| Summary: | The blue jack mackerel Trachurus picturatus (Bowdich, 1825) specimens (N = 155) were collected during the MEDITS survey, done along the eastern side, precisely, of the Croatian fishing ground in July 2018. Biometrical analysis of ten morphometric and five meristic characters, as well as genetic analysis proved that the collected specimens were blue jack mackerel. The total length (TL) and weight (W) of all observed specimens ranged from 9.2 to 33.7 cm (12.15 ± 2.95 cm) and from 5.79 to 384.94 g (17.64 ± 39.42 g), respectively. All calculated length–length relationships were linear (r > 0.923). Sex was determined only on two larger specimens (28 cm < TL < 32.8 cm), which were females. In the length–weight relationship, positive allometry was established (b = 3.1789). Based on 37 partial cytochrome b sequences, the overall haplotype diversity (h) of 0.812 ± 0.048 and nucleotide diversity (π) of 0.0064 ± 0.0007 indicated high levels of haplotype and low nucleotide diversity. The obtained sequences were compared to previously published research within the Northeast Atlantic Ocean and the Mediterranean Sea, confirming the absence of genetic structure among these populations.
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