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 inexp ensive method of ageing individuals of T. s. murphyi. The evidence of the o tolith age-weight relationship is presented according to: (a) the persisten ce and progression of modes in otolith weight frequency distributions of fi shery-caught individuals during 1996, and (b) establishing a direct relatio nship between the otolith weight and age. The fish were aged using the dens ity of daily growth increments, while the number of modes in the otolith we ight frequency data (OWFD) were analyzed using MIX and MULTIFAN computer so ftware. 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), unde r the assumption that modes in the data represent year classes. Using MULTI FAN, 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 lin ear and statistically significant (r(2) = 0.42, n = 44, P < 0.05). The inve rse 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 t hat the results support the idea that otolith weight is related with the ag e of jack mackerel and that it may be a useful tool of rapidly estimating t he age of individuals and probably the age structure of jack mackerel. Howe ver, alternative methods of ageing should be applied because real ages of j ack mackerel are far of clear. (C) 2001 Elsevier Science B.V. All rights re served.