Population Dynamics and Fisheries Potential of Anadara Tuberculosa (Bivalvia: Arcidae) Along the Pacific Coast of Costa Rica
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Rev. Biol. Trop. Vol. 54 (Suppl. 1): 87-99, September 2006 Population dynamics and fisheries potential of Anadara tuberculosa (Bivalvia: Arcidae) along the Pacific coast of Costa Rica Amanda Stern-Pirlot1 & Matthias Wolff2 1 Leibniz Institute for Marine Research, IfM-GEOMAR, Düsternbrooker Weg 20 24105 Kiel, Germany +49 (431) 600- 4580; [email protected] 2 Centre for Tropical Marine Ecology, Fahrenheitstrasse 62 8359 Bremen; [email protected] Received 08-IX-2005. Corrected 13-X-2005. Accepted 30-III-2006. Abstract: The present study aims to describe the population dynamics and state of the fishery of the mud cockle Anadara tuberculosa (locally know as piangua) in three harvesting areas along the Pacific coast of Costa Rica (Chomes, Purruja and Rincón) and to put the results in context with this species in other Costa Rican areas as well as with other Anadara spp. around the globe. The study is based on a five-month sampling period from October, 2003 through April, 2004 during which monthly length frequency measurements and tagging experi- ments were carried out. Von Bertalanffy growth parameters, K and L∞, (0.14 and 63.15, respectively) revealed a growth performance (f’= 2.75) which is in the range of reported values for this family of bivalves, indicating the accuracy of these values. Estimated exploitation rates range from 0.62 (Rincón) to 0.76 (Purruja) and exceed sustainable levels at all study sites. The strong overharvesting in these areas, also mirrored by the low average harvested cockle sizes relative to less exploited areas of Costa Rica and elsewhere, is due to the relatively small clam populations in each fishing area, the easy accessibility and the high economic importance of the species to the local resource users. If conservation measures are not implemented it is probable that A. tuberculosa may follow its formerly abundant and bigger sister species, Anadara grandis (chucheca), on its way to local extinc- tion. Rev. Biol. Trop. 54 (Suppl. 1): 87-99. Epub 2006 Sept. 30. Key words: Anadara tuberculosa, bivalve, Costa Rica, Golfo Dulce, Gulf of Nicoya, mangrove, phi prime, small scale fishery, von Bertalanffy. Anadara tuberculosa (Sowerby, 1833) is a Costa Rican cockle harvesters, or piangue- “blood cockle” that occurs in mangrove estu- ros, travel into the mangroves daily at low aries along the Pacific coast of the Americas tide on foot or in a small row boat and walk from Baja California Sur, Mexico to Northern among the prop roots probing the mud with Peru and is the most important commer- their hands in order to locate the cockles. When cially harvested mollusk along this coastline other marketable bivalves, such as Protothaca (MacKenzie 2001). asperrima (almeja blanca) and Mytella guya- In Costa Rica, A. tuberculosa -commonly nensis (mejillón chora) are encountered during known as the piangua- is harvested in the man- piangua harvesting, they are collected as well. grove channels of the Gulf of Nicoya, Sierpe- The species are collected in a bucket or sack, Térraba and Golfo Dulce and in other smaller carried out of the mangroves into the channel to mangrove complexes along the Pacific coast be rinsed, and carried back to town. The length (Fig. 1). Individuals are found at about 15cm of a workday depends on the magnitude of deep in level muddy substrate among the prop the tide; during neap tides piangueros usually roots of Rhizophora spp. within and around the work 2-3 h and during spring tides 4-5 h. There margins of the mangroves. are an estimated 500 piangueros in Costa Rica Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 54 (Suppl. 1): 87-99, September 2006 87 and Báez 1983). However, despite its commer- cial importance for coastal dwellers, the fishery potential, present harvest rate and population dynamics of this species in its natural habitat have as yet not been assessed. Previous studies, (Campos et al. 1990, de la Cruz 1994, Silva-Benavides and Bonilla- Carrion 2001) as well as anecdotal reports from piangua harvesters, indicate that most areas of Costa Rica have seen a decline in the abundance and sizes of A. tuberculosa over the past 10-20 years, suggesting that the fishing pressure on this species is too high. In addition, based on preliminary information on the study Fig. 1. Primary A. tuberculosa harvesting regions of Costa sites, it is assumed that the local A. tuberculosa Rica, indicated by gray shading. Sampling sites for the stocks are subjected to differential exploitation present study (Chomes, Rincon, and Purruja) are also rates, with the highest exploitation to be found labeled. at Purruja, and the lowest at Rincón. The aims of the present study are therefore to establish