A phenomenon of undulating fluctuations of nekton abundance in the Kuroshio system is discussed on example of japanese sardine Sardinops melanostictus, as the most abundant and the most fluctuating species. The so-called «sardine epochs» are distinguished according to this species abundance. The last such epoch ended in the early 1990s. Since 2014, structural changes occurred again in the nekton communities of the Pacific waters off Kuril Islands, caused by expansion of southern fish and squid species, primarily japanese sardine and chub mackerel Scomber japonicus, and decreasing in abundance of japanese anchovy Engraulis japonicus and saury Cololabis saira. The scope of these changes allowed Russian fishermen to resume the fishery on japanese sardine and chub mackerel in the Russian exclusive economic zone since 2016. Annual catch of Japanese sardine increased steadily from 6,700 t in 2016 to 315,500 t in 2020. Over these 5 years, Russian fishermen landed 531,700 t of sardine and 167,900 t of chub mackerel. However, many Russian fishery forecasters believe that these reconstructions is only a «rehearsal» of the upcoming «sardine epoch», being based on formal climatic indices, without delving into the mechanisms of abundance fluctuations. The authors note that new «sardine epochs» cannot be predicted as completely similar to the previous ones. Several hypotheses on causes of the beginning and end of japanese sardine blooms are considered critically, and the conclusion is made that mechanisms which determine its year-classes strength are still unclear, as well as the reasons of undulating fluctuations of this species and some other nekton species abundance, because of high complexity of this problem.

Stocks of demersal and pelagic fish species are assessed for the bottom layer over the outer shelf and upper continental slope between Cape Olyutorsky and Cape Navarin (northwestern Bering Sea) on the data of bottom trawl survey conducted aboard RV Professor Levanidov over the isobaths 20-400 m in the summer of 2019. The total biomass of demersal fish in the surveyed polygon was estimated as 682,262 t; the portion of pacific cod was 51.50 %, arrowtooth flounder — 9.80 %, great sculpin — 9.64 %, rock sole — 4.60 %, alaska skate — 4.57 %, flathead flounders — 2.56 %, yellow irish lord — 2.30 %, and < 2 % for other species. The total biomass of pelagic fish species in the bottom layer was estimated as 759,639 t (species-specific coefficients of catchability were used); the portion of adult walleye pollock was 85.12 %, its juveniles — 9.94 %, pacific herring — 4.67 %, and other pelagic species — 0.27 % in sum. Mean ratios of the species stock between the surveyed polygon and other areas of the northwestern Bering Sea, as the Gulf of Anadyr and the deeper continental slope (below 400 m), were evaluated using the data of bottom trawl surveys conducted in 2005-2017. Some species as yellow irish lord, saffron cod, pacific halibut, arrowtooth flounder, great sculpin, kamchatka flounder, rock sole, and aleutian skate distributed mostly within the polygon and their mean biomass in the outside areas varied from 0.5 % to 69.2 % relative to the biomass in the polygon surveyed in 2019. On the contrary, the portions of pacific cod, greenland halibut, alaska skate, and walleye pollock were larger outside the polygon — from 102.9 to 190.4 %, and almost entire stocks were in the outside areas for alaska plaice, flathead flounders, and pacific herring — from 533.4 % to 1380.5 % relative to the biomass accounted within the polygon. The stocks assessed in 2019 reflected both the state of populations and their spatial and bathymetric redistribution, mostly because of the St. Lawrence Cold Water Pool shrinkage at the bottom of the Gulf of Anadyr. The stocks fluctuations are reasoned mainly by natural factors, rather than fishery impact.

Dynamics of demersal fish biomass is considered on results of trawl surveys conducted in Peter the Great Bay (Japan Sea) in 1977-2015. Fishery did not have a significant impact on this complex of fish species, including both fluctuating species (walleye pollock, arabesque greenling, pacific herring) and species with stable stock, because of low intensity. Annual landings in the bay were much lower than recommended volumes (TAC). However, a tendency to the total biomass decreasing was observed, accompanied with growth of the sculpins (Cottidae) portion in the ratio of species biomass. The sculpins prey on juveniles of many other species, so maybe their higher abundance was one of reasons for general depletion of the benthic fish community. Other predators, as marine mammals, were not abundant in Peter the Great Bay. For rational resource management, the fishery impact should be distributed proportionally between populations, taking into account their state, so both traditional commercial objects and predators with growing abundance should be landed. This ecosystem approach requires permanent monitoring to all groups of fish species.

