Growth and population structure of Argopecten purpuratus at La Rinconada Marine Reserve, Antofagasta, Chile

Abstract

Continuing studies at our Antofagasta (Chile) laboratory provided data from March 2001 to May 2003 on population parameters of Argopecten purpuratus in the scallop bed occupying the nearby La Rinconada Marine Reserve. Population growth parameters of L_∞ = 120.38 mm and K = 0.9668 were obtained using marking and recapture methods. The high recapture percentage (60–80%) of marked individuals indicates a low natural rate of mortality, as well as a sedentary behavior. Annual population estimates confirmed the sedentary behavior of the scallops, with most of the individuals concentrating towards the center of the bed at densities between 9–14.8 ind m^–2. Scallop abundance showed interannual variation, with total individual counts ~10.1 × 10⁶ in 2001, 8.2 × 10⁶ in 2002, and 12.4 × 10⁶ in 2003. Evaluations showed a reduction in mean size within the population from one year to the next, the mean length of 75.8 mm in 2001 decreasing to 62.1 mm in 2002 and to 51.7 mm in 2003. This decrease also occurred in the number of individuals of legal extraction size (90 mm), from 2.6 × 10⁶ individuals in 2001 to 3.7 × 10⁵ in 2003. Application of the growth parameters to the population structure in the bed showed recruitment of a new year-class of 6.5 × 10⁶ individuals in 2002 and 11.2 × 10⁶ in 2003. Subtraction of these numbers from the total numbers present in both years showed losses of about 8.4 × 10⁶ individuals in 2002 and 7.0 × 10⁶ in 2003. Based on the population growth parameters and supported by analysis of the catch curve using the Fisat program, these losses concerned the size range of scallops over 60 mm in length with a mortality Z = 2.15 in 2002 for individuals ranging from 67.5 to 115.5 mm in length and Z = 3.08 in 2003 for those ranging from 64.3 to 112.5 mm. The results suggest that the loss of scallops from these size ranges was due to human intervention (clandestine harvesting), also reported by wardens in the field. Illegal harvesting of this resource contravenes Chilean national legislation designed to conserve A. purpuratus and maintain its rational exploitation.