Bottom Trawl Surveys Research Articles

Can an invader support commercial fishing? A case study of walleye pollock Gadus chalcogrammus in the western Chukchi Sea

Based on research of bottom and mid-water trawl surveys conducted onboard Russian research vessels in 2003–2020, data on the spatial and vertical distributions of walleye pollock Gadus chalcogrammus in the western Chukchi Sea are presented. The response of walleye pollock distribution related to water temperature, as well as the features of its distribution are also presented. In the water column, schools were mainly comprised of juveniles, while medium-sized and large individuals were scarce. Approximately 98% of the total number of juveniles and 100% of the large pollock caught by bottom trawls were observed in the southern part of the study area, while most of medium-sized fish were found in the northern part. In the water column, walleye pollock was found within the upper 65 m layer. Near the bottom, the majority of small-sized fish and all large individuals were observed at depths shallower 100 m, while over 70% of medium-sized walleye pollock were found at depths than 100 m. In the water column, juveniles were observed at temperatures from −0.4 to 3.3 °C with largest concentrations within a temperature ranges of 0.9–1.8 °C) and 3.0 °C. Near the bottom, walleye pollock were recorded at a temperature from −1.2 to 4.3 °C with maximum number within the ranges 2–3 °C, > 3 °C, and 1–2 °C. A few individuals were found at bottom temperatures below 0 °C. In the water column, the abundance was low during the entire study period. Acoustic estimates were notably higher compared to those based on the results of bottom trawl surveys. The recent dramatic increase of walleye pollock abundance in the western Chukchi Sea appears to reflect a complex of factors, including warming of bottom temperatures, decreasing of ice cover, increasing of northward currents, better foraging conditions, and increasing abundance and biomass in the Bering Sea. An attempt is made to determine the prospects for commercial fishing of walleye pollock in the western Chukchi Sea. Bottom trawl surveys in 2019–2020 indicated an estimate of total allowable catch of 37,200 t for the 2021 and 2022 fishing seasons; in 2021 over 4000 t were caught. However, this kind of fishing cannot be considered sustainable due to several reasons, including the inability to forecast the number of fish migrating from the Bering Sea, the limited fishing season due to harsh climatic conditions, the remoteness of main ports, and the uncertain population status of its stocks.

Feeding ecology of Decapterus maruadsi in the southern coastal area of Zhejiang based on stomach contents and stable isotope analysis.

Decapterus maruadsi is an important economic pelagic fish in the southern coastal area of Zhejiang. According to the bottom trawl surveys conducted in May, August, November 2020 and January 2021, the feeding habits of D. maruadsi in the southern coastal area of Zhejiang were examined by both stable isotope (carbon, δ13C, and nitrogen, δ15N) and stomach content analyses. Results showed that the δ13C value of D. maruadsi ranged from -17.76‰ to -15.25‰, with a mean of (-16.55±0.60)‰, while the δ15N value ranged from 9.06‰ and 13.03‰, with a mean of (11.76±0.88)‰. There was a significant negative correlation between the δ13C values and fork length, and a positive relationship between the δ15N with fork length. Results from stomach content analysis showed that the main prey groups of D. maruadsi were fish, shrimp, crabs, Cephalopod, Polychaete, and small crustaceans. As for the stable isotope analysis, the nutritional contribution rate of shrimp was the highest (40%-84%) among all prey groups, followed by Polychaete, small crustaceans, crabs, Cephalopods and fish. Significant ontogenetic dietary changes were found for D. maruadsi. As the fork length of D. maruadsi increased, it tended to eat prey from higher trophic levels.

Broadband acoustic quantification of mixed biological aggregations at the New England shelf break.

