Age, growth and validation of otolith morphometrics as predictors of age in the forkbeard, Phycis phycis (Gadidae)

The traditional counting of annual growth zones on thick otoliths of gadiform fish requires special processing methods and is for that reason difficult, expensive and time-consuming. Therefore, this study analysed an alternative method such as the use of otolith morphometrics as predictors of age in the forkbeard, Phycis phycis. We examined sagittal otoliths of this fish species collected from the south-eastern Adriatic Sea (Elafiti Islands) between February 2008 and February 2009. Females and males ranged in size from 19.4 to 45.8 cm and from 14.0 to 42.0 cm of the total length (LT), respectively. Ground otoliths displayed a concentric pattern of alternating opaque and translucent zones which were used to estimate fish ages. Edge-type analysis confirmed the formation of a single growth annulus per year.The relationship between the total length and age was described by the von Bertalanffy growth model: LT = 59.08[1 − exp (−0.24(t + 0.33))] for females and LT = 75.18[1 − exp (−0.15(t + 0.70))] for males. Observed maximum age for both sexes was 5 years although most of the sampled fish were 2 years old. All measured otolith morphometric parameters (length, width and mass) were linear with fish age. The linear model explained between 61.3% and 69.0% of the variation in age. The most precise age estimations of analysed population were obtained from the otolith mass, followed by the otolith width and length. Moreover, the effectiveness of the simple otolith mass measuring technique for estimating the age of studied species was the same as of the laborious preparation and reading otoliths. It should be emphasized, however, that presented models were fitted to P. phycis ages 1–5 so extrapolation of data beyond this estimated range is not recommended.

► The interpreting annual growth rings in ground otoliths of Phycis phycis is a valid ageing method.
► Thick and dense otoliths of gadiform species require special processing methods.
► Of all otolith measures, otolith mass is the most useful P. phycis age predictor (up to 5-years old fish).
► No difference was found between age structures (1–5) estimated by otolith mass and counting annuli.
► Calibrated morphometric model allows quick and precise estimation of P. phycis age structure (1–5).