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Harvesting the beach clam Tivela mactroides: short and long-term dynamics

Abstract

Small-scale fisheries are frequently overlooked for research and management, and their social and environmental impacts are often overlooked as well, preventing the implementation of appropriate actions for their sustainability.Additionally, the dynamics of beach clam fisheries and their importance for local communities are not well understood. A study on the population biology of the clam Tivela mactroides in Caraguatatuba Bay, southeasternBrazil, revealed intense harvesting of this resource by both residents and tourists. To assess the extent and dynamics of clamming, the number of harvesters was recorded during the course of the day in vacation and nonvacationperiods throughout 2003-2005 and 2007-2008; the number of other beach users, weather conditions, and tide height were also recorded. The overall amount of clams harvested was estimated based on censuses of clammers and interviews to calculate the amount of clams harvested per collecting event. The intensity and dynamics of theharvesting activity varied on all the temporal scales evaluated. The estimated amount of clams harvested per year decreased from the first (24.6 kg/year) to the second (8.8 kg/year) group of sampling years, presumably due to clammass-mortality events and smaller shell sizes in 2007-2008, although clam abundance increased enormously. Vacation periods (presence of tourists) influenced the number of harvesters and the daily dynamics of clamming activity, although this relationship was only evident during 2003-2004. The number of harvesters increased with the number of tourists, except in periods of very high tourist activity, when harvesting decreased. Clamming was more widespread during the day under high tourist activity but during nonvacation times was concentrated in morninglow-tide periods. Weather had a partial influence on clamming, with harvesters absent only during intense rain occasions. The understanding of the dynamics of this Tivela mactroides fishery highlights key points for planning andimplementing management measures, which will involve continuous monitoring of stocks, harvesting, and food safety. (AU)

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