Trawl Survey Data Research Articles

Bottom temperature warming and its impact on demersal fish off the Pacific coast of northeastern Japan

Global climate change occurs not only at the ocean surface but also at the ocean bottom, which is the main habitat of demersal fish. To clarify the current status of bottom temperature warming off the Pacific coast of northeastern Japan, we examined gridded bottom temperature fields from 2003 to 2019. These fields were created by a newly developed gridding method using flexible Gaussian filter weighting with time, distance, and depth. Spatially averaged bottom temperature had a strong, significant warming trend of 0.083 to 0.115°C yr-1 in depth zones of 150-300 m, indicating bottom temperature warming. Corresponding to the warming, increases in landing amounts were found for warm-water species such as searobin in the middle region of our study area (37°50’-39°N). Seasonal catch amounts suggest that ribbon fish and swimming crab recently began to overwinter and reproduce in the area. The distribution shifts of non-target species in fisheries were also analyzed using bottom otter trawl survey data from the area from 2003 to 2019. Northward distribution shifts and increases in density were observed in blackbelly lantern shark and bighand grenadier, indicating that bottom temperature warming led to habitat expansion. Conversely, darkfin sculpin and jelly eelpout shifted northward with decreasing density, suggesting that bottom temperature warming had a negative effect on them. Deepsea bonefish shifted deeper into colder waters with increasing density and mean body weight. Thus, changes and responses of demersal fish to bottom temperature warming in the area were revealed.

Distributions of threatened skates and commercial fisheries inform conservation hotspots

Bycatch in commercial fisheries is a pressing conservation concern and has spurred global interest in adopting ecosystem-based management practices. To address such concerns, a thorough understanding of spatiotemporal relationships among bycatch species, their environment and fisheries is required. Here we used a generalized linear mixed model framework incorporating spatiotemporal random effects to model abundance patterns for 3 skate species caught as bycatch in commercial fisheries (thorny skate Amblyraja radiata, winter skate Leucoraja ocellata and smooth skate Malacoraja senta), as well as 10 target species on the Scotian Shelf, NW Atlantic. Spatiotemporal estimates of relative abundance for at-risk skates within the years 2005-2015 were modelled from research trawl survey data and overlaid with those for target species to identify hotspots of bycatch risk. In addition, abundance estimates for at-risk skates within the years 1975-1985, a period of higher stock abundance, were used to identify areas of previously important habitat. Historically, skate species densely occupied areas near Sable Island and Banquereau Banks, Georges Bank and the Bay of Fundy. Bycatch hotspots between at-risk skates and commercial targets were identified in regions across the Scotian Shelf. These hotspots were independently validated by predicting species presence from at-sea observer data that monitor skate bycatch directly. We discuss spatial relationships between target and bycatch species, highlighting limitations of at-sea observer programmes that this method helps to address. This framework can be applied more broadly to inform ecosystem management and priority areas for conservation or fisheries regulation.

Bigger juveniles and smaller adults: Changes in fish size correlate with warming seas

Abstract Increasing sea temperatures are predicted to decrease body size of marine ectotherms based on the temperature size rule. This will impact fisheries yields, but empirical evidence of the process is still limited. We used fishery‐independent bottom trawl survey data from 1970 to 2017 to examine the trends of length‐at‐age of four commercially important demersal fish species (cod, haddock, whiting and saithe) in two study areas facing increasing sea temperatures: the West of Scotland and the North Sea. We then compared the trends of length‐at‐age with annual bottom sea temperatures. The mean length‐at‐age of adults declined over the study period, in all species and in both areas, except for cod in the West of Scotland. A common trend of decline in adult length was inversely correlated with bottom sea temperatures. Correlations with temperature at seven yearly time‐lags were significant and negative in the North Sea. Correlations were only significant at lags of 1 and 2 years in the west of Scotland, where sea temperature warming was twice as slow. The mean length‐at‐age of juveniles concurrently increased, which has not hitherto been reported. This trend, shared by all species and both regions, correlated positively with rising temperature, suggesting that our study species have a faster growth rate due to increased temperatures. Synthesis and applications. We examined the body size of a range of commercially exploited fish species, at different age groups, from two management regions. We found that juvenile fish have been getting bigger and adults smaller in both regions. These changes were correlated with rising sea temperatures, providing empirical evidence that global warming is affecting the size of commercial fish species. The effects of these changes on productivity of fish populations and fisheries yield now require investigation. Temperature changes should, therefore, be included into forecasts used in fisheries science in order to mitigate the impact of global warming and maximise sustainable yields.

