Ecology, culture and utilisation of the mussel, Brachidontes recurvus (Rafinesque), in the context of an integrated management approach to Boca del Rio-Mandinga estuarine system, Mexico

Abstract

The ecology and culture potential of the mussel, Brachidontes recurvus (Rafinesque) was investigated in the estuarine system of Boca del Rio-Mandinga, Veracruz, Mexico in order to establish the basis for its possible cultivation and utilisation in the context of integrated resource management. This large ecosystem consists of three lagoons connected by channels or Esteros (total area, 2350 ha); its dynamic hydrobiological characteristics are mainly determined by the interaction of tidal cycles with the rainy and north wind seasons. This results in important spatial and temporal changes in temperature and salinity distribution in the system which influence the life cycle of Brachidontes recurvus. A decrease in salinitv and an increase in temperature triggers the main spawning period of this mussel in August, September and October, although the mussel population is reproductively active year round as shown by gonad development indices (Condition Factor and Seed Mean Gonadic Index), and the temporal occurence of mussel larvae and spatfall. Recruitment of mussel spat occurs over most of the year on the sub-tidal mussel beds, whereas spat settement on aerial mangrove toots appears to be determined by salinity stratification and substratum availability. Natural overcrowding limits growth of wild mussels and their mortality is caused by prolonged, extreme changes in salinity or by asphyxia or predation. Asphyxia kills approximately 80 % of the mussels forming aerial mangrove root clusters because pans of the clusters are knocked off by water currents and the mussels sink into the soft muddy susbtratum and suffocate. Based on the physical and physiological requirements of Brachidontes recurvus. Laguna Redonda has suitable physiographic and hydrobiological conditions for rearing mussels stocked in polypropylene onion bags suspended from wooden racks. Based on a preliminary analysis, socioeconomic factors also seem to be favourable for mussel culture. Mussel seed (30 mm) can be obtained from the natural population, using polypropylene onion bags as collectors or as by-catch from the local oyster fishery. Using this culture method, mussels can attain a minimum commercial size (50 mm) in 6 month at an avenge growth rate of 4 mm/month. Annual minimum yields of 20 tons of mussels from a one hectare culture area in the most favourable zone of the estuarine system can be expected, but production for direct human consumption may be limited at present to a demand of approximate 20 kilos/week by local restaurants. Two identified environmental constraints on commercial mussel culture are potential losses from bird predation and organic pollution caused by the discharge of untreated sewage into the estuarine system. Concentrations of coliform bacteria above the maxima recommended by international health agencies were registered in the water column and in mussel flesh analyzed during this study, especially when salinity was low. While a small volume of cultivated mussels, after proper cleaning and cooking, could be consumed in local restaurants, nutrition trials indicated that larger quantities could be used successfully as a dietary component for cultured caridean and penaeid prawns. A scheme for including mussels as part of the food chain in an integrated multispecies production system is proposed. The culture of Brachidontes recurvus should be established in Laguna Redonda, as part of this approach which includes an estuarine aquatic polyculture system (fish-molluscs-crusutceans) and an integrated fish farm operable on a family basis using traditionally reared animals and plants. Preliminary assessment indicates that optimisation of energy flow could be obtained by recycling non utilised energy produced by both sub-systems, through integration. These proposals and the information presented on Brachidontes recurvus, are provided as a baseline for the future testing of possible ecodevelopment schemes for Boca del Rio-Mandinga in order to establish a sustainable, multiple use management policy for this important tropical coastal ecosystem.