At the New England shelf break, cold, less saline shelf water collides with warmer saltier slope water to form a distinct oceanographic front. During the Office of Naval Research Sediment Characterization Experiment in 2017, the front was mapped by narrowband (18 and 38 kHz) and broadband (70-280 kHz) shipboard echo sounders. The acoustically determined cross-shelf velocity of the front ranged in amplitude from 0.02 to 0.33 m/s. Acoustic surveys revealed aggregations of scatterers near the foot of the front. Acoustic backscatter in conjunction with Northeast Fisheries Science Center bottom trawl surveys identified longfin squid (Doryteuthis pealeii) and mackerel (Scomber scombrus) as the most likely scatterers in the aggregations. A mixed species scattering model was developed and further refined by the use of a matching method used for distribution of the lengths of each species. The mean length of squid and mackerel, respectively, using the matching method was 4.45 ± 1.00 and 20.25 ± 1.25 cm compared with 6.17 ± 2.58 and 22.76 ± 1.50 cm from the trawl data. The estimated total biomass of the aggregation was a factor of 1.64 times larger when using the matching method estimated length distribution compared to the trawl length distribution.

Acoustic observations of nekton and zooplankton along the Northeast continental shelf break

Observations of fish and squid (nekton) and crustacean and gelatinous animals (zooplankton) at the shelf break using active acoustic systems (e.g., echosounders) have been made along the United States shelf break from the mid-Atlantic to New England since the early 2000s as part of a biennial bottom trawl survey (NOAA’s Northeast Fisheries Science Center), since 2012 as part of the Atlantic Marine Assessment Program for Protected Species (AMAPPS), and recently as part of the Task Force Ocean New England Shelf Break Acoustic project. The shelf break can be a boundary between seabed-oriented species on the continental shelf and pelagic species in the open ocean, as well as transitional habitat for some species to exploit. These data have been collected using multifrequency, narrowband (e.g., 18, 38, 70, 120, and 200 kHz) and wideband (18–250 kHz) scientific echosounders with hull-mounted transducers to primarily map the prey species of mid- to upper-trophic level foragers and predators. Classification of acoustic data to biologically meaningful metrics and how these can be integrated with oceanographic features and spatial and temporal distributions of predators will be explored in relation to the broader goals of developing ocean acoustic networks. [Work supported by the Office of Naval Research.]

The current state of stocks, distribution and fishing оf saffron cod (Eleginus gracilis, Gadidae) in the southwestern part of the Bering Sea

The results of bottom trawl surveys and simulation assessments indicate relative stable state of saffron cod stock in the southwestern part of the Bering Sea. According to model calculations the total stock of saffron cod in 2020 amounted to 163,8 thousand tons. The analysis of the Danish seine catches showed that saffron cod forms commercial concentrations mainly in the Litke Strait and the Gulf of Corfu. In 2010-2020 the total catch of the saffron cod ranged from 4,3 to 14,5 thousand tons averaging 8,5 thousand tons. The most effective fishing by Danish seine is carried out in September and October with average catches of 9,1 and 11,5 tons per day respectively.

A no-take zone and partially protected areas are not enough to save the Kattegat cod, but enhance biomass and abundance of the local fish assemblage

Abstract To supplement catch and effort regulations with the purpose to rebuild the cod (Gadus morhua) stock in Kattegat, Sweden and Denmark established a large (426 km2) year-round no-take zone (NTZ) surrounded by partially protected areas (PPAs) in 2009. The purpose of these spatial regulations was to prohibit cod fishing on the spawning grounds and to displace fisheries bycatch of cod from areas where mature cod aggregate in the Kattegat. The aim of this study is to evaluate the effects of the established NTZ and PPAs on the local fish assemblage, including cod. Based on a spatially high-resolution bottom trawl survey in the Kattegat (covering 2008–2021), multivariate analyses revealed significant shifts in the fish assemblage. A closer analysis indicated that six to seven fish species, including cod increased in the NTZ relative to control areas depending on if abundance or biomass was used as dependent variable. Univariate analysis showed that two flatfish species dab (Limanda limanda) and lemon sole (Microstomus kitt), and Norway lobster (Nephrops norvegicus) significantly increased in biomass in the NTZ, and turbot (Scophthalmus maximus) in the PPA relative to the control areas. These results suggest that the NTZ protected even relatively mobile species in an open sea system, such as the Kattegat. However, neither cod abundance nor biomass showed a significant increase as an effect of the NTZ and PPA despite two relatively strong year classes in 2012 and 2013, which possibly would have helped the recovery of the cod stock. As assessed by the International Council for the Exploration of the Sea in 2022, Kattegat cod continuously suffer from being severely overfished with low recruitment, and high discard rates in the mixed N. norvegicus fishery, is considered the major driver behind the reinforced depletion of the stock.