There is believed in classical fishery ichthyology, that slower growth can be a reason for decreasing of fish abundance. A sharp decrease occurred recently in the pink salmon abundance on the eastern coast of Sakhalin Island that inspired this study of growth in the pink generations with different survival during the marine period of life. The growth rates were determined using the scale measurements for the pink salmon returned to the southeastern coast of Sakhalin in 2005-2018. The growth parameters (length increments, variability and asymmetry of size composition) were compared with the survival rate of corresponding year-classes. Slower growth coincided with lower survival for feeding fry only, i.e. in the early marine period of pink salmon life. So, the hypothesis is not confirmed about a «crucial size» for salmon juveniles that must be reached until the end of summer-autumn feeding to survive during wintering. That means that previous estimations of size-selective mortality of pink juveniles are not realistic because of both inadequate samplings and reconstructions in size composition during the juveniles growth.

The shelf and continental slope of the western Bering Sea, being among the most productive areas of the Far-Eastern Seas of Russia, became exploited by commercial fishery half a century ago, after introduction of 200-mile exclusive economic zones in 1977 and relocation of the Russian fishing fleet from the eastern Bering Sea to its western part. In 2010-2019, about 20 % of the total catch of sea fish in the Far-Eastern basin (excluding pacific salmons) were caught in the West Bering Sea fishery zone. Among the fishery districts of the Russian Far East, this area is currently the 1st one by annual catch of pacific cod, grenadiers, sculpins and sablefish, 2^nd — by catch of walleye pollock, halibuts and skates, 3^rd — by catch of rockfishes, and 4^th — by catch of pacific herring and flounders. Features of specialized fishery in the West Bering Sea fishery zone in 2010-2019 are clarified. Now 48 types of the specialized fishery can be distinguished here, while 96.3 % of the average annual landing is provided by the following 8 most important types: walleye pollock midwater trawl fishery (77.8 % of mean annual catch); pacific herring midwater trawl fishery (6.1 %); pacific cod bottom longline fishery (4.7 %); walleye pollock Danish seine fishery (2.7 %); grenadiers bottom longline fishery (2.4 %), pacific cod Danish seine fishery (0.9 %); squids bottom trawl fishery (0.9 %); and pacific cod bottom trawl fishery (0.8 %). All these types of fishery are highly specialized and portion of the target objects in the catches ranges from 76 to 96 % (according to official statistics), while the by-catch accounted as 4-24 %. The is no specialized fishery on such objects as rockfishes, arrowtooth and kamchatka flounders, and sablefish in the West Bering Sea fishery zone, but they are landed as by-catch. The fishery statistics with the data sorting to specialized fishery and by-catch can be used quite effectively for the stocks assessment and determining acceptable level of their exploitation, with recommendations for fishery regulation, including evaluation the possible level of non-specialized by-catch.

Growth of golden king crab in the northern Okhotsk Sea is considered. Parameters of von Bertalanffy growth equation were estimated using SLCA method (Shepherd's length composition analysis): the asymptotic carapace width W_∞ was 252.0 mm for males and 165.3 mm for females; the growth rate K was 0.081 and 0.130, respectively. Relative increment per molt for males with carapace width of 116-154 mm was estimated as 11.6 % for the carapace width and 10.9 % for the carapace length. Directly determined (by tagging) dependence of males molt frequency on their size was approximated by logistic curve of molting probability with the threshold of 164 mm carapace width for 50 % probability of annual molt. Mean probability of annual molt for recruits was estimated as 87 %, for pre-recruits — as 92 %. Taking into account the data on increment per molt and annual molt probability, the growth curve for males had the parameters: W_∞ = 296 mm and K = 0.073. The age of recruits was estimated as 9 years by SLCA approach and as 8 years on the results of tagging. The growth equations parameters were determined for the first time for golden king crab in natural habitat in the Russian waters. These results could be used in models and for theoretical studies of Crustacean life history.

