Reproductive conditions of walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Gadidae) off the northeastern coast of Sakhalin Island,Sea of Okhotsk

The spatial distribution of eggs and larvae of the walleye pollock Theragra chalcogramma is considered in respect to dynamics of oceanologic processes, nutrients, chlorophyll а and zooplankton off the northeastern coast of Sakhalin Island in spring 2012. It is shown that the effect of severe temperature regime in the near-bottom horizons in the western Sea of Okhotsk during the spawning period of walleye pollock becomes milder due to specific features of water dynamics. The egg distribution is determined by mesoscale eddies in the region. The species survival depends on the effect of such environmental factors as freshwater discharge from the Amur River, eddy structure in waters of the Sea of Okhotsk, and dynamics of phytoplankton and zooplankton development.     

Variability of the Characteristics of the First Scale Annulus and Length of Year-Old Fish in the Bering Sea Walleye Pollock Theragra chalcogramma

Geographic and interannual variability of the number of annuli and the radius of the first scale annulus, as well as the retrospective length of year-old walleye pollock from the western, northern, and eastern parts of the Bering Sea, was examined using data for the years 1995, 1996, 1998, and 1999. The characteristics of the first scale annulus were varying. By these parameters, walleye pollock from the western Bering Sea significantly differs statistically from walleye pollock of the eastern Bering Sea. With respect to number of annuli, the radius of the first ring, and the retrospective length of year-old specimens, the walleye pollock from the Navarinskii region occupies an intermediate position between fish from the western and eastern parts of the Bering Sea. Interannual variability of the three parameters was found for walleye pollock from the Navarinskii region.     

Population genetic structure of walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Gadidae,Pisces) from the Bering Sea and Sea of Okhotsk

Walleye pollock Theragra chalcogramma Pallas occupies a central place in ecosystems of the North Pacific and is an important target species of fisheries. The species is characterized by daily vertical, spawning, feeding, and wintering migrations and spawning occurring under the sea ice. Since population structure estimation by the tagging with recapture is inefficient in walleye pollock, the pollock resources are difficult to estimate by conventional methods, requiring population genetic studies with molecular markers. The population genetic structure of five spawning aggregations from the Bering Sea was for the first time studied with ten microsatellite loci: Tch5, Tch10, Tch11, Tch12, Tch14, Tch16, Tch17, Tch19, Tch20, and Tch22. A spatially distant sample from the Sea of Okhotsk was used as a reference group. Polymorphism for the markers reached 100%, and heterozygosity of individual loci ranged from 41 to 95% in different populations. It was shown the aggregations of interest are in goodness-to-fit the Hardy-Weinberg equilibrium (HWE) at hole, while the Sea of Okhotsk sample demonstrated a sex bias: the heterozygosity at Tch16 in males was significantly lower than in females. The highest discriminative power was observed for Tch10, Tch20, and Tch22. F _ST genetic distances between populations were typical for marine fishes. A mixed composition was supposed for the sample from the region of the underwater Shirshov Ridge, which serves as a natural partial geographic barrier between the Olyutor-Karagin and Koryak walleye pollock stocks. With the Shirshov sample excluded, F _ST scatter plots and the spatial autocorrelation approach supported isolation by distance for the aggregations. An influence of abiotic factors on the population structure was assumed for walleye pollock of the Bering Sea.     

Spawning of walleye pollock (<Emphasis Type="Italic">Theragra chalcogramma</Emphasis>) off the Southwestern coast of Kamchatka

The results of ichthyoplankton surveys conducted in 2007–2008 showed that waters off Southwest Kamchatka and North Kuril Islands were areas of mass spawning of walleye pollock. The peak spawning occurred during the last 10 days of April and in early May, which was much later than the peak at the main spawning site off West Kamchatka. The spawning activity of walleye pollock near the southwestern shores of Kamchatka is a regular event, as the analysis of archive materials shows. This gave us grounds to suspect the existence of a southern site that coincided well with the mass spawning in the Shelikhov Gulf in its timing and scale, but was missed during standard ichthyoplankton surveys conducted in early April. After analyzing the growth rates of spawners, the assumption was made that the southern spawning was performed by the East Kamchatkan walleye pollock population, whose mass spawning usually occurs in late April-early May. According to the data of 2008, the estimated biomass of walleye pollock spawning in the area of the Ozernovskaya basin in late April was nearly 600000 tons. Regular monitoring of the southern spawning is proposed by means of additional ichthyoplankton surveys south of 53° N, including the Okhotsk Sea waters of the North Kuril Islands, in late April—early May.     

