Abstract
Maurolicus muelleri is an important component of mesopelagic ecosystems; nevertheless, we possess only limited knowledge about its biological features. We collected samples of M. muelleri from six scientific surveys between 2019 and 2021 in three geographical areas in the Northeast Atlantic waters (from South to North): the Bay of Biscay, the Celtic Sea, and the Norwegian Sea. Geographical variations in otolith growth, fish age, length, weight, and condition, as well as length–weight relationships and von Bertalanffy growth models (VBGMs), were investigated. Length-weight relationships revealed differences associated with the fish’s origin, paralleling the annual and daily otolith growth. VBGM parameters increased progressively northwards, in accordance with Bergmann’s rules. Fish length was positively related to the otolith radio, and Lee’s phenomenon was undetected. The impact of environmental variables, such as temperature and food availability, is debated, with these considered potential drivers of this variability. Populations may belong to separated units, either genetically or morphologically, representing differences in biological parameters as a signal of geographical divergence.