It is shown that in Peter the Great Bay, the southern part of the range of the threaded sculpin Gymnocanthus pistilliger, its maximum size is larger than in the more northern waters off western Kamchatka. Reliable records of the species in trawl catches begin when individuals reach an absolute body length of 20 cm. Since males reach smaller sizes than females, their records, especially in the north, are less comprehensive. The growth rates of fish in Peter the Great Bay are higher, which is why the fish of the same age are larger here than in the waters off the Kamchatka Peninsula. The weight of individuals of the same size in the bay is also larger. The best growth rates in the area located to the south is determined by more favorable conditions of the aquatic environment. Despite the differences in growth rates, the maximum age of fish reached in populations is similar. The sex ratio with a predominance of females is also similar.

On the background of water structure, general patterns of the sandfish Arctoscopus japonicus distribution in Peter the Great Bay are determined on the data of bottom trawl surveys in spring, summer and fall seasons. When the larval stage is finished, the underyearlings of sandfish appear in the trawl catches since July. Depth of their catches by bottom trawl in July-August is 23–60 m, under the temperature of 8–16 о С. Localization of the catches at the upper benthic front corresponds to their pelagic spreading with concentration on the seasonal thermo-pycnocline, so the belts of high catches by bottom trawl are located along the line of the thermocline contact with the bottom. They convert to bottom dwelling in fall season, being concentrated below the benthic front and shifting deeper gradually with the front. The adults dwell at the bottom, permanently in the subsurface waters. Their feeding starts near the coast in May-June when they aggregate below the upper benthic front, avoiding the remnants of the cold Bottom Shelf water mass. Under thickening of the warm surface layer in July-September, they retreat deeper and spread widely at the shelf bottom within the area occupied by the Subsurface Shelf water. Pre-spawning aggregations of adult sandfish are formed at the shelf edge and upper continental slope in October, whereas the non-spawning fish stay in this month on the shelf together with the underyearlings. The adults migrate to the coastal zone in November for spawning on seaweeds. The southern part of the bay is presumably more favorable for the spawning of sandfish, as the underyearlings are absent in its northern part in summer. The sandfish adults wintering is available outside the bay, so the southward migration is supposed in late autumn and the backward migration in spring.

The Elegant sculpin Bero elegans is a plentiful but little-studied fish species that lives in the coastal shallow waters of the north-western part of the Japan Sea. In the study reported here, we assessed for the first time the life span, growth pattern, and age at maturity of this fish, based on growth and maturation observations and an analysis of the structure of otoliths from fish captured in Paris and Zhitkov bays, Russky Island, Peter the Great Bay, Sea of Japan in the period 2018–2022. The life span is estimated to be 47 months for females and 40 months for males. Males generally grow faster than females, but groups of slow-growing and fast-growing males that form during maturation are found. Both males and females spawn for the first time at 2 years of age. The von Bertalanffy’s growth function with three constants was used to calculate growth ( L t = estimated length as a function of time t [months]; L ∞ = theoretical asymptotic length; t 0 = theoretical time at 0 length; k = constant of linear growth), with the following values obtained: L ∞ = 176.294, k = 0.033436, t 0 = − 2.3394 at R = 0.95 for males; L ∞ = 118.389, k = 0.0536, t 0 = − 2.3464 at R = 0.94 for females (where R is otolith radius).

We identified Gymnocanthus detrisus as the absolute dominant species in the taxocene of sculpins (family Cottidae) caught below the 20-m depth in the aquatic area between Cape Povorotny and Cape Mapatsa. Myoxocephalus polyacanthocephalus, Gymnocanthus herzensteini, Enophrys diceraus, and Triglops pingelii are dominant as well. Subdominant species are Triglops septicus, Icelus cataphractus, Hemilepidotus gilberti, Myoxocephalus brandtii, Myoxocephalus jaok, Taurocottus bergii, and Gymnocanthus pistilliger. Other types of fish identified in this area are insignificant in abundance. Sculpins aggregate at greater depths in the southern aquatic areas, from Cape Povorotny to Olga Bay, and from Olga Bay to Cape Belkin, than in the northern areas, between Cape Belkin and Cape Zolotoy, particularly between Cape Zolotoi and Cape Mapatsa. We attribute this phenomenon to the differences in water regimes. The highest densities of sculpins are generally typical for the southern area. Conversely, the lowest densities are typical in the northern area. However, some species prefer northern areas. The minimum depths of the habitat for different species vary from less than 20 m to 82 m; the maximum depths range from 80 m to 530 m. Due to the high species diversity, aggregations of sculpins are observed in a wide bathymetric range from 20 to 300 m, with peak values at depths of 150–200 m. Sculpins live in summer at water temperatures above 0.4ºC. The shallow water species of sculpins tolerate much greater temperature ranges then deep water species. Hence, the stenotherm increases in deep water species.