Trends in the abundance of Celtic Sea demersal fish: Identifying the relative importance of fishing and environmental drivers

The Celtic Sea is a productive fishing ground, therefore identifying the relative importance of fishing and environmental factors on fish stock dynamics is crucial for developing our understanding of sustainable yields and to operationalize Ecosystem Based Fisheries Management (EBFM). We investigated the effect of environmental variables and fishing on the relative abundance inferred from catch-per-unit-effort (CPUE), of twelve demersal stocks (i.e., cod, haddock, whiting, anglerfish, hake, megrim, plaice, sole, lesser-spotted dogfish, spurdog, Trisopterus spp., skates and rays) in the Celtic Sea from 1997 to 2019 (23 years). Annualized time series (1997-2019) of net primary production, bottom temperature, copepod abundance (Calanus finmarchicus and Calanus helgolandicus) and North Atlantic Oscillation index were used to characterize key environmental variables. Fishing exploitation rates (F/FMSY) were used to represent fishing pressure and CPUE trends derived from an International Bottom Trawl Survey (IBTS) were used to infer abundance. We used redundancy analysis to identify key explanatory variables and then dynamic factor analysis to assess their relationships with the CPUE series and identify underlying patterns in the unexplained temporal variation. Our results show that for the majority of demersal fish species, the CPUE trends were strongly influenced by fishing exploitation rates. The gradual reduction in exploitation rates observed throughout the study period most likely led to the partial recovery of cod, spurdog, hake, megrim, plaice, whiting, Trisopterus spp., and the skates and rays. In addition, exploitation patterns on one stock influenced CPUE trends of other demersal stocks (e.g., hake, megrim, plaice, lesser-spotted dogfish, sole). We also observed that the CPUE of whiting, hake and plaice increased when C. finmarchicus were abundant in the plankton. We infer from our findings in the investigated time series that the recovery of cod, spurdog, hake, megrim, plaice, whiting, Trisopterus spp., and the skates and rays in the Celtic Sea remains dependent on controlling fishing mortality, and this would not, at least for now, be confounded by the environmental conditions.

Effects of Round Goby on the Benthic Invertebrate Community and on Diets and Growth of Yellow Perch and White Perch in Oneida Lake, New York

AbstractFood web changes from the introduction of nonnative species can be complex, with sometimes unexpected effects or little effect due to food web interactions. Invasive Round Goby Neogobius melanostomus became common in samples in Oneida Lake, New York, by 2014. Data from a long‐term monitoring program allowed us to document the Round Goby expansion through time. Using shoreline seine, fyke net, bottom trawl, and video surveys, we estimated that Round Goby density reached over 25,000 fish/ha 6 years after detection and subsequently varied between 4,110 and 26,565 fish/ha. Concurrent benthic invertebrate sampling and long‐term fish diet sampling allowed us to assess the impacts of Round Goby on densities of several benthic invertebrate taxa and on invertebrate consumption by fish. Significant decreases in density after Round Goby arrival were found for amphipods (Amphipoda), caddisflies (Trichoptera), snails (Gastropoda), and dreissenid mussels Dreissena spp. Invertebrate densities after Round Goby arrival ranged from 19% to 48% of densities in the pre‐Round Goby period. For amphipods, snails, and, in some cases, caddisflies and chironomids (Chironomidae), frequency of occurrence in the diets of Yellow Perch Perca flavescens and White Perch Morone americana was lower after Round Goby became established. The decreased consumption of these invertebrates did not lead to decreases in fish growth; growth increased for some age‐classes of Yellow Perch and all age‐classes of White Perch. Despite potential Round Goby predation, densities of burrowing mayflies Hexagenia spp. expanded during this time and likely helped to offset reductions in consumption of other invertebrates by Yellow Perch and White Perch, as did consumption of Round Goby. Long‐term monitoring shows that Round Goby decreased the density of several benthic invertebrate species and decreased the consumption of these invertebrates by fish, but the effect on growth of Yellow Perch and White Perch was offset by consumption of Round Goby and burrowing mayflies.