A Method for Differentiating between the Migrating Population Complexes of Sea of Okhotsk Pink Salmon in Pacific Waters off the Kuril Islands Based on the Gonadosomatic Index

An original method is proposed for rapidly differentiating between regional pink salmon runs with different timings of spawning and states of gonads (gonadosomatic indices, GSI). The results of differentiation obtained by the original and traditional methods are compared. The concept of spatial niches for various temporal groups of Sea of Okhotsk pink salmon in Pacific waters off the Kuril Islands is critically revised on the basis of the proposed new method. The existing concept is based on the traditional use of certain numerical markers (GSI) for samples at the sites of trawl hauls. The width of the spatial overlap zone in the distribution of temporal groups is shown. The method was tested using data of trawl surveys conducted aboard the R/V Professor Kaganovsky in May–July 2018, during the anadromous migration of pink salmon. It shows high agreement with the estimates of the proportions of the contribution that the regional pink salmon groups make to the runs and catches according to the official statistics on fishing and the escapement of spawners to rivers. The assumption on the tendency of the late-maturing pink salmon forms to migrate through the southern part of Pacific waters off the Kuril Islands is confirmed. The specific (along transects) and cumulative portion of early-maturing pink has been shown to increase with distance off the Kuril straits into Pacific waters.

Assessing pinniped bycatch mortality with uncertainty in abundance and post-release mortality: A case study from Chile

The effects of human-caused mortality, such as fisheries bycatch, of endangered, threatened and protected (ETP) species of marine mammals can be evaluated using population model-based stock assessments. The information available to conduct such assessments is often very limited. Available data might include fragmented time-series of abundance estimates, incomplete data on bycatch for the fisheries that interact with ETP species (often few years and low observer coverage), and perhaps some data on scale and trends in fishing effort. Such data are challenging to use as the basis for stock assessments, which generally assume that estimates of removals (bycatch, in our context) through time are available for at least the most recent decade or two. This paper describes a stock assessment method for use with sparse observer data on bycatch mortality, applied within the context of a Bayesian estimation framework. The method produces estimates, with associated measures of precision, of population size and historical time-series of bycatch mortality that are consistent with the observer and abundance data. It provides a rigorous way to account for the uncertainty arising from animals that are caught but released alive and then die subsequent to release, given a post-release mortality rate prior. Observer data from industrial and artisanal purse seine and trawl fisheries and survey data for South American sea lions (Otaria byronia) and South American fur seals (Arctocephalus australis) off Chile are used to illustrate the method.

Temporal and spatial variability in size-at-maturity for the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi)

Growth in size stops at the terminal molt (molt-to-maturity) for the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi) and understanding size-at-maturity (SAM) dynamics is critical to manage their fisheries, which target and retain large mature males. Bottom trawl survey data from 1982 to 2017 were analyzed with correlations and spatial regressions to identify temporal and spatial patterns of SAM and size-specific proportions mature (PM) for newshell (recently molted) crab of each sex of each species. While substantial annual variability in SAM was estimated, no long-term directional trends were estimated for either species. Recruitment variability was likely the major driver of temporal trends in SAM for both species, rather than variable ocean temperatures or other possible factors. Recruitment abundances and SAM were negatively correlated for female snow crab but positively correlated for male snow crab and both sexes of Tanner crab. Male and female snow crab SAM had similar periodicities as snow crab recruitment. SAM spatial patterns were relatively stable at decadal time-scales for both species. Annual temporal variability in PM was moderate to negligible for both species and decadal-sale variability was modest, indicating temporally stable size-specific maturation probabilities. Comparison of SAM and PM regression results show strong effects of ontogenetic movement on spatial patterns of SAM for male snow crab. Intra- and inter-species comparisons allowed for broader understanding of SAM dynamics. Strong periodicity of male snow crab SAM may have utility for fisheries management. Elucidating the mechanisms responsible for periodic recruitment for these species will help provide a more complete understanding of both SAM dynamics and general population dynamics. With increasing ocean temperatures in the EBS, the mean size of mature crab for these species may increase but would certainly be accompanied by a decrease in snow crab abundance, the more stenothermic of the two species.

