Abstract

In central-south Chile, the jack mackerel (Trachurus symmetricus) resource conforms disperse aggregations in a reproductive condition in oceanic waters, and obtaining representative samples to estimate populational. attributes as size composition and mean size is difficult. In this paper, the precision of these attributes was analyzed through resampling techniques and the effective sampling size for jack mackerel was determined for the major reproductive activity in oceanic waters by using data from spawning surveys carried out in 2000 and 2001. In the 2000 survey, the observed sampling size was 1109 individuals and the effective sampling size was estimated in 1096 individuals. The average length was 27.3 cm, with variance of 0.17 and confidence intervals at 95 % between 26.7 y 27.8 cm. In 2001, the observed sampling size was 1370 individuals and the effective sampling size was 355, with an average size of 28.2 cm, variance of 0.58 and confidence intervals at 95% between 27.6 and 28.9 cm. It is concluded that the populational attributes of jack mackerel were correctly determined in terms of their precision, describing satisfactorily the length composition in the area and period of the studies.