Feeding ecology of Amblychaeturichthys hexanema in Haizhou Bay based on linear mixed model

Many traditional economic marine fishery resources have been declining in the coastal waters of China. Amblychaeturichthys hexanema has become one of the dominant species in Haizhou Bay and some other coastal areas of China, playing an important role in marine food webs. We analyzed the feeding strategy of A. hexanema based on the bottom trawl surveys conducted in the Haizhou Bay and adjacent waters in spring and autumn of 2011 and 2013-2016. The feeding habits and main influencing factors of A. hexanema were studied using linear mixed model (LMM). The results showed that A. hexanema fed on 14 prey groups, with Amphipoda, shrimps, Bivalvia and Ophiuroidea as the main prey groups. A total of 37 species were identified in the preys. The dominant prey species were Gammarus sp., Leptochela gracilis, Monoculodes sp., Moerella iridescens and Alpheus distinguendus. Results of the linear mixed model showed that the feeding intensity of A. hexanema in Haizhou Bay was influenced by body length, water depth, survey season, and bottom water salinity. Among them, water depth and bottom salinity had significantly negative effects, whereas the impact of body length was significantly positive. The feeding intensity was significantly higher in spring than that in autumn. Results of cluster analysis showed that A. hexanema had different feeding habits between two length groups as divided by a threshold of 70 mm body length. The change of feeding habit was mainly attributed to the variations in predation ability and food availability. Feeding strategy analysis showed that trophic niche of A. hexanema had large components between phenotypes and low intraspecific competition for prey. This study would contribute to our understanding of life history characteristics and ecological habits of A. hexanema, and provide scientific support for the sustainable utilization and management.

Estimating and accounting for fish losses under the footrope of a survey trawl: The case of northern shelf anglerfish

Anglerfish, or, monkfish, (Lophius piscatorius and L. budegassa) are two of the most valuable commercial fish species in northern Europe. The stock which occupies the northern European shelf is monitored by an annual bottom trawl survey which aims to estimate absolute abundance. This estimation includes corrections for herding by the trawl gear but also requires an estimate of the capture efficiency of the net. To determine the latter, losses of fish under the footrope were quantified by 1 cm length class using trawls fitted with sub-footrope collection bags. The results demonstrate clear length dependency with the smallest fish being the most likely to escape under the gear. Overall, approximately 27 % of the anglerfish were lost under the footrope, with approximately 77 % of those below 30 cm being found in the ground gear collection bags. A length-based retention model for the gear was fitted here with a day-night effect, following appropriate model selection. This estimated higher proportions of fish escapes at night than during the day. The model was then used to estimate total stock numbers at length using data from the 2006 monkfish survey data to examine their impact on the stock estimates. As expected, this demonstrated a significant increase in the abundance of smaller recruiting fish when escapement under the footrope is accounted for. The estimates at length and age will provide better inputs for future developments of an age- or length-based analytical stock assessment which in turn will contribute to better stock management.

Spatio-temporal variations in species composition and diversity of fish communities in the adjacent waters of Changshan Islands, China

In order to understand species composition and diversity of fish communities in the ecotone between the Yellow Sea and Bohai Sea, we examined species composition and temporal and spatial variations of fish species diversity using the relative importance index (IRI), species diversity index and k dominance curve based on bottom trawl survey data of fish resources in the adjacent waters of the Changshan Islands in October 2016, January, May and August 2017. The results showed that 77 fish species were captured, mainly temperate water fishes, demersal fishes and migratory fishes. The dominant species exhibited obvious seasonal variation. The dominant species in spring and winter were Lophius litulon, and those in summer were pelagic species such as Scomber japonicus and Engraulis japonicus. A total of 46 migratory fish species were found throughout the year. The seasonal species migration indices were all higher than 100, while the species migration index was the largest in autumn. The species diversity index showed high spatial and temporal dynamics, with the highest species richness in spring and the highest Shannon diversity and evenness in autumn. There was a significant negative correlation between species richness and sea surface temperature in summer, significant positive correlations between species richness and depth and bottom temperature in winter, and a significant positive correlation between Shannon diversity index and sea bottom temperature. In short, fish community showed high species diversity, rich migratory species and obvious temporal and spatial heterogeneity in the adjacent waters of the Changshan Islands, which is an ecotone between the Yellow Sea and the Bohai Sea.