Cold-Water Soft Corals (Cnidaria: Nephtheidae) as Habitat for Juvenile Basket Stars (Echinodermata: Gorgonocephalidae)

Nephtheidae soft corals are a common component of benthic ecosystems in the Northwest (NW) Atlantic and in the Arctic, but little is known about their functional roles. Here we investigated their role as habitat for basket stars (Ophiuroidea). By examining over 4500 soft coral colonies of Drifa glomerata, Duva florida, Gersemia spp., and Pseudodrifa racemosa collected during trawl surveys in the regions of Newfoundland, Labrador and West Baffin Bay, we show that Nephtheidae soft corals are habitat for juvenile Gorgonocephalus sp. basket stars. Juvenile Gorgonocephalus sp. were found on all soft corals species, but Drifa glomerata was the soft coral species with the highest occurrence of associated juvenile basket stars (32%), with a maximum of 111 basket stars found on a single 4.8 cm tall (contracted state) colony. Individual basket star disc diameter did not change with colony height, but it varied with season for individuals found on D. glomerata. On the other hand, colony height was positively related with maximum disc diameter (D. glomerata), and presence and number of basket stars on a colony (D. glomerata and D. florida). One very young ophiuroid was found within one polyp of a D. florida colony collected in July. We also examined bottom trawl survey data from 2011-2017 from Newfoundland to Baffin Bay, and other locations in the Eastern Canadian Arctic and found that for our largest database, 45% of trawl sets that contained basket stars also contained corals, of which 67% were Nephtheidae soft corals. In this study we show that all four Nephtheidae soft coral taxa studied act as habitat for juvenile basket stars, which are conspicuous components of benthic communities in cold-water environments. Soft corals are commonly exposed and vulnerable to bottom fishing gear in the North Atlantic. We urge future studies evaluating the nature of this relationship, the potential role of soft corals as nurseries, and the potential degree of dependency/preference between basket stars and soft corals in relation to other organisms.

Growth and Distribution of Amphioctopus fangsiao (d’Orbigny, 1839–1841) in Haizhou Bay, Yellow Sea

Octopus fisheries have prospered in many areas of the world ocean over the last six decades. Despite degradation and overexploitation of Chinese coastal ecosystems, octopus has become a stable Chinese domestic fishery species. Among the octopus species in the Bohai and Yellow, East and South China Seas, Amphioctopus fangsiao (d’Orbigny [in Ferussac & d’Orbigny], 1839–1841) plays an increasingly important role both economically and ecologically. However, no systematic research has been conducted on its biology and population dynamics in the China Seas. In this study, we characterized the growth and distribution of A. fangsiao in Haizhou Bay, Yellow Sea based on four years of trawl survey data. As the results, A. fangsiao is characterized by allometric growth and an overall sex ratio of 0.97 with a slight male bias. Small octopus can be observed through a whole year, suggesting that A. fangsiao may have an extended or continuous spawning season. A. fangsiao may migrate short distances along with seasonal changes and coastal currents, considering they overwinter in the offshore water and spawn in the inshore water. In addition to fishing pressure, annual variations in the population structure and biomass appeared to be influenced by ambient water temperature. These results provide basic biological information for a better understanding of the population dynamics and the ecological importance of A. fangsiao.