For the first time, we report the finding of egg masses and an egg-guarding male of the whip sculpin Gymnocanthus intermedius (Temminck et Schlegel, 1843) in Peter the Great Bay (Sea of Japan). Bottom eggs were found in the post-wintering period during a scuba dive at a depth of 12 m. Exact data on the depth of occurrence and structure of the egg masses, the number of eggs per mass, and the sex of the guarding parent fish are provided. The characteristics of the eggs and the substrate on which the egg masses were laid are described. The diagnostic features of the male that allowed it to be assigned to G. intermedius are given.

— The rare species Icelinus pietschi Yabe, Soma et Amaoka, 2001 was first discovered in the Russian waters in the Sea of Japan (the Peter the Great Bay). The characters and description of the analyzed specimen are reported. The species I. pietschi inhabiting the Sea of Japan may be supposedly found from the Peter the Great Bay to the water region off the south coastline of the Sakhalin Island and along the continental coastline northward to the Vladimir Bay and southward to the northern part of the Korean Peninsula.

Two intraspecific groups of the plain sculpin Myoxocephalus jaok inhabiting the Russian territoral waters in the Sea of Japan, representing the northern and southern species groups, have been revealed. In a summer season, the species groups are separated from each other by a wide water area extending from 45° to 48° N. The northern species-group nucleus in this period is located in the Tatar Strait inlet, while the southern species group inhabits the Peter the Great Bay. In the summertime, the plain sculpin prefers the upper slope shelf area with water column depths ranged from 80 m (for the southern species group) to 60 m (for the northern species group). The juvenile fish tend to be found in shallower waters as opposed to the mature fish. In a cold season, the majority of fish stocks of the southern species group is still congregating in the Peter the Great Bay, shifting to the shelf edge and the continental slope. Some porton of male fish remains at spawning grounds in the coastal zone to the springtime to guard egg clutches. The specimens of the nothern group leave the shallow-water Tatar Bay inlet at massive level to congregate over the winter in the locations of a deep-water area off the mid-coastal zone of the Sakhalin Island. The specimens of the northern group reach the body length of 67 cm, while the sizes in the southern fish group are larger, reaching 75 cm in length.

Studies conducted in April−October showed that the Japanese sandfish Arctoscopus japonicus occurred in Peter the Great Bay in the near-bottom water layers at a depth of 1.5 to 550 m at a temperature of –0.2 to 16.5°C; however, distribution of the species varied during this period significantly. After a period of pelagic development, young fish with a length (FL) from 3.5 cm pass to the benthic mode of life and leave in July the coastal area. In summer, juveniles live at a depth of approximately 25–55 m; in the autumn, they spread over the shelf. Maturation of some individuals in the first year of life was not observed in the bay; it is typical only for the southern areas of the range. In the warm season, the majority of older fish adhere to the bathymetric range of 40–60 m and adjacent depths. In the autumn, before spawners approach the coast for spawning they descend into deeper waters and penetrate into the lower part of the shelf and onto the continental slope.

— The purplegray sculpin Gymnocanthus detrisus occurs at the depths between of 17 and 595 m at the temperature from –1.1 to+13.2°C off the mainland coast of the Russian waters of the Sea of Japan. In the warm period, the fish are concentrated in the middle and lower parts of the shelf, mainly at a temperature of 1.2–2.0°C, in the cold period, in the lower part of the shelf and the upper part of the continental slope at 0.6–1.4°C. The water area in the north (the waters of the Tatar Strait) is intensively used only in the warm season. The main aggregations are formed in the south (Peter the Great Bay) and in the areas of the central part of the surveyed region. Juveniles are more eurythermal than adult individuals and more adults prefer the shelf zone in all seasons, where the temperature regime is less stable than on the mainland slope.

