Stereology is the tridimensional interpretation of bidimensional sections of a structure, widely used in fields such as mineralogy, medicine, and biology. This paper proposes a general software to do stereological analysis, called STERapp, with a friendly graphical interface to enable expert supervision. It includes a module to estimate fish fecundity (number of mature oocytes in the ovary), which has been used by experts in fish biology in two Spanish marine research centers since 2020 to estimate the fecundity of five fish species with different reproductive strategies and oocytes characteristics. This module encloses advanced computer vision and machine learning techniques to automatically recognize and classify the cells in histological images of fish gonads. The automatic recognition algorithm achieved a sensitivity of 55.6%, a specificity of 64.8%, and an average precision of 43.1%. The accuracies achieved for oocyte classification were 84.5% for the maturity stages and 78.5% for the classification regarding presence/absence of the nucleus. This facilitates the analysis and saves experts’ time. Hence, the SUS questionnaire reported a mean score of 81.9, which means that the system was perceived from good to excellent to develop stereological analysis for the estimation of fish fecundity.

Fish fecundity is one of the most relevant parameters for the estimation of the reproductive potential of fish stocks, used to assess the stock status to guarantee sustainable fisheries management. Fecundity is the number of matured eggs that each female fish can spawn each year. The stereological method is the most accurate technique to estimate fecundity using histological images of fish ovaries, in which matured oocytes must be measured and counted. A new segmentation technique, named the multi-scale Canny filter (MSCF), is proposed to recognize the boundaries of cells (oocytes), based on the Canny edge detector. Our results show the superior performance of MSCF on five fish species compared to five other state-of-the-art segmentation methods. It provides the highest $F_1$ score in four out of five fish species, with values between 70% and 80%, and the highest percentage of correctly recognized cells, between 52% and 64%. This type of research aids in the promotion of sustainable fisheries management and conservation efforts, decreases research’s environmental impact and gives important insights into the health of fish populations and marine ecosystems.

Previous studies have highlighted possible cryptic biodiversity in the genus Neoscopelus. This hypothesis was tested using new morphological, molecular and biogeographical data on species of this genus caught in the north Atlantic between 2010 and 2022. The information obtained has been combined with available data in an integrative approach, including a review of morphological characters reported in the ichthyological literature and DNA-based species delimitation analyses. The main outcome of the present study is the description of Neoscopelus serranoi sp. nov. from the Atlantic and southwestern Indian Oceans. The new species is morphologically very similar to Neoscopelus microchir from which differs in having a shorter anal-fin base, a shorter pelvic fin, more dorsal and pectoral-fin rays, less anal-fin rays, fewer gillrakers and fewer isthmus and lateral photophores. They also differ in geographic distribution, with the new species occurring in the Atlantic Ocean and the near southwestern Indian Ocean, whereas N. microchir was originally described from Japanese waters of the Pacific Ocean. A literature review of available morphological data between geographic areas for Neoscopelus macrolepidotus and Neoscopelus microchir showed a large intraspecific overlap and no boundaries. However, molecular species delimitation based on the mitochondrial COI gene revealed the existence of cryptic diversity in both species, with eight to ten molecular operational taxonomic units (MOTU), compared to three valid species. Neoscopelus serranoi sp. nov. was considered an independent MOTU in all analyses performed, supporting the morphological identification as a new species. These results highlight that the taxonomy of Neoscopelus is far from settled and show that a greater sampling effort is needed to resolve the uncertainties and to describe unknown putative species. This also exemplifies the virtues of integrative taxonomy in delving into the systematics of deep-sea fishes.

The Galician shelf (northwestern Iberian Peninsula) is a highly dynamic area with an important multi-species fisheries industry that exploits resources from several habitats, characterized by being not only highly diverse, rich, and productive but also seasonally and interannually variable. Early life stages of different species are distributed throughout the year, with fluctuating abundances and community composition. Likewise, the influence of environmental factors and processes on larval production and survival remains unknown. Sampling was carried out in July 2012, and all the larvae obtained were identified to establish the specific composition of the community in a summer upwelling scenario. The results show no zonation in the species distribution, a consequence of the mixing effects of the upwelling and eddies, with high diversity but low abundance, which render in a slight predominance of a few species. Due to the dependence of planktonic populations on upwelling events, which was not highly pronounced in 2012, we cannot conclude that this was a typical conformation of the Galician summer larval fish community, but it is a first approach to comprehend the community composition.

The large estimates of mesopelagic fish biomass have long fuelled harvesting interests in the relatively untouched twilight zone of the ocean. The silvery lightfish—one of the most abundant species inhabiting the North Atlantic mesopelagic layer—is a candidate for such a fishery despite its enormous ecological importance and the insufficient knowledge about its population genetic structure. To address this knowledge gap, 863 individuals sampled across the North Atlantic Ocean and into the Mediterranean Sea were genotyped using a panel of 170 genome‐wide SNP loci. Analyses revealed habitat‐driven differentiation into three main units: Mediterranean Sea, oceanic samples, and Norwegian fjords. These groups were not completely isolated from each other as introgression from the Mediterranean Sea was detected in the Eastern Atlantic façade extending from Moroccan waters northward to 47° N, within an otherwise genetically homogeneous oceanic cluster. The complex topography of the Greek Seas seemed to shape the genetic structure in the Mediterranean Sea whereas along the Norwegian coastline, sills did not appear to hinder genetic exchange among fjords ranging 200 km apart, likely reflecting a combination of the position of the species in the water column and its swimming ability. This genetic information should be integrated with ecological and demographic properties to outline the management boundaries of this species prior to any eventual fishery attempt.

This study investigates the reproductive biology and growth of Notacanthus bonaparte based on 158 specimens collected in October 2023 from the Porcupine area, NE Atlantic. The specimens measured between 172 and 495 mm total length. Sagitta otoliths were extracted, photographed, measured and sectioned, revealing age estimates ranging from 4 to 20 years. The growth rate coefficient (k) was calculated as 0.05±0.02 (1 year–1). Gonads were dissected and analysed histologically, confirming that Notacanthus bonaparte in the NE Atlantic is an iteroparous species with group-synchronous oocyte development. Length at maturity (L50) was estimated to be 280 mm of total length for both sexes combined. The parameters of the length-weight equation were a=15.15 and b=3.32, indicating positive allometric growth. This study offers new insights into the life history of Notacanthus bonaparte and provides valuable information on the population dynamics of this deep-sea species.