Current state of stock and fishery of red king crab Paralithodes camtschaticus at West Kamchatka

Current state of the red king crab population at West Kamchatka is considered on the data of bottom trawl surveys and fishery statistics since 2013. Some biological parameters of the crabs and assessments of the commercial stock are presented; recent condition of the red king crab fishery and evaluation of the total allowable catch are discussed. The red king crab fishery was resumed in 2013, after 8 years of ban caused by the population decrease. During the ban, the stock restored and exceeded the value observed before the ban: the total accounted abundance of red king crab in 2017 was about 244 . 106 ind. that was the highest level for the last two decades, with the number of commercial males estimated as 110 . 106 ind. — this amount was never observed before on the shelf of western Kamchatka, at least in four decades of surveys. Recently the northern forbidden district and Khayryuzovsky district of the shelf zone lost their role of the main areas of crabs reproduction and their juveniles feeding, in favour to the Ichinsky district located southward. Recent fishery of red king crab is fruitful and sustainable. In the last 3 years, the major part of annual catch is transported alive to ports of China and South Korea. For this purpose, the caught crabs are sorted strictly aboard fishing vessels and the largest males are released back to the sea. Possibly, year-to-year increasing of the males size on the West Kamchatka shelf is caused by this sorting and releasing; another effect of the large-sized crabs releasing is relatively low number of recruits in the last couple of years. The fishery elimination coefficient is estimated in the range 5–16 % of the total accounted stock for 2013–2019, that shows a highly precautionary approach. Gradual decreasing of the crab commercial stock is forecasted for the nearest future, though the stock will remain above the target value. Current state of the red king crab population at West Kamchatka is not considered as dangerous, and ongoing intense fishery of this valuable species within the recommended volumes will not affect this sustainable state.

Assessing the spatiotemporal persistence of fish distributions: a case study on two red mullet species (Mullus surmuletus and M. barbatus) in the western Mediterranean

Understanding the spatiotemporal persistence of fish distributions is key to defining fish hotspots and effective fisheries-restricted areas (FRAs). Hierarchical Bayesian spatiotemporal models provide an excellent framework to understand these distributions, as they can accommodate different spatiotemporal behaviour in the data, primarily due to their flexibility. The aim of this research was to characterize the fundamental behavioural patterns of fish as persistent, opportunistic or progressive by comparing different spatiotemporal model structures in order to provide better information for marine spatial planning. To illustrate this method, the spatiotemporal distributions of 2 sympatric Mullidae species, the striped red mulletMullus surmuletusand the red mulletM. barbatus, were analysed. The occurrence of each species, its conditional-to-presence abundance and median length were analysed using Mediterranean trawl survey data from the western Mediterranean between 2000 and 2016. Results demonstrate that there are various common hotspots of both species distributed along the Iberian coast. The convenient persistent spatiotemporal distribution of these hotspots facilitates the configuration of a network of connected FRAs for red mullets in the study area.

Impacts of the sampling design on the abundance index estimation of Portunus trituberculatus using bottom trawl

In the survey of fishery resources, the sampling design will directly impact the accuracy of the estimation of the abundance. Therefore, it is necessary to optimize the sampling design to increase the quality of fishery surveys. The distribution and abundance of fisheries resource estimated based on the bottom trawl survey data in the Changjiang River (Yangtze River) Estuary-Hangzhou Bay and its adjacent waters in 2007 were used to simulate the “true” situation. Then the abundance index of Portunus trituberculatus were calculated and compared with its true index to evaluate the impacts of different sampling designs on the abundance estimation. Four sampling methods (including fixed-station sampling, simple random sampling, stratified fixed-station sampling, and stratified random sampling) were simulated. Three numbers of stations (9, 16 and 24) were assumed for the scenarios of fixed-station sampling and simple random sampling without stratification. While 16 stations were assumed for the scenarios with stratification. Three reaction distances (1.5 m, 3 m and 5 m) of P. trituberculatus to the bottom line of trawl were also assumed to adapt to the movement ability of the P. trituberculatus for different ages, seasons and substrate conditions. Generally speaking, compared with unstratified sampling design, the stratified sampling design resulted in more accurate abundance estimation of P. trituberculatus, and simple random sampling design is better than fixed-station sampling design. The accuracy of the simulated results was improved with the increase of the station number. The maximum relative estimation error (REE) was 163.43% and the minimum was 49.40% for the fixed-station sampling scenario with 9 stations, while 38.62% and 4.15% for 24 stations. With the increase of reaction distance, the relative absolute bias (RAB) and REE gradually decreased. Resource-intensive area and the seasons with high density variances have significant impacts on simulation results. Thus, it will be helpful if there are prior information or pre-survey results about density distribution. The current study can provide reference for the future sampling design of bottom trawl of P. trituberculatus and other species.

