Comparison of age readings by two otolith preparation techniques and readers

During the 2012 CCAMLR toothfish ageing workshop, otoliths from 31 Antarctic toothfish were exchanged between Russia and New Zealand ageing programmes to compare consistency in ages estimated by different readers and using different methods. Both “break and burn” and “bake and embed” techniques were used to prepare otoliths and each were read by an experienced reader from each program. The resulting four-way comparison enabled differences in preparation method to be distinguished from differences in interpretation of otolith banding patterns. Results suggest broad agreement in ages determined by each reader and with each method. However, there were enough inconsistencies in preparation technique and in interpretation of the break and burn preparation method to warrant further coordination and comparisons before merging data. Bake and embed preparations were consistently interpreted by both readers. This experiment highlights the importance of monitoring and comparing ageing protocols within and between fish ageing programmes. The criteria for determining similar age interpretation used here (mean paired age difference statistically equal to zero, overall CV less than 10%, and slope of regression across ages equal to 1) were useful diagnostics in interpreting age comparisons.