Population structure of the chub mackerel (Scomber colias) in the NE Atlantic inferred from otolith elemental and isotopic signatures

The Atlantic chub mackerel (Scomber colias) is one of the main small pelagic fish resources in the North-East (NE) Atlantic. At present, for fisheries management purposes, a single stock of this species in the NE Atlantic is considered. Over the last years, fishing pressure has greatly increased in this species, but the information regarding its population structure, necessary in order to maintain a sustainable fishery, is still limited. One hundred and eighty individuals of S. colias were collected off the Portugal mainland (Matosinhos, Sesimbra and Portimao) and in the Atlantic Oceanic Islands (Azores, Madeira and Canaries) from January to March 2018. Following age estimation by counting the annual growth increments, 30 pre-selected individuals of the same age group (2 years old) from each sampling location were used for further otolith chemical analyses. Data were analysed by univariate and multivariate statistics to assess the degree of separation between the individuals of these geographic regions. Overall reclassification accuracy rate, obtained from a quadratic discrimination function analysis, was 80 % suggesting that fish passed through their life enough time in heterogeneous water masses to have a distinct chemical signature. Moreover, fish from the islands were fully discriminated (100 % reclassification rate). Regional differences were driven mainly by Sr:Ca, Na:Ca, Ba:Ca, Mg:Ca, Li:Ca, Mn:Ca, delta C-13 and delta O-18, showing a clear separation of four population-units of S. colias in the NE Atlantic, namely Azores, Madeira, Canaries and Portugal mainland. The hereby results provide support for treatment of these fisheries as different regional management units.