Age estimation and back-calculation of fish length of the European hake, Merluccius merluccius (Linnaeus, 1758) in Sardinian waters

Aim of the present study is to evaluate the growth parameters for the European hake, Merluccius merluccius, in Sardinian waters by means of otolith annual ring analysis, providing also an indirect validation through the use of back-calculation. A total of 283 individuals (of which 150 males, ranging from 8.5 to 42.1 cm TL, and 133 females, that ranged from 11.2 to 83.3 cm TL), collected during the MEDITS survey in 2012 and 2013, were selected for the age reading. The determination of the age was carried out on the whole otolith for fish with total length smaller than 20 cm while for the others a transversal section across the core was prepared. Hyaline rings count was performed twice by two different readers, showing good reproducibility (%CV= 13.9; IAPE= 10.69). The estimated von Bertalanffy parameters (± s.e.) were Linf= 53.26 ± 10.15, k= 0.17 ± 0.06, t0= -1.73 ± 0.26 and Linf= 102.77 ± 13.67, k= 0.09 ± 0.02, t0= -1.46 ± 0.13 respectively for males and females. Back-calculation, carried out on otolith sections through the measurement of the ventral radius of each annulus, has provided an estimation of fish size at each year. Mean length at each age class (1-6 for females, 1-4 for males) obtained with the two methods was compared individually showing no statistically significant differences (t-Student, p-value> 0.05), except for age class 1 for both sexes. This result is probably due to the difficulty, well-known in literature for the European hake, in the identification of the first year annulus, which could be confused with one of the false rings around the nucleus. Nevertheless, this procedure certainly offers a valuable mean for corroboration of the age determination when other validation analysis are not achievable