Assessment of the consumption of walleye pollock eggs by nekton and jellyfish in the northern Sea of Okhotsk in the spring

In the northern Sea of Okhotsk, nekton and jellyfish consumed as many as 831 × 10⁹ walleye pollock eggs per day in 2011. The nekton exerted the highest pressure, viz., 98.3% of the overall predation on pollock egg by aquatic animals. Of the entire quantity of consumed eggs, 55.9% were eaten by herring, 35.9% by walleye pollock, 6.5% by Sakhalin sole, and 1.7% by jellyfish. Among jellyfish, scyphomedusae Cyanea capillata and Chrysaora melonaster, as well as the hydromedusa Tima sachalinensis consumed the largest quantities of eggs. The total consumption of pollock egg by aquatic animals in 2011 was estimated at 42.4 × 10¹², or 11.4% of the entire quantity of eggs that were spawned by walleye pollock in the waters of the northern part of the sea. The total amount of pollock eggs that were eaten by herring and pollock together for 51 days in 2011 amounted to 38.9 × 10¹², which was 5.7 times as much as that in 2002. Thus, a significant growth of predation on pollock eggs by their main consumers, viz., herring and walleye pollock, was observed in 2011. This was caused by an increase in the populations of both species during the recent years and also by a higher concentration of pollock eggs.     

Quantitative composition,distribution, and feeding of large jellyfish (<Emphasis Type="Italic">Scyphozoa et Hydrozoa</Emphasis>) on the West Kamchatka shelf in summer

The total biomass of jellyfish on the shelf of the eastern Sea of Okhotsk in the summer is estimated as 1672700 tons according to the results of hydroacoustic measurements and 901000 tons by the method of squares. The use of hydroacoustic technologies makes evaluation of the actual stock and range of medusae more accurate, and the further enhancement and perfection of the hydroacoustic method based on multifrequency measurements enables one to obtain more reliable estimates. A significant increase (nearly 25 times) of the total jellyfish biomass takes place in the summer period. Cyanea prevailed in biomass in the spring and Chrysaora melanaster prevailed in the summer. Some species showed considerable expressed spatial differentiation of distribution and affinity to certain environmental conditions. The studied species were almost exclusively zoophages. Their algal diet consisted mainly of diatom algae. Scyphomedusa’s diet mainly included the so-called “peaceful” zooplankton, viz., euphausiids and copepods (as a rule, over 50% the mass), at the same time carnivorous zooplankton, saggits, amphipods, and small medusae also formed a substantial share of their diet. One individual of the predominant jellyfish species consumes a total of 6.1 to 70.5 kcal during its lifecycle, which corresponds to 79.1–513.0 g of raw organic material, assuming 70% assimilability. The relatively low demand for food of this sort can be explained by the low caloric value of the jellyfish body, 96–97% of which consists of water. The distribution and composition of the jellyfish prey show that scyphomedusae exert the greatest influence on the nekton community, as they concentrate in the shelf area of the eastern part of the sea, at walleye pollock spawning sites. There the larvae of bottom invertebrates, including commercially valuable organisms, such as crab and shrimp, are also consumed. In the summer, jellyfish eat nearly 100 billion eggs and 20 billion larvae of walleye pollock, as well as 130 billion decapod (mostly crab) larvae each day, which corresponds to 0.03% of the eggs and 0.003% of the larvae of walleye pollock and 0.003% of the decapod larvae in the estimated stock.     