Additional deepwater sampling of prey fish in Lake Michigan annual bottom trawl survey reveals new insights for depth distribution dynamics

Beginning in 2013, sites at the 128-m bottom depth contour were added to the sampling design of the annual Lake Michigan bottom trawl survey for prey fish, which has been conducted by the U.S. Geological Survey Great Lakes Science Center (GLSC) each fall since 1973, to better assess fish depth distributions in a changing ecosystem. The standard sampling design included bottom depths from 9 to 110 m, although the GLSC also sporadically conducted bottom trawl tows at the 128-m bottom depth contour during 1973–1988. Enactment of this new sampling design in 2013 revealed that mean biomass density of deepwater sculpins (Myoxocephalus thompsonii) at the 128-m depth exceeded the sum of mean biomass densities at shallower depths, indicating that the bulk of the deepwater sculpin population is residing in waters deeper than 110 m. Thus, our findings supported the hypothesis that the depth distribution of the deepwater sculpin population had shifted to deeper waters beginning in 2007, thereby explaining, at least in part, the marked decline in deepwater sculpin abundance since 2006 based on the standard sampling design. In contrast, our results did not support the hypothesis that the slimy sculpin (Cottus cognatus) population had shifted to deeper waters sometime after 2000. A portion of the burbot (Lota lota) population may have also shifted in depth distribution to waters deeper than 110 m after 2007, based on our results. Our findings have served as an impetus to further expand the range of depths sampled in our bottom trawl survey.

Seafloor litter generated by coastal and offshore fisheries operations in the South Sea of Korea

Seafloor litter sources, distribution and density were assessed in a bottom trawl survey of the South Sea around Jeju Island by the Ara-ho training ship of Jeju National University. Samples were taken from 14 transects at a depth range of 60–120 m in areas with sandy and muddy substratum. Generally, mean densities varied from 26.9 items/km2 to 62.4 items/km2 and 104.8 kg/km2 to 370.9 kg/km2. Mean densities of total litter sampled was 46.3 items/km2 and 228.6 kg/km2. Derelict gears were the most common litter items (92 % of total litter) with mean densities of 44.3 items/km2 and 228.1 kg/km2. Gillnets and traps were the most derelict with densities reaching 4.9 items/km2 and 99.2 kg/km2 and 25.0 items/km2 and 89.1 kg/km2 respectively. The results of this study indicate that significant level of pollution in the South Sea are generated from commercial fishing activities and gillnets and traps being high risk derelict gears.

Detection of fish species from marine protected areas of the North Sea using environmental DNA.

This report describe the first application of environmental DNA-metabarcoding approach for the assessment of fish species diversity in two marine protected areas of the North Sea: the Doggerbank and the Sylt Outer Reef. We collected 64 water samples and detected 24 fish species. We discuss qualitative differences between MPAs and compare the results with those obtained from bottom-trawl surveys in the same areas. We found three additional species to those documented in the same year with trawls, including the critically endangered European eel.

First record and otolith morphometric description of an adult lightfish, Ichthyococcus ovatus (Actinopterygii: Stomiiformes: Phosichthyidae), caught in the Strait of Sicily (central Mediterranean Sea)

On July 2018, one specimen of Ichthyococcus ovatus (Cocco, 1838) was caught in the Strait of Sicily during the International Bottom Trawl Survey in the Mediterranean (MEDITS). The adult I. ovatus measured 49 mm in total length and weighed 1.44 g. In this context, the presently reported study constitutes the first and deepest record of an adult of I. ovatus as well as the morphometric description of its sagittal otoliths. In addition, we provide an age estimation as well as an update of the geographical distribution of this bathypelagic species around the Mediterranean Sea. Based on the growth increments of sagittal otoliths, the estimated age was five years. Specifically, the otolith from the presently reported specimen of I. ovatus tended to be elliptic in shape related to aspect ratio and high rectangularity while circularity showed high complexity of otolith contour complexity. The absence of economic value of rarely reported species may underestimate their abundance. Therefore, more studies and research surveys would be necessary to fill the information gap on the biology of these deep-water species.

