Otolith shape analysis as a tool to infer the population structure of the blue jack mackerel, Trachurus picturatus, in the NE Atlantic

The blue jack mackerel, Trachurus picturatus, is an economically important fishery resource of the NE Atlantic, commonly captured around the Macaronesian islands of Azores, Madeira and Canaries, but also along coastal Portugal mainland. Despite its importance for the fisheries industry, information regarding stock delineation for management and conservation purposes is still scarce. This study examines whether otolith shape is efficient to infer about the population structure of the species and if the obtained data is consistent over time. Individuals of T. picturatus were collected in 2013 and 2016 from six important fishery regions in the NE Atlantic: Azores, Madeira, Canaries and Portugal mainland Matosinhos, Peniche and Portimao. Otolith shape indices and elliptic Fourier descriptors were investigated by uni and multivariate statistical analyses. Data showed regional differences in the otolith shape analysis, with differences between the years. Fish sampled in 2013 showed an overall low re-classification rate (43%) with a high overlap of individuals within sampling locations, suggesting a single population unit. However, samples from 2016 allowed the discrimination of three distinct spatial groups with a moderate overall correct re-classification (62%) - group 1 with samples from the Azores; group 2 with samples from the Canaries; and group 3 with samples from Madeira and Portugal mainland. The hereby data suggests that fish population structure could be dynamic over time, recommends a year-classes follow-up of the population units in the main fishing areas and stresses the need of holistic methodological approaches to study fish stocks. Furthermore the hereby observed complex spatial dynamic structure of the species demands region specific requirement actions to ensure a rational management of the fishery.