General distribution patterns for japanese sandfish Arctoscopus japonicus in Peter the Great Bay in spring, summer, and fall, which are related to the water structure, are determined from the bottom trawl survey data. When the larval stage is finished, the sandfish underyearlings appear in trawl catches since July and are recorded at the depths of 23–60 m in July–August at water temperature of 8–16°С. Judging from localization of catches at the upper benthic front, they lead a pelagic mode of life in summer, concentrating above the seasonal pycnocline, so belts of high catches by bottom trawl are located along the line of pycnocline contact with the bottom. In the fall season, the underyearlings pass to bottom dwelling below the upper benthic front. The adults dwell permanently at the bottom, in the subsurface shelf waters. Their feeding starts near the coast in May–June, avoiding the remnants of the cold bottom shelf water, and they gradually retreat deeper in July-September. Prespawning aggregations are formed at the shelf edge and on the upper continental slope in October. The spawners migrate to the coastal zone in November for spawning on seaweeds. For wintering, the sandfish adults may leave Peter the Great Bay, moving to the more southern areas.

The species composition of sculpins of the genus Malacocottus in the Sea of Japan is discussed. It is shown that only Malacocottus gibber Sakamoto, 1930 inhabits this sea. The name M. microphthalmus Popov, 1933 is a junior synonym of M. gibber. A new Russian name for M. gibber is proposed: yaponomorskii myagkii bychok.

In the waters of Primorye in the spring-summer period, the Okhotsk snailfish Liparis ochotensis is distributed in the depth range from 50 (minimum analyzed) to 741 m; prefers the upper part of the continental slope (200–500 m). In spring, juvenile specimens, to a greater extent than adult fish, gravitate towards shelf waters, while in summer they live mainly on the continental slope. In both seasons, the Okhotsk snailfish prefers temperatures above 0.3°С: in the Peter the Great Bay, mainly to 1.2°С, in the waters of Northern Primorye to 2.0°С. The species reaches 82 cm in body length and 7.2 kg in body weight. The largest fish are found in the summer in the south of the region, in Peter the Great Bay. By the nature of its nutrition, it is a nektobentoichthyovore: decapods (74.6%) dominate the diet, mainly shrimps, fish, mysids and euphausiids play a secondary role. As the snailfish grows, the daily ration decreases from 4.5 to 1.6% and, on average, in individuals 11−70 cm in length it is 2.0% of body weight. The seasonal dynamics of the feeding intensity is poorly expressed, which is determined by living in relatively stable abiotic conditions.

Great sculpin Myoxocephalus polyacanthocephalus is distributed in the Russian waters of the Sea of Japan at the depths between 6 and 640 m at the temperature from –1.2 to +15.5°C. Its seasonal migrations are mainly bathymetric. The latitudinal migrations are observed only in the northwestern part of the Tatar Strait, and they have a southern direction in the fall and northern direction in the spring. In the winter, the majority of great sculpins avoids negative water temperature in the upper part of the shelf and prefers its lower part and upper areas of the continental slope. In the summer, the fish are concentrated in the upper and middle parts of the shelf. The juveniles are more eurythermal than adult individuals, and they are distributed in the shelf zone over the entire year.

The results of the study on the age structure of the population of the mud skate Bathyraja taranetzi from the Pacific waters off the northern Kuril Islands are presented. The modified method of staining the preparations of mud skate vertebrae with Harris hematoxylin, which significantly improves the distinctness of annual growth increments compared to the previously used ones, has been tested. Approximation models are proposed that make it possible to identify the true annual growth increments on the vertebrae, and consider the relationships between the radius of rings and age, body length and age, body weight and age at the time of formation of the annual increments. According to our data, the age of maturation of individuals of the mud skate turned out to be higher than that known for the population in the eastern part of the Bering Sea and also estimated with vertebrae, and significantly exceeded the results of age determination with caudal thorns obtained for the western part of this species range. It turned out to be about the same for males and females, 12–14 years. The estimated lifespan of individuals of this species was also significantly higher than previously believed.