Potential impact of landing on the stock and biological state of Tanner crab is analyzed for two fishery districts at Kamchatka in 2013-2020. In the Kamchatka-Kuril fishery subzone, the number of commercial males without limbs and with old shell increased in catches that may be associated with active operations of alive-crab vessels in 2015-2017. In the Petropavlovsk-Commander fishery subzone, cases of injured Tanner crabs catch became frequent after resumption of the crab harvesting in 2017, as well as the cases of commercial crabs with old shell, that also could be explained by operations of alive-crab vessels. The operations of such vessels are distinguished by strict sorting of caught crabs with removing the individuals of lower quality. As the result, the crabs without limbs and commercial males at late stages before molting were accumulated in the areas of sorting. The problem can be solved by measures for total registration (weighting) of catch before the start of sorting, concerning to all crabs with commercial size landed aboard. To avoid degradation of the Tanner crab population, development of catch processing aboard fishing vessels is recommended, with transportation of frozen products instead of live crabs.

Morphological signs (32 plastic and 7 meristic) and biological parameters (size, weight, age and sexual structure, fecundity) are considered for weather loaches of gen. Misgurnus inhabiting the Pad Bolshaya (Jewish Autonomous oblast), with the body length Ad 60.2-182.4 mm, body weigh 2.3-55.3 g, age 1-8 years and sex ratio about 1 : 1 (53 % males, 47 % females). Sexual dimorphism is found for antedorsal, anteventral and ventroanal distances, tail fin base length, base length and height of pectoral, pelvic and anal fins, and maximum body height. Ontogenetic variability of morphological indices in dependence on body length is detected for 26 of 32 plastic signs, whereas the dependence of anteanal, antedorsal, pectoventral, and anteventral distances, and maximum and minimum body height on the body length is not statistically significant. Males grow slower than females. Age of puberty is 4-6 years, mean individual absolute fecundity is 7409 eggs. The analysis concluded that the weather loaches Misgurnus in the Pad Bolshaya belonged to the species Misgurnus nikolskyi (Vasil’eva, 2001).

Hydrological and chemical surveys were conducted in the Uglovoy Bight in October, 2019, February, May and June, 2020 (in total 120 stations) and chemical analyses of water from 13 small rivers running into the bight were done on October 21-22, 2020. Extremely high concentration of nutrients was detected in the Peschanka, Saperka and Gryaznukha Rivers that was obviously caused by waste waters discharge. These rivers were the main source of the bight eutrophication. Within the bight, the highest anomalies of chemical parameters, as low oxygen content, low pH, high concentrations of nutrients (N, P, Si), high turbidity, and high CO₂ partial pressure were observed close to these rivers mouths, in particular under the ice in winter, when wind mixing was absent. The hypoxia disappeared in the warm period of year because of wind mixing. High concentrations of total nitrogen (10.0-40.0 μmol/L), total phosphorus (1.5-2.0 gmol/L), dissolved organic carbon (3-5 mgC/L), and chlorophyll a (0.5-2.0 μg/L) in all seasons were the results of active production-destruction processes, obviously with prevalence of organic matter destruction, since the water in the bight was undersaturated with oxygen and supersaturated with carbon dioxide — the bight accumulates and mineralizes organic matter from terrestrial and riverine discharge. Underwater photographs did not detect Zostera meadows at the bottom, which were observed in the northwestern Uglovoy bight in the past. Comparison of historical data on episodic studies in the bight with results of the surveys indicates degradation of its ecosystem, with such signs as disappearance of seagrass, hypoxia in winter, and CO2 flux into the atmosphere. Reduce in water exchange between the bight and the Amur Bay caused by construction of the underwater pipeline in 1982 and the bridge in 2012 is suggested as a reason of the degradation.

On the data of long-term surveys, mean biomass of plankton in the epipelagic layer of the Bering Sea is evaluated as 821.3 mg/m³ (1058.2 mg/m³ in shelf areas and 760.6 mg/m³ in the deep-water areas) and the stock as 245.1•10⁶ t WW (64.4 • 10⁶ t over the shelf and 180.7 • 10⁶ t in the deep-water sea). By taxa, the average annual portions are: 55.1 % for copepods, 26.3 % for arrowworms, 10.8 % for euphausiids, 3.2 % for medusas, and 2.9 % for amphipods. The dominant species are the arrowworm Sagitta elegans (26.3 %) and the copepod Eucalanus bungii (19.7 %); other mass species are: large-sized copepods Neocalanus cristatus (10.4 %), Neocalanus plumchrus + Neocalanus flemingeri (7.8 %) and Calanus glacialis + Calanus marshallae (5.1 %), euphausiids Thysanoessa raschii (3.5 %) and Thysanoessa longipes (3.4 %), small-sized copepods Metridia pacifica (3.5 %) and Oithona similis (3.5 %), medusa Aglantha digitale (3.2 %).

