Age structure and growth of bluefin tuna (Thunnus thynnus, L.) in the capture-based aquaculture in the Mediterranean Sea

The principal objective of this study was to estimate the age and growth, length-weight relationships and condition factors of the Mediterranean bluefin tuna (Thunnus thynnus) in a capture-based aquaculture farm and to reveal possible differences among farmed (reared in sea cages for 18 months), fattened (reared in sea cages for 6-7 months) and wild specimens. Moreover, to determine the age composition of the fish destined for fattening and farming operations with the current lack of demographic information makes the bluefin tuna stock assessment extremely difficult. For this purpose, a total number of 2096 specimens (ranging from 102 to 295 cm in fork length and from 33 to 540 kg in round weight) were sampled, from the Greek Bluefin Tuna Farm (GR 01/2004), in the Ionian Sea (38 degrees 26' 0.07 N, 21 degrees 1' 48.85 E), during the five year period 2007-2011. In addition, length and weight literature data from wild specimens captured in the central Mediterranean Sea were analyzed. The results revealed high percentage difference in weight between wild and reared in sea cages bluefin tuna of the same fork length, which reached a maximum of 43.9% in large specimens. The condition factor of the reared in sea cages bluefin tuna ranged from 1.24 to 3.16 with a mean value of 2.04 +/- 0.19. The mean condition factor of the farmed specimens (1.92 +/- 0.17) was lower than that of the fattened specimens (2.08 +/- 0.15). This difference was consistent both in younger and older specimens in the sample. Estimated ages that were obtained using the caudal vertebrae of 619 reared in sea cages specimens ranged from 5 to 18 years. It was observed that the age of fish has a significant impact on the condition factor. The comparison among the mean condition factor values of each age class revealed that older fish (10-20 years old) present higher values than younger ones (5-9 years old). With a view to estimating the precision of the aging method, the Average Percent Error (APE) and the Coefficient of Variation (CV) were calculated (APE = 1.89% and CV = 2.46%). The von Bertalanffy growth model was fitted to mean lengths at estimated ages and the growth parameters were determined (L-infinity = 360.3 cm, k = 0.083 yr(-1), t(0) = -0.942 yr). (C) 2013 Published by Elscvicr B.V.