Population dynamics of blue crab (<i>Paralithodes platypus</i> Brandt, 1849) in the northwestern Bering Sea

State of the blue crab stock and production is considered for the Russian sector of the Bering Sea on the data of bottom trawl and trap surveys conducted by TINRO in 2005–2017 and fishery statistics. The commercial stock was assessed for 2018 by cohort analysis. The absolute majority of commercial males within the West Bering Sea fishing zone concentrated in two areas located at Koryak coast and at Cape Navarin, whereas the commercial aggregations were absent in the Olyutorsky Bay and Karaginsky Bay. The stock distribution between these two areas was unstable; recently the aggregation at Koryak coast increased to 72 % of the total number of commercial males in 2017. This dynamics is conditioned by both changes in reproduction and intensity of fishery. The total commercial stock of blue crab had increased since 2014 and reached the maximum of 18.133 × 106 ind. (males) in 2017 that exceeded significantly the level of 2005–2013. The blue crab fishery extends in the Bering Sea, since 2010 the portion of used TAC increased from approximately 93 % in 2011–2015 to 98 % in 2016–2017 and 99 % in 2018. State of the population is evaluated as satisfactory and stable.

Features of biology and prospects of modern fishery for pacific herring <i>Clupea pallasii</i> on the shelf of southern Kuril Islands

Biological characteristics and dynamics of stocks and fishery are considered for pacific herring in the South-Kuril fishing district on the data of research surveys and fishery statistics for 2014–2021. Features of the herring wintering and the rates of linear growth and maturation are analyzed. The stock recovery on the shelf of southern Kuril Islands is detected that corresponds to recovery of the whole Sakhalin-Hokkaido population of this species. A sharp increase of the herring abundance in this area is observed nowadays for the first time since the 1940s. According to results of cohort modeling, the commercial biomass of herring in the South-Kuril district has increased from several tens of ton in 2014–2017 to 9,700 t in 2021. The basis of the stock consists of fish aged 3–4 years, 25–29 cm long, that prevailed (76–82 %) in the catches in 2020–2021. The stock increasing is accompanied by expansion of the herring distribution, the aggregations density increasing in spring, complication of the size and age structure, heightening of the average size and age in catches of fixed nets in the spawning season, mass appearance of the juveniles in catches of bottom trawl survey in 2020, and progressive growth of the landing in 2018–2021. The usual place for the herring wintering in this area is the eastern side of the deep-water canyon off the western coast of Kunashir Island. According to updated data, the spawning migrations of herring begin in early-middle April. The producers migrate mainly from the wintering sites to the southern Kunashir Strait, and then along the Pacific side of Kunashir Island to their spawning grounds. The migration along the Okhotsk Sea coast is secondary. The spawning occurs at Kunashir Island in the period from late April to middle May (the timing varies from year to year; the peak of spawning was observed in early May in 2020, but in middle May in 2021). Unlike other populations, the herring of the South-Kuril area do not make extended migrations after the spawning, but feed on adjacent shelf for a long time. Fishery on herring develops rapidly in the South-Kuril fishing district: the annual catch was 140 t in 2018, but reached 1010 t in 2021. Trawl fishery of pacific herring has the highest prospects for coming years. The fishing by Danish seines can be intensified in the waters between Kunashir and Shikotan in the pre-spawning period, though the landing by fixed nets on the spawning grounds can be stabilized if the number of coastal fishing teams will be increased.

The influence of age and cohort on the distribution of walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea

The spatial distributions of marine fish populations are influenced by environmental conditions, intrinsic properties of the populations, and prior distribution. The influence of these factors may not be consistent across age classes. For this study, age composition estimates for walleye pollock ( Gadus chalcogrammus) collected on bottom-trawl surveys in the Bering Sea were used to estimate range correlation indices, population centers of gravity, and effective area occupied. Age-specific density maps suggested a circular ontogenic migration during the summer feeding season, with the youngest and oldest groups most broadly distributed. Range correlation analysis among age groups and year classes provided clear evidence of a population cohort effect in the spatial distribution of the population. Variance decomposition analysis indicated that the variance in the spatial distribution of age groups during summer was influenced by the initial distribution of that cohort as recruits. Model-based analyses showed that extrinsic temperature variables affected the youngest and oldest age classes the most, but provided no indication of age-related effects for intrinsic population factors. This study showed that both cohort and age-specific factors are important drivers of spatial distribution.