Evaluation of the productivity of walleye pollock generations in the northern Sea of Okhotsk

Based on data on the abundances of various age groups in the northern Sea of Okhotsk in 1984–2008, the productivity of 33 generations of walleye pollock born between 1975 and 2007 was evaluated in two ways. The first approach allowed a retrospective estimation by abundance of the first nine one-year age groups, and the second one was a perspective estimation using the abundance index of the nearest recruitment—3- and 4-year-olds—that enabled us to evaluate the productivity of a generation before it enters into spawning and commercial stocks. These productivity estimates were compared to the data of annual catches and the dynamics of spawning stock for many years. A high value of walleye pollock stock was shown to be supported by several productive and highly productive generations, a medium value—by one productive and one moderately productive or by two moderately productive generations, and a low value—by one productive, or moderately productive, and low productive generations. In addition, overall mortality rates were calculated for generations with various productivity levels. The greatest variability of mortality rates was observed in immature walleye pollock; the maximum mortality rate was typical for the recruitment of highly abundant generations, and the minimum, for the least abundant ones. The lowest mortality was recorded at the age of mass maturation. Mature individuals of generations with various productivity levels showed insignificant differences in overall mortality rates.     

Spatial distribution,size composition,and dynamics of catches of the Okhotsk skate <Emphasis Type="Italic">Bathyraja violacea</Emphasis> in the North Pacific Ocean

The results of the long-term (1963–2015) studies of spatial and vertical distributions, dynamics of abundance and size composition of the Okhotsk skate Bathyraja violacea in the North Pacific Ocean are presented. This species is the most abundant at depths of 100–400 m; in the cold season, it migrates to the larger depths for wintering, while it migrates to shallower depths in the warm season. Specimens of the length 12 to 132 cm with prevalence of skates of 50 to 80 cm long were recorded in catches. The difference between males and females in body weight and length was not considerable. Condition factor of specimens decreases from January to August and begins to increase in September. Males dominate among the skates up to 30 cm long, females dominate in the size groups of 30–60 cm and >70 cm, while an almost equal sex ratio is observed in the size group of 60–70 cm. The dynamics of the catches of the Okhotsk skate vary in different regions: in the western Bering Sea and in the Sea of Okhotsk, the catches consistently increased during the study period, the catches increased to the mid-1980s–early 1990s in the Kuril and Kamchatka waters of the Pacific Ocean, and then a decrease has been observed; in the eastern Bering Sea, the peak of catches occurred in the mid-1970s, and then they decreased subsequently.     

Effect of spawning ground location on the transport and growth of chub mackerel (Scomber japonicus) eggs and larvae in the East China Sea

Chub mackerel (Scomber japonicus) are an important pelagic fish species within the China Sea. Annual recruitment of this species is determined primarily by survival in the early life history stages. Minor changes in the physical marine environment can have a significant effect on the growth and survival of eggs and larvae, thereby affecting recruitment of population. To model this interaction, we constructed a bio-physical dynamic model of the early life history of chub mackerel in the East China Sea (ECS). The physical model was based on the unstructured grid Finite Volume Coast and Ocean Model (FVCOM) and simulated the 3-D physical fields. The biological model was based on individual-based models (IBMs) in which the early life stages of chub mackerel were divided into five stages based on age or length. The model was parameterized using functions describing spawning, growth, and survival for chub mackerel in the ECS. Using this coupled physical and biological model, driven by the March–July climatological forcing, we tracked super individuals from spawning grounds to the nursery grounds to evaluate the influence of the physical environment at each of the spawning locations (western, normal, eastern) on the transport and survival of chub mackerel. The model suggests that spawning location has a significant effect on larval transport, although the larvae were generally advected northeastward to enter the Japan/East Sea through the Tsushima/East Strait or southeastward with the Kuroshio Current which then flows along the eastern Japanese coast. Spawning to the west was highly influenced by the Taiwan Warm Current (TWC) during early transport when the larvae were advected northward and then northeastward. The speed of drifting during this period was relatively slow. The model predicted that a large number of eggs and larvae would enter and transit through China’s coastal waters (Changjiang River Estuary, Hangzhou Bay, and the Zhoushan Islands). Under this scenario, the majority of larvae were transported to the northern nursery grounds, 79% to the nursery at Jeju Island and 10% to the nursery at Tsushima Strait. In contrast, only 11% were transported to the southern nursery grounds in the Pacific Ocean and Kyushu. Larvae spawned at the eastern spawning ground were primarily influenced by the Kuroshio Current which transported the larvae southeastward. Kuroshio acts as a barrier, restricting larvae from being advected to the interior of the western Pacific Ocean. Under such circumstances, almost no eggs and larvae were retained in the coastal waters of China. Instead, the larvae were rapidly transported northeastward from the Chinese shelf towards the coast of Japan. The model predicted that a large number of larvae would be transported to the southern nursery grounds in the Pacific Ocean and Kyushu, before entering the Pacific Ocean and Japan Sea. In total, 36% of larvae were transported to the Pacific Ocean nursery, 45% to the northern nursery grounds of Jeju Island and Tsushima Strait, and 27% to the Jeju Island nursery. The three simulations assumed the same number of eggs were spawned (2.17 × 10¹²) and the survival of larvae at the western, normal, and eastern spawning grounds was 0.0306%, 0.0353%, and 0.0234%, respectively. The average length was 123.7, 126.0, and 123.5 mm, respectively. Our results suggest that larvae spawned in different regions encountered different physical environments and were subject to different transport processes. These differences explain the changes in survival and growth observed between larvae from the different areas. Survival and growth was highest for chub mackerel that were spawned at the normal spawning location and subject to suitable water depths and temperatures during transport.     

