Analyses of morphological characters of the scaly-headed gunnels reveal that Russian waters are inhabited by only one species from this group, Pholis crassispina, while Ph. nebulosa is absent. The article also provides values of important diagnostic characters for both species and clarifies the range of Ph. crassispina in Russian waters.

This study aimed to understand the feeding habits and trophic level of the fat greenling Hexagrammos otakii off the Gangneung coast of Korea. In total, 421 individuals were collected using gillnets between September and December 2023 and between March and August 2024. The total length of the specimens ranged from 20.0 to 47.8 cm. The most important prey of H. otakii was fish, with Ammodytes personatus being the most frequently consumed species. The mean estimated trophic level of H. otakii was 4.20 ± 0.73, ranging from 3.46 ± 0.52 to 4.34 ± 0.75. H. otakii predominantly fed on fish across all seasons and exhibited an ontogenetic dietary shift in which the proportions of brachyuran crabs and polychaets in the diet decreased, while the proportion of fish increased with growth. The stomach fullness index of H. otakii was the highest in spring and showed an increasing trend with growth. Permutational multivariate analysis of variance analysis revealed significant differences in diet composition according to season and size class. However, no notable interaction was detected between these two factors.

The variety of life strategies among representatives of the genus Salvelinus suggests the existence of resident (lacustrine or riverine) and migratory forms. Resident brook populations of char are characterized by small size and short lifespan (up to 8–11 years), low fecundity, and often a juvenile (pedomorphic) appearance. Such populations are most common among the northern Dolly Varden char S. malma, less common among the southern Dolly Varden char S. curilus, and are extremely rare among the Arctic char S. alpinus. In 2024, an unusual population of dwarf char S. alpinus complex was discovered in a small watercourse in the upper Kolyma River basin. The fish were collected in shallow channels of the Enkon River in the middle of a wide, flat valley (aufeis glade) in the middle reaches of the Burkandi River at an altitude of approximately 950 m above sea level. The captured dwarf brook char specimens are characterized by extremely slow growth, a juvenile appearance, and a short lifespan compared to lacustrine stocks. Similar stocks can be expected to be found in other sections of watercourses isolated from the main channel by physical barriers. In northeastern Asia, potential habitats for similar dwarf char populations may include sections of rivers and streams located above the aufeis, which are common in this area.

Unlike a natural river, where the conditions determining the characteristics of downstream migration change gradually from upper to lower reaches (river continuum), in a regulated river, environmental barriers are formed—a reservoir and a dam—which significantly change these conditions. In such transformed systems, downstream migration is not a random but a regular process, resulting in the selective removal of migrants (emigration) from the limnic to the lotic zone. Emigration is most pronounced among fish inhabiting pelagic habitats. Littoral habitats, rich in landmarks and shelters, significantly inhibit emigration. This selectivity is determined not only by differences in the migratory behavior of fish but is largely linked to the impact of the three-dimensional hydraulic structure in the water intake zone on adjacent ecological zones of the reservoir. The altered environmental situation has different effects on the migratory behavior and distribution of juvenile fish of different taxa. Thus, the relatively similar nature of the downstream migration of juvenile Leuciscidae and Percidae in natural rivers changes significantly and differently in regulated rivers. These changes are associated with both the transformation of the hydraulic and biotopic structure of the habitat and the emergence of new environmental barriers that act as selective mortality factors. Analysis and ranking of factors influencing variability in downstream migration and emigration of juvenile fish from reservoirs provides information necessary for developing approaches and technologies for the protection and reproduction of fish populations in regulated rivers. These approaches are based on ecological methods for protecting fish during migration, the most challenging period of their life cycle.

Microplastic pollution of the marine environment is a global environmental problem requiring in-depth study, understanding of its qualitative and quantitative characteristics, the conditions promoting its accumulation, and the potential impacts on living organisms, including ichthyofauna. In total, 101 water samples were collected in 2019–2021 using a Manta net (335-µm mesh size) to assess the distribution and microplastic concentration on the sea surface of the offshore and coastal areas of the Russian Arctic Shelf. No significant concentrations of microplastics or extensive “garbage patches” similar to those observed in the Pacific and Atlantic Oceans were detected in the area under study. The average concentration of microplastics in the surface waters of the western Chukchi Sea was 0.015 particles per cubic m (pcs./m3), in the East Siberian Sea, 0.019 pcs./m3, in the Laptev Sea, 0.035 pcs./m3, in the Kara Sea, 0.036 pcs./m3, in the eastern Barents Sea, 0.039 pcs./m3. The maximum microplastic concentration in the surveyed seas did not exceed 0.201 pcs./m3, 1–3-mm particles being predominant. The most frequently recorded particles were those of polyethylene, polypropylene, low- and high-density polyethylene and polystyrene, which corresponds to the volumes of their production and application as these are most common polymers in Europe. These estimates of microplastic pollution in the Arctic waters may be conditionally considered as background (baseline) indicators. This is especially relevant facing the active development of shipping and operating the Northern Sea Route as well as for assessing potential eco-risks to aquatic organisms.

Biological parameters of the fish, gonadal state, and ultrastructure of oocyte envelopes and spermatozoa have been studied in Japanese goatfish Upeneus japonicus from Nha Trang Bay. The females are larger than the males: their body length–body weight ratios are significantly different. The ovarian state indicates continuous oogenesis. The average batch fecundity is 3301 oocytes. The females (50% of the fish) reach sexual maturity at a fork length of 104 mm. The egg envelope includes a single-layer zona radiata and a chorion. In the ovulated oocytes, their thickness is 2.0–2.5 and 0.1–0.3 µm, respectively. The spermatozoon head has a pointed bend in its apical sharpened part. Among other studied species of the family Mullidae, a similar spermatozoon structure is observed in U. tragula and U. heterospinus.

The blue runner Caranx crysos (Mitchill, 1815) is the leading fish resource for artisanal fishing in the Colombian Caribbean. We asked whether there were distinguishable stocks to generate helpful information for the correct management of the species. We used several statistical approaches to search for a consensus. Several stocks have been identified on the basis of body and otolith measurements, but the statistical approaches have not reached unanimity. Either there were two stocks, one for the Santa Marta–La Guajira area and the other for the Tolú–Urabá area, or each locality harbored a distinct stock with some overlap of Tolú fish with Santa Marta–La Guajira fish. The management of C. crysos should consider these results, as fish stocks are likely to have distinctive demographic characteristics.

This paper presents a morphological description of Myoxocephalus scorpioides from the Chaun Bay of the East Siberian Sea. A comparative analysis of the studied individuals with representatives of the species from the Bering Sea, as well as with the literature data on a combined sample of M. scorpioides from the Chukchi Sea and Bering Strait, has revealed a geographical variability of morphometric characters and a high degree of stability of a number of meristic characters (number of rays in the 1st dorsal fin and anal fin and total number of vertebrae). The significance of some taxonomic characters has been clarified. It is shown that characteristics such as the absence of postorbital and occipital tubercles, the presence of barbels in their place, the presence of bone plaques on the sides of the body, and the lengths of the upper and lower jaws, can be used for taxonomic purposes with high reliability.