In the autumn months of 2018-2019, food spectra of salmons were quite diverse in the Okhotsk Sea. Amphipods prevailed in the diet of juvenile pink and chum salmon that corresponded to these zooplankton species abundance in the southern part of the sea; other significant portions of their diet were presented by euphausiids, copepods, arrowworms, pteropods, and oikopleura. Decapods, fish and squids were in the diet of masu, coho and chinook salmon. Portion of zooplankton was about 100 % in the diet of pink salmon fingerlings, but only 98 % in the diet of chum salmon fingerlings with 2 % portion of squids. Grazing by salmons was insignificant: 0.2-0.3 % of the total zooplankton biomass within the periods of observation (18 days in 2018 and 13 days in 2019), or 0.04-2.0 % for certain taxonomic groups, except of pteropods and oikopleura with the consumed portions of 4-7 % and 7-12 %, respectively. However, the daily consumption by salmons relative to daily production of some groups was significant in October, 2018 and reached 97 % for amphipods and > 200 % for euphausiids in the northeastern Okhotsk Sea (biostatistical areas 6-8) and 48 and 27 %, respectively, in the entire surveyed area. This ratio was twice lower in October, 2019, as 24 % for amphipods and 12 % for euphausiids. Daily consumption of copepods and arrowworms never exceeded 2 % of their daily production. The most favorable conditions for nekton feeding in fall of both years were observed in the northwestern Okhotsk Sea, and the worst conditions — in its northeastern part, with moderate conditions without deficit of prey in the central part (biostatistical area 9) and southern part (biostatistical areas 12, 13), where the biomass of forage plankton exceeded the biomass of nekton in 30-97 times in 2018 and in 56-76 times in 2019.

Total biomass and production are evaluated for taxonomic groups of zooplankton sampled with Juday net (mesh size 0.168 mm) from the 0-200 m layer in the Bering Sea in 1986-2018. The annual mean stock of zooplankton is assessed as 245.1 . 106 t WW, or by taxonomic groups: copepods — 135.0 . 106 t (55.1 %), chaetognaths — 664.4 . 106 t (26.3 %), euphausiids — 30.8 . . 106 t (10.8 %), medusas — 7.9 . 106 t (3.2 %), and hyperiids — 7.0 . 106 t (2.9 %). The gross annual yield is estimated as 1378.6 . 106 t for copepods (P/B = 9.3), 422.5 . 106 t for chaetognaths (P/B = 6.0), 212.5 . 106 t for euphausiids (P/B = 9.0), 132.4 . 106 t for net medusas (P/B = 16.7), and 90.9 . 106 t for hyperiids (P/B = 12.3). The total annual yield of 2281.9 . 106 t, on average, was formed as a sum of 107.4 . 106 t produced in winter (P/B = 1.0), 889.6 . 106 t — in spring (P/B = 2.4), 755.2 . 106 t — in summer (P/B = 2.8), and 529.6 . 106 t — in autumn (P/B = 2.2). This annual yield was produced mostly by non-predatory zooplankton (1613.8 . 106 t), but also by predatory zooplankton (668.1 . 106 t). Mean annual P/B-ratio for the entire zooplankton in the Bering Sea was 8.5 that was in 1.4 times lower than in the Okhotsk Sea (P/B = 11.8), mainly because of predominance of less productive Copepoda species.

Effect of estradiol on development of gonads in resident form males of cherry salmon is investigated. The fish aged 0+ with initial weight of 3.48-10.70 g were treated with the hormone for 30 days, then reared for 8 months. Estradiol was given with pelleted food in concentration of 100 mg/kg. The gonads were examined histologically during entire period of the experiment. The control group included 184 specimens (90 females, 59 anadromous males and 35 males of resident form). The experimental group of 50 females and 48 males was taken randomly in 3.5 months after the end of hormonal exposure. The fund of germ cells in gonads of the experimental group consisted only of gonias. Males of resident form were identified in 6 and 8 months after the end of hormonal exposure, they were distinguished by both size of the testes and presence of germ cells at different stages. Portion of the males of resident form was similar in the control and experimental groups — 37.0 % and 40.7 % of total number of males, respectively.

Choosing of mathematical model for describing of the pacific salmon progeny dependence on the parents abundance is discussed. Results of different approximations are interpreted. The analysis could be useful for forecasting of the pacific salmon stocks by specialists without deep skills in numerical modeling (like the author).