Growth parameters (L∞, K, t0)  and natural mortality of the Patagonian sprat (Sprattus fueguensis) Chiloé inland sea were estimated. This area corresponds to the northern limit of the species distribution in the South Pacific Ocean. A total of 1,584 otolith were analyzed between July 2005 and December 2006. A covariance analysis ruled out significant differences (P = 0.654) in the otolith size-fish length relationship between males and females. The otoliths showed high monthly proportion of hyaline edge type in August 2005 and from July to October 2006. The hyaline band formation from mid-winter to spring coincided with the breeding season extending from September to December. This suggests an annual periodicity of annuli formation, likely as a consequence of channeling energy into reproduction. The growth parameters were L∞= 17.71 cm; K=0.78 yr-1; t0 = -0.46 yr,  while the maximum observed age was 6 years. Natural mortality given growth parameters and species maturity, was estimated at 1.0 yr-1. The modal decomposition of monthly length distribution, analyzed by a statistical-numerical algorithm (MIX approach) gave similar results to those obtained from otolith readings.