Evidence of a relationship between age and otolith weight in the Chilean jack mackerel, Trachurus symmetricus murphyi (Nichols)

The relationship between age and otolith weight was examined in the Chilean jack mackerel, Trachurus symmetricus murphyi (Nichols). The purpose was to present evidence for the relationship in order to obtain a rapid and inexpensive method of ageing individuals of T. s. murphyi. The evidence of the otolith age-weight relationship is presented according to: (a) the persistence and progression of modes in otolith weight frequency distributions of fishery-caught individuals during 1996, and (b) establishing a direct relationship between the otolith weight and age. The fish were aged using the density of daily growth increments, while the number of modes in the otolith weight frequency data (OWFD) were analyzed using MIX and MULTIFAN computer software. According to MIX, there were between 8 and 10 modes in the OWFD and the average otolith mass growth rate (mg yr(-1)) was 8.256 mg yr(-1), under the assumption that modes in the data represent year classes. Using MULTIFAN, the best fit was a 10 age-class model and the otolith mass growth rate ranged between 7.22 mg yr(-1) in the first two age classes and 7.07 mg yr(-1) in the last age classes. The estimated ages of 52 fish ranged between 1.4 and 12.7 yr, and the relationship between otolith weight and age was linear and statistically significant (r(2) = 0.42, n = 44, P < 0.05). The inverse relationship, using the otolith weight as the independent variable, was also linear, but the intercept was statistically equal to zero, The slope of a regression forced to pass through the origin was 0.119 yr mg(-1) with a 95% confidence interval between 0.104 and 0.134 yr mg(-1). We concluded that the results support the idea that otolith weight is related with the age of jack mackerel and that it may be a useful tool of rapidly estimating the age of individuals and probably the age structure of jack mackerel. However, alternative methods of ageing should be applied because real ages of jack mackerel are far of clear. (C) 2001 Elsevier Science B.V. All rights reserved.