A. tuberculosa’s von Bertalanffy in (MacKenzie 2001), however because harvest- situ growth parameters, K and L∞, and to find ers often live and work in very remote areas the natural and fisheries mortality and exploita- and there are few centralized landing sites for tion rates for this species in different fishing this species, the number of harvesters and their areas along the Costa Rican coastline. A similar landings are difficult to approximate. Pianguas study es available for the west indian topshell are sold usually to middlemen by the dozen Cittarium pica for the Caribbean coast of Costa or 100 individuals for an average of US$ 0.05 Rica (Schmidt et al. 2002). Additionally, an per whole piangua. The pianguas are then attempt is made to put the population dynamics made into a ceviche (a cold salad with onions, of A. tuberculosa in Costa Rica in context with peppers, and lime juice); a ceviche with ten this species in other areas as well as with other piangua meats typically sells in tourist estab- Anadara spp. around the globe. lishments for around US$ 3.00. The largest body of scientific work on A. tuberculosa in Costa Rica has focused on MATERIALS AND METHODS reproductive biology (Cruz 1982, Cruz and Palacios 1983, Cruz 1984, Ampie and Cruz Description of Study Sites 1989, de la Cruz 1994). Information from Mexico is found in Baqueiro Cárdenas and Gulf of Nicoya, study site Chomes Aldana-Aranda (2000). Additionally, there have been some studies on microbial contamination The Gulf of Nicoya is a tropical estuary (Wong et al. 1997) in A. tuberculosa as well as (Voorhis et al. 1983) with an area of approxi- descriptive studies on the size structure of the mately 1 550 km2 that can be divided into population in Sierpe-Térraba and elsewhere in two distinct regions: the mangrove-fringed Latin America (Squires et al. 1975, Baquiero upper gulf with a depth of less than 20 m, 1980, Campos et al. 1990, Vega 1994). Two and the lower gulf with a depth ranging from growth increment studies have been published 25 to 100 m (Kress et al. 2002). The entire in which A. tuberculosa were grown under gulf is subject to rainy and dry seasons from artificial conditions (Madrigal 1980, Villalobos May to November and December to April, 88 Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 54 (Suppl. 1): 87-99, September 2006 respectively. In the rainy season, the average erecta. Behind the mangroves to the north are rainfall is 600 mm/month and salinity in the man-made aquacultural ponds used for shrimp upper gulf ranges from 14 to 30 ‰, with the rearing and other ponds for salt extraction. The lowest salinities recorded in the estuaries at southern border of the main channel is lined river mouths. In the dry season, average rainfall with shrimp ponds that are no longer used and is 50 mm/month and salinity ranges from 28 to there are no mangroves present. Shallow water 32 ‰ (Kress et al. 2002). Of Costa Rica’s 41 temperature in the estuary is uniform year- 292 ha of total mangrove area, 15 176 ha are round (around 28 °C) (Kress et al. 2002). found in the upper Gulf of Nicoya (Jiménez 1999). More information on the gulf’s ecosys- Golfo Dulce, study sites Rincón and Purruja tem can be found in Vargas (1995) and Wolff et al. (1998). Golfo Dulce is the southern-most embay- The Gulf of Nicoya is the most important ment along the Pacific coast of Costa Rica, cen- commercial and subsistence fishing area in tered on 8°30’N and 83°16’W, and has an area Costa Rica with fisheries primarily for shrimp of approximately 750 km2. It is characterized and finfish such as red snapper and sea bass as “fjord-like” in that it has a 50-70 m sill that (Vargas 1995, Vargas and Mata 2004). A tro- separates the deep (>200 m) inner basin from phic model is available for this ecosystem the open ocean. The depth of the inner basin (Wolff et al. 1998, Wolff 2006). There is also a combined with this shallow sill limiting water small trap fishery for the blue crab Callinectes exchange with the ocean causes anoxic condi- arcuatus and, of course, bivalve harvesting tions within the gulf to occur at depths of 60 m from intertidal areas. The finfish and shrimp and below (Hebbeln et al. 1996). It is the only fisheries have been in decline for the past 20 such embayment with these hydrographic char- years as the number of fishermen and fishing acteristics in the tropical Americas and one of boats has been increasing steadily (Vargas only four known in all of the tropics (Richards and Mata 2004). In response to this, the Costa 1965). Surface salinity in the small estuaries Rican Fisheries Institute (INCOPESCA) has within Golfo Dulce ranges from 0 to 25 ‰, instituted a yearly 3-month ban on finfish fish- lower on average than in the Gulf of Nicoya ing and a complete ban on shrimp trawling in due to higher annual rainfall (200 mm/month the upper gulf.