Life-history traits as growth, age at first maturity, reproduction and condition as well as the size distribution of the commercial and the discarded catch, have been studied for the blue whiting, Micromesistius poutassou, from the NW Mediterranean. This is a species of commercial interest in the area. In the last decade its landings remained at very low levels and the stock has not shown signs of recovery. Samplings were conducted monthly in the northern Catalan coast on board a commercial bottom trawler, from October 2016 to October 2018. The results showed a high variability in the size at age. The size at first maturity was around 18 cm for both sexes that is achieved when the species completes its first year of life, that is, the spawning season following its birth. Females and males showed different growth curves, with females reaching higher sizes than males. From 25 cm TL females became dominant in the population. The readings of lapilli otoliths of very young individuals (from 42–87 days old) allowed an accurate estimate of the growth parameters. The diameter of the first annual ring in the sagitta otoliths was defined for the NW Mediterranean (9.8 ± 0.74 mm). The diameter of the observed two false annual rings were also provided. This study has also shown a close relationship between reproduction, condition and stomach fullness trends, which, in turn, have been observed to be related with the changes in the species growth during the year. Finally, the study allowed knowing the very high amount of blue whiting discards. The main component of catches (in numbers) corresponded to discarded individuals, and, in addition, the minimum landing size implemented in the study area is smaller than the size at first maturity.