The European anchovy ( , Linneaus 1875) is a coastal pelagic species of great importance. Not only does it support an important fishing industry, but it is also fundamental in marine growth chains. The characteristics of this small fish are: a short life cycle, seasonal migrations, high growth rates, first reproduction at an early stage, a long egg-laying period and schooling behaviour (shoal or group). All this gives rise to considerable inter-annual fluctuations – given the variability of stock replacement by juvenile fish, these being highly vulnerable to environmental conditions. European anchovy catches have dropped alarmingly over the last few decades, going from 83,615 tons in 1965 to 4,456 tons in 1982 and only 950 tons in 2005. This is what Pablo Cermeño Villanueva focuses on in his PhD, undertaken at the Faculty of Science and Technology of the UPV/EHU, research work that provides a tool to determine the age of anchovies with greater accuracy on a monthly or even weekly basis, as opposed to merely annually, and enabling studies to be undertaken of the earliest phases of life, which are those that most bear on survival and, thereby, recruitment. The PhD, entitled was led by Doctor Beatriz Morales-Nin of the Institute for Advanced Studies of the Mediterranean (IMEDEA) and developed by the AZTI-Tecnalia Foundation under the leadership of Mr Andrés Uriarte. The PhD was defended at the Department of Zoology and Animal Cell Biology, obtaining summa cum laude. Pablo Cermeño Villanueva is Doctor in Biology from the University of the Basque Country and President of the Basque AMBAR (Society for the Study and Conservation of Marine Fauna) and works for the World Wildlife Foundation (WWF). The application of daily growth techniques enables the discovery of aspects about growth and its relationship with the environment. The development of the technique in this current research will enable a clarification of when and how stock replacement by juvenile European anchovy in the Bay of Biscay occurs and understanding of the relationship between growth and mortality in the first phases of the development of the species. Determining the age in days of the juveniles enables obtaining the distribution of these juveniles or survivors of the larval stage according to their month of birth; comparing this data, not only at an inter-annual level, but also showing the variations produced throughout the year their relation with the oceanographic conditions present at the different periods of the year. The aim of the PhD thesis was to develop and apply this technique to the study and management of the European anchovy. To this end and by means of a series of experiments using chemical marking during captivity, for the first time the daily formation of growth rings in this species has been validated. The chemical marking was carried out, at least during short periods of captivity, using the soluble complex of calcium that both juvenile and adult anchovies deposit as a daily ring of growth in their otoliths. The daily growth rings deposited in the otoliths form a complex pattern of daily growth which were analysed using both scanning electron microscopy (SEM) as well as optical microscopy with image analysing. In this way, a reproducible methodology for measuring age in days is proposed and which can serve as a guideline for further or other research. Thus, not only the distribution of survivors over a specific year is obtained, but also - and for the first time ever – a growth curve for the first months of life of the European anchovy.