Applied research to fisheries resources has the mission to bring new insights into the biology and dynamics of exploited populations, so that we approach as much as possible to the actual condition in which these resources are found. This requires knowing the life cycle and other biological parameters which characterize each exploited species or stock. Thus, biological aspects related to reproduction, growth and population structure provide the basic information to correctly assess and diagnose the condition in which the resources are. Yet, despite the enormous effort to obtain reliable parameters, there are still certain biological uncertainties in some species. Because of this it is often necessary to assume values of certain parameters that are not duly verified. European hake (Merluccius merluccius; Linnaeus, 1758) is a clear example of a species that presents serious difficulties when estimating its age and so this is the subject of ongoing studies.