A spawning ground of the Mud Skate Bathyraja taranetzi and the Okhotsk Skate B. violacea was recorded for the first time from Pacific waters off the northern Kuril Islands. It was located at 50°25′7″ N, 157°22′7″ E, and a depth of 350–351 m; the water temperature was 3.8°C. At the spawning ground, egg cases and recently hatched juveniles were found in large numbers.

— In the summer period of 2017, the threaded sculpin Gymnocanthus pistilliger occurred at depths from 14 (minimum depth of trawling) to 64 m in the Sea of Okhotsk off the western coast of Kamchatka. Areas of the maximum G. pistilliger concentrations are only partially similar in terms of fish mass and abundance. The maximum average catches in terms of biomass were recoded between 55° and 56° N, where a high abundance of the species was also recorded. The maximum average catches in terms of abundance were obtained southerly, between 53° and 54° N. The species is the most common and abundant to the depths of 40−50 m. The length of specimens varies within 6−26 (on average 18.3) cm. The ratio of males to females is 0.4 : 1. The smallest sizes of specimens of both sexes were recorded in the depth range of 14−20 m; the average sizes of males and females increase with depth. In the summer period, G. pistilliger occurs in a wide temperature range between 0.4 and 12.1°С on the West Kamchatka shelf. The highest catches of the species were recorded at the temperature of 1.0−3.5°С in the main part of the bathymetric interval, and at 3.5−6.0°С in a shallow zone. According to the results of summer surveys off the western coast of Kamchatka the estimated abundance of G. pistilliger was 146 million ind., and the biomass was 11200 t.

In the northwestern part of the Sea of Japan, hunchback sculpin Malacocottus gibber occurs in the depth range from 160 to 940 m (maximum trawling depth) at a water temperature of 0.1–2.6°C except its atypical capture at a depth of 79 m. In the southern area (below 47°10′ N), it prefers the depth range of 300–500 m in the warm season and depths over 400 m in the cold season. In the northern area (above 47°10′ N), it tends to live at depths from 500 to 700 m throughout the year; in the cold period, its density increases in this area as a result of M. gibber approaches from smaller depths. M. gibber forms denser concentrations and begins to occur at a shallower depth under higher temperature conditions in the southern area than in the northern one. Its maximum length is 31 cm. It reaches maturation at a length of 15–18 cm; the proportion of males (in particular, adult individuals) is higher in this size group than among juveniles. On the whole, the population is dominated by females at a sex ratio of 1.5 : 1.0. M. gibber is a euryphage that feeds mainly on northern shrimp, squids of the genus Gonatopsis, polychaetes, gammarids, and euphausiids. In the spring–summer period, the average daily ration of individuals 11–25 cm long is 2.5% of body weight; no significant size-age and seasonal dynamics is observed in the feeding intensity.

It was revealed that the southern border of the range of spectacled sculpin Triglops scepticus runs in the region of 37°23′ N. It has been confirmed that the southern border of the range of Jordan’s sculpin T. jordani is located in the South Korean waters; of ribbed sculpin T. pingelii range, in the North Korean waters. In the Russian waters of the Sea of ​​Japan, Jordan’s sculpin occurs at depths of 14–230 m and water temperature of –1–13.8°C; ribbed sculpin, 30–424 m and –0.3–9.7°C; spectacled sculpin, 57–586 m and 0.3–4.3°C, respectively. In the warm period, the concentrations are somewhat shifted towards shallow waters; in the cold period, towards deep waters. The body length and weight of one-sized individuals increase with the degree of deep-water preference. The body length of Jordan’s sculpin reaches 20 cm; of ribbed sculpin, 23 cm; of spectacled sculpin, 26 cm. In all three species, females are larger than males. Females dominate populations of Jordan’s and ribbed sculpins while in spectacled sculpin the share of males is higher. In the northwestern part of the Sea of Japan spectacled and ribbed sculpins are planktivores consuming mainly hyperiids and euphasiids while Jordan’s sculpin is nectobenthivore with food dominated by mysids and small shrimps. During the spring—summer period mean values of daily rations of adult specimens of Jordan’s, ribbed and spectacled sculpins are 3.3, 2.4 and 1.8% of body weight, respectively. The biomass of g. Triglops species in the Russian zone of the Sea of Japan is about 2100 tonnes.