Some biological features and productive characteristics of <Emphasis Type="Italic">Oithona similis</Emphasis> (Copepoda) in the Sea of Okhotsk and western Bering Sea

The biological peculiarities of Oithona similis were described based on the materials from 29 complex surveys that were conducted by the TINRO Center in the Sea of Okhotsk and the western Bering Sea from 1986 to 2005 and the production of the species was estimated. In this area, O. similis produces four generations a year, viz., one in the spring, one in the fall, and two in the summer. The peak of spawning in May and June coincides with the maximum development of phytoplankton. Somatic production of O. similis in both seas becomes higher during the summer season. In this species, the largest increment in body weight begins with the copepodite stage CIV, when basic nutrients are accumulated. The average specific daily production of O. similis in the epipelagic zone of the Sea of Okhotsk and Bering Sea was 0.0092 and 0.01 in the spring, 0.043 and 0.031 in the summer, and 0.023 and 0.014 in the fall, respectively. The total somatic production of this species reached 28.318 million tons in the Sea of Okhotsk and 4.811 million tons in the Bering Sea. The Sea of Okhotsk is more favorable for the development of O. similis.     

First data on the biology of <Emphasis Type="Italic">Hadropareia middendorffii</Emphasis> (Perciformes,Zoarcidae) in the Tauysk Bay,the Sea of Okhotsk

Data of long-term studies (1997–2013) on biology of Hadropareia middendorffii from different areas of Tauysk Bay, the Sea of Okhotsk are generalized. This species is widespread over the entire northern coast of the Sea of Okhotsk, and it prefers sandy-pebbly grounds. Maximum sizes and age comprise 267 mm, 73 g, and 7+; in catches, individuals with a length of 9.1–12.0 cm, weight 0.1–10.0 g, at an age of 1+ to 3+ dominate. In general size–age characteristics of females and males as a whole are rather similar, and sex ratio is nearly equal. Hadropareia middendorffii spawns from the second half of June to the first 10-day period of August; its fecundity varies within 67–219 eggs.     

New data on sexual dimorphism and reproductive biology of Alaska skate <Emphasis Type="Italic">Bathyraja parmifera</Emphasis> from the northwestern Pacific Ocean

New data on sexual dimorphism and features of reproductive biology of Alaska skate Bathyraja parmifera from the western Bering Sea and the northern Sea of Okhotsk are presented. Based on comparative analysis of 11 external morphological characters, the males differ from the females in the length of the disc and snout length (to the eyes, to the mouth, and to the nostrils). In the western Bering Sea, 50% of the females and 50% of the males reach the first sexual maturity at 84.5 cm TL and 85.2–90.2 cm TL, respectively; in the Sea of Okhotsk, 50% of the males reach their first sexual maturity at 80.8–83.8 cm TL (the assessment depends on the features of the method). Among smaller skates (less than 45 cm TL), the males prevail, but the proportion of the females increases in larger size groups; the sex ratio becomes equal in the exemplars 60–80 cm TL, but the males prevail again among the largest fishes (more than 80 cm TL).     