Importance of fish species in the bottom and near-bottoml biotopes of the Far-Eastern Seas and Pacific waters of Russia

Importance of fish species in the benthal ichthyocoenoses (up to depths of 2000 m) is discussed for the Russian waters in the Far-Eastern Seas and adjacent North-West Pacific on the data of bottom trawl surveys conducted by Pacific Fish. Res. Inst. (TINRO) in 1977–2010. The inverse Simpson index (or polydominance index) was chosen as a measure of dominance; Sorensen-Chekanovsky similarity index was used for comparative analysis of the dominance structure in ichthyocoenoses. The importance is determined for the top 20 species ranked by biomass (94.9 % of the total biomass of all demersal fish on the shelf and 95.8 % — at the continental slope, on average). Poor evenness under strong domination of 2–4 species is noted in the fish communities within the range of 0–2000 m. Walleye pollock Theragra chalcogramma dominate in all regions whereas the subdominants are pacific herring Clupea pallasii, pacific cod Gadus macrocephalus, atka mackerel Pleurogrammus monopterygius, or okhotsk atka mackerel P. azonus, depending on the region. The pollock domination is interrupted at the continental slope of the Bering Sea, where giant grenadier Albatrossia pectoralis dominate. The subdominants at the continental slope of all regions are greenland halibut Reinhardtius hippoglossoides matsuurae, popeye grenadier Coryphaenoides cinereus, pacific herring, pacific sleeper shark Somniosus pacificus, scale-eye plaice Acanthopsetta nadeshnyi, and blackfin flounder Glyptocephalus stelleri. Generalized graphs of the ichthyocoenoses similarity between the studied regions (Okhotsk Sea, Bering Sea, Japan Sea, and Pacific waters at Kamchatka and Kuril Islands) by species structure and composition of the dominant species are presented, separately for the shelf and continental slope.

Changes in Distribution Patterns for Larimichthys polyactis in Response to Multiple Pressures in the Bohai Sea Over the Past Four Decades

Understanding patterns of change in the distribution of species among their critical habitats is important for analyzing population dynamics and adaptive responses to environmental shifts. We investigated spatio-temporal changes in small yellow croaker (Larimichthys polyactis) using eight alternative models fitted to data from bottom trawl surveys conducted in the Bohai Sea each spring (spawning period) and summer during 1982–2018. These models included different combinations of local sea temperature, fishing pressure, and individual climate index (i.e., North Pacific index, NPI, and West Pacific index, WPI) as explanatory variables. Selection of the most parsimonious model for each season was based on Akaike’s Information Criterion (AIC). The model with NPI as its only explanatory variable was used as a base case for pre-analysis. In spring, a spatio-temporal model with sea temperature as a quadratic effect, plus the spatially varying effects of a climate index and fishing pressure was selected, as the AIC value of this model was reduced by 41.491 compared to the base case model without these effects. In the summer after spawning, the spatio-temporal model with WPI as a climate index covariate lagged by 1-year best explained the spatio-temporal distribution patterns of the stock. The results suggested that small yellow croaker populations significantly decreased in biomass in the Bohai Sea over the study period. A statistically significant northeastward shift in the center of gravity (COG) and a contraction in the distribution range occurred in summer throughout the study period (p<0.05). During the spring sequence (1993–2018), a statistically significant northeastward shift in the COG was also found (p<0.05). Our results showed that biomass-density hotspots of small yellow croaker in both seasons have shrunk or disappeared in recent years. Overall, these findings suggest that the spatio-temporal patterns of the populations in their spawning, feeding and nursery grounds have been influenced over the past 40 years by multiple pressures, and population density in the southwestern areas of the Bohai Sea declined faster and more drastically than in the northeastern areas. This study has important implications for developing targeted spatial conservation measures for small yellow croaker at various stages of its life history under different levels of stress.