Population parameters of the trigonal tivela Tivela mactroides (Bivalvia: Veneridae) from Caicara Beach, Anzoategui, Venezuela

Aspects of population dynamics of the trigonal tivela Tivela mactroides, an important fisheries resource, were studied from October 2002 to September 2003 at Caicara Beach, Anzoategui, Venezuela. The substratum consists mainly of fine-grained sand (100-250 mu m; 80.33%). The mean density of the clam on the shoreline was 53.83 ind m(-2), but there were significant monthly differences. The population structure was bimodal, with a dominant group from October 2002 showing a high growth rate. Maximum population biomass was recorded in March 2003 (25.12 +/- 4.44 gDW m(-2)) and minimum in July 2003 (2.37 0.41 gDW m(-2)). The condition index was maximum in July 2003 (26.26 +/- 4.64%) and minimum in March 2003 (16.48 +/- 13.23%), with a monthly mean of 17.55 +/- 3.02%. The mean dry tissue yield was 23.58 +/- 5.05%. The growth parameters estimated, L-infinity = 44.32 mm and K = 0.165 month(-1), indicate an average longevity of 18 months. The strong monthly decline in density and biomass, with adult mortality of 0.20 month(-1), reveals that the population is seriously affected by human extraction, and we therefore recommend reducing the intensity of exploitation.