Specific features of distribution and some features of biology of Korean flounder <Emphasis Type="Italic">Glyptocephalus stelleri</Emphasis> (Pleuronectidae) in waters off Kamchatka in the Sea of Okhotsk

According to materials of 1997–2002, the characteristic of the spatial-bathymetric distribution, size and age composition, size and sex structure, and feeding of the Korean flounder Glyptocephalus stelleri in the summer period in waters off Kamchatka in the Sea of Okhotsk (the site from 51°15′ to 57°20′ N, depths of 10–300 m) is given. It is shown that throughout the observation period, maximum catches of this flatfish were constantly observed at two sites of the western Kamchatka shelf (51°20′–53°00′ N and 55°00′–56°00′ N) in the zone exposed to the effect of anticyclonic gyres in the range of depths 101–200 m at the near-bottom temperature below 2°C. Unlike other habitat areas, off the coast of western Kamchatka, G. stelleri in the summer time is quite frequently found at negative temperature values, which is obviously determined by the specifics of thermal conditions of the eastern part of the Sea of Okhotsk. It was established that, in this species of flatfish, sex dimorphism is distinctly pronounced in sizes—males are considerably smaller than females whose relative number among individuals with a length larger than 50 cm reaches 100%. The main items of feeding of G. stelleri at the western Kamchatka shelf in the summer period are oligochaetes.     

The First Finding of the Actinia Synandwakia hozawai in the Sea of Okhotsk

The finding of the actinia Synandwakia hozawai in the coastal waters of northern Sakhalin (Sea of Okhotsk) suggests a wider range of this species, which was previously only known to inhabit the coastal waters of eastern Japan. Data are presented on the morphology of the S. hozawai specimen from the Sea of Okhotsk and the types of its nematocysts.     

Variation in the diet of Arctic Cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) in the Pacific Arctic and Bering Sea

Arctic cod, Boreogadus saida (Lepechin, 1774), is a nodal species in Arctic marine foodwebs as an important prey of many birds, marine mammals, and other fishes, as well as an abundant predator of zooplankton and epibenthic fauna. We examined the summer diet of Arctic cod across a latitudinal gradient extending from the southern limit of their distribution in the eastern Bering Sea to the northern margins of the eastern Chukchi Sea (ECS) continental shelf. Specimens were collected from demersal and pelagic trawls conducted between 1999 and 2012, and across a range of predator sizes (3–26 cm). Arctic cod diets vary with body size and between regions within the study area, and appear to vary between years in the eastern Bering Sea, indicating opportunistic feeding habits. Constrained Analysis of Principal Coordinates was conducted on ECS demersal samples and revealed consumption of fish and decapod crustacea were positively correlated with Arctic cod length while consumption of euphausiids and copepods had the opposite relationship. The demersal Arctic cod diet in the northern latitudes of the ECS was dominated by copepod consumption (47% by weight, %W), but copepods were less important (12–26%W) in the central and southern latitudes of the ECS and in the northern and eastern Bering Sea—areas where diets were more varied in their composition. High levels of variation in the diet of Arctic cod highlights the need to monitor Arctic cod diets to identify consistently dominant prey types and potential future changes to trophic relationships related to climate change or increasing anthropogenic activity.     

