Validity of using translucent zones of otoliths to age the pilchard Sardinops sagax neopilchardus from Albany, Western Australia

Abstract

Otoliths (sagittae) of the pilchard (Sardinops sagax neopilchardus) from the Albany region of Western Australia were examined on a monthly basis during 1989 to 1991. When read consecutively, with the month of capture known, a clear annual cycle of an increase in marginal increments to October followed by a decline was recorded. This pattern was, however, severely distorted when samples were re-examined after having been jumbled and the month of capture concealed. Younger age groups (≤3) showed evidence of multiple rings being formed annually, but not in all years or individuals, and this was not evident in older individuals (≥4). Agreement between readings was low (43-53%), especially between different readers. Resultant growth curves showed that males grow to a smaller size (L_∞ =164, K = 0.805, Φ = 1.03) than females (L_∞ = 174, K = 0.584, Φ= 0.95). Agreement between counts of annuli and ages estimated by using otolith weight and length was only 54% for all individuals, but increased to 76% for individuals assigned the same age by different readers. Given the poorer predictions of future catch rates from the relative number of 2-year-olds in the commercial catch using annuli (implying lower accuracy) and increased complexity in preparation, otolith weights will be the method adopted for future ageing.