Otolith weight as an estimator of age in the Patagonian grenadier, Macruronus magellanicus, in central-south Chile

Abstract

Is the otolith weight (OW) of Patagonian grenadier, Macruronus magellanicus, useful for estimation of age? This question is addressed analyzing the otolith mass growth rate through both monthly otolith weight–frequency data (OWFD) and the relationship between OW and observed age. Samples were obtained from the purse-seine fishery in spring and summer of 1996 and 1997 in the central-southern area off Chile (36–40°S). We considered that a constant otolith mass growth rate among cohorts is necessary to support the hypothesis that OW is directly related to the fish age. The modal structural analysis of OWFD was done by sex using the MIX computer software. The mean OW of the identified cohorts progressed and persisted along the time axis with a constant otolith mass growth rate. This growth rate was not statistically different among cohorts, nor between sexes, and the estimate for both sexes was 32.6 mg per year. The otoliths sampled in 1996 were used to determination of age by interpretation of annuli in the surface of whole otoliths. The relationship between OW and observed age was linear and statistically significant, and the slope of regressions forced to pass through the origin were 31.8, 31.0, and 31.4 mg per year for males, females and both sexes respectively. We concluded that the results support the hypothesis that OW is related with the age of M. magellanicus.