Sex-biased egg cannibalism in spawning walleye pollock: the role of reproductive behavior

We studied inadvertent egg cannibalism in spawning stocks of walleye pollock, Theragra chalcogramma, in the Gulf of Alaska and the Bering Sea between 1986 and 1996. Male pollock had on average 3 times more eggs in their stomachs than females. For both sexes adult fish of average body length had more eggs in their stomach than did smaller or larger fish. When maturity of fish was taken into account, actively spawning males had the highest numbers of eggs in their stomachs. We found weak evidence for a diel variation; the number of eggs per stomach was high in both sexes during the day and lower during the night. We suggest two possible explanations for this phenomenon. Sex-biased egg cannibalism may reflect the differential time spent in layers of high egg densities. Hydroacoustic and trawl catch data from both areas suggest that males aggregate deeper than females. Spawning takes place in the deeper layers of the fish aggregation, so males spend more time in high egg densities. Alternatively, males may be more active than females and increased gill ventilation and/or drinking rates may be responsible for the differences in egg cannibalism.     

Interannual differences in growth and hatch date distributions of age-0 year walleye pollock Theragra chalcogramma (Pallas) sampled from the Shumagin Islands region of the Gulf of Alaska, 1985 – 2001

Daily increments of age-0 year walleye pollock Theragra chalcogramma otoliths from 1985 to 2001 were examined for interannual variability in growth and hatch dates. Fish were collected in summer and autumn surveys near the Shumagin Islands in the western Gulf of Alaska (GOA). Hatch date distributions of these fish were compared with hatch dates of larvae from spawning aggregations located in Shelikof Strait and the Shumagin Islands based on their spawning times. The hatch date distributions of age-0 year fish captured from the Shumagin Islands area were similar to those observed for larvae from the Shelikof Strait spawning group. Age-0 year fish whose hatch dates corresponded to the Shumagin Islands spawning, which occurred earlier in January and February, were not found. Sea surface temperature was associated with variability in hatch date distribution and growth.     

Spatial and length distribution,sex composition,and feeding of the notched-fin eelpout <Emphasis Type="Italic">Zoarces elongatus</Emphasis> (Perciformes: Zoarcidae) in the Sea of Okhotsk

Based on the data from six bottom trawl surveys carried out on the shelf of the Sea of Okhotsk in 1997–2007, the features of the distribution, size and sex structure, and food composition of the notched-fin (east) eelpout Zoarces elongatus were analyzed. It was established that this species regularly occurs in trawl catches only in the eastern part of the sea and that the density of its schools decreases with depth. It was recorded that females of the notched-fin eelpout are larger than males. The sex ratio of the fishes varies from year to year. Females prevail at depths less than 25 m and males prevail at depths greater than 25 m.     

Changes in vertebral-column characters and in swimming abilities of the walleye pollock <Emphasis Type="Italic">Theragra chalcogramma</Emphasis> (Pallas) under the pressure of fishing activities

After 30 years of intensive walleye pollock fishing in the Sea of Japan and the Sea of Okhotsk, the average number of vertebrae has increased significantly in the caudal section of the spinal column. The maximum frequency of occurrence of “multivertebrae” phenotype (33 caudal vertebrae) has increased from 3–5 to 76–78%. The number and frequency of occurrence of the abdominal vertebrae did not change. Experimental testing of walleye pollock on its ability to resist the flow justifies our suggestion that the number of vertebrae in the caudal section is an important adaptive feature. We argue that the changes in phenotypic structure of the studied population are the result of intensive fishing.     

New data on the reproductive biology of shortspine thornyhead <Emphasis Type="Italic">Sebastolobus alascanus</Emphasis> (Sebastidae) in the northwestern Pacific

New data on fecundity, sex ratio in different size groups, and the rate of sexual maturation of shortspine thornyhead Sebastolobus alascanus in the northwestern part of the Pacific Ocean are provided. Individual absolute fecundity of the studied females varies in the range of 175 000–821000 (on average, 392500) eggs. In Pacific waters off the Kuril Islands and Kamchatka, among individuals with a length to 41 cm, males dominate; in size groups of 42–59 cm, sex ratio is almost equal; among larger fish, females prevail. In the western part of the Bering Sea among analyzed fish with a length to 44 cm, sex ratio is almost equal; among larger individuals, females dominate. Half of males and females of shortspine thornyhead off the eastern coast of the northern Kuril Islands and southeastern Kamchatka mature at a length of approximately 41 cm that can be recommended as a commercial size for the given species in this area.