest was considered the number of ani maLs of a species that can be removed (per kilometer) by humans every year without altering the size of the stand ing population and was determined using Robinson and Redford’s (1991) harvest model. To calculate produc tion (P, the addition to the population through births and immigntions), these authors assumed that realistic maximum figures would occur at 60% carrying capacity to accommodate variation related to density depen
dence and birth rates. Hence,
“max (0.6 D X Imax) — 0.6 D,
where D is the population density and imax the maxi
mum finite rate of increase of the species. Maximum fi
1 em + beTm — bC