Spatio-temporal niche of dominant shrimp species in the Zhongjieshan Islands Marine Protected Area, China

To understand the characteristics and living conditions of shrimp population in the Zhongjieshan Islands Marine Protected Area (MPA), according to the trawl survey data of the area in January, April, July and October 2018, we analyzed the spatio-temporal niche of dominant shrimp species based on the indices of relative importance, Shannon index, Pianka formula and Pearson correlation coefficient. The results showed that 16 shrimp species from 10 families and 13 genera were collected and identified. Palaemon gravier, Solenocera crassicorni, Alpheus distinguendus, Parapenaeopsis hardwickii and Alpheus japonicus were dominant species, belonging to four fami-lies and four genera. The temporal niche breadth of the dominant species varied greatly, while their spatial niche breadth was stable. The temporal niche breadth was significantly correlated with temperature suitability. The spatio-temporal niche breadth value of A. japonicus, the small shrimps, was highest. The temporal niche overlap value between P. hardwickii and S. crassicornis was the largest, indicating that the time synchronization was high and the seasonal competition was severe. There was a significant spatial niche overlap in all dominant species, indicating that the dominant species of shrimp had the same spatial homology, with strong regional competition. The spatio-temporal niche overlap value between P. hardwickii and P. gravieri was the largest, indicating that they had high similarities in physical and biological environment selecting, spawning and migratory habits, and fierce inter-specific competition. Our results could provide reference for the application of spatio-temporal niche of marine organisms.

Effects of latitude gradient and seasonal variation on the community structure and biodiversity of commercially important crustaceans in the Yellow Sea and the northern East China Sea

To evaluate the spatio-temporal variations in the community structure and biodiversity of commercially important crustaceans in the Yellow Sea and the northern East China Sea (NECS), the seasonal and regional changes in species composition, biomass structure, biodiversity and distribution of commercially important species were analyzed using bottom trawl survey data during 2014–2015. The results showed that the latitudinal gradient was obvious in species richness, dominant species and biodiversity. The indices of biodiversity increased with the decreasing latitude. When the sampling sites shifted south by one latitudinal degree, Margalef’s richness index (D), Pielou’s evenness index (J′) and Shannon–Wiener diversity index (H′) increased by 0.10, 0.03 and 0.09, respectively. The biomass proportion of the cold-temperate species represented by Crangon affinis declined with the decreasing latitude, and the warm-temperate species represented by Ovalipes punctatus and Portunus trituberculatus increased. Because of the growth regulation of crustaceans and the fishing moratorium, the biomass of commercially important crustaceans in the Yellow Sea and NECS was highest in October and August, respectively. Salinity had a more significant influence on H′ of commercially important crustaceans than other environmental factors (including zooplankton density, sea bottom temperature and water depth). Overall, the results of this study contribute to a better understanding of community dynamics of crustaceans in the Yellow Sea and NECS, and provide evidence to verify the latitudinal gradient theory in biodiversity.

Community size spectra provide indicators of ecosystem recovery on the Newfoundland and Labrador shelf

Species- and size-selective overexploitation often have ecosystem-wide impacts that are evident in community size spectra. To both derive potential ecosystem targets for community rebuilding and assess contemporary indicators relative to these targets, we constructed theoretical size spectra to predict pristine biomass densities using a combination of species- and size-specific nitrogen stable isotope signatures and a range of trophic efficiencies and primary productivity estimates within and among 3 sub-regions of the Newfoundland and Labrador shelf. Theoretical size spectra were compared to empirically derived size spectra using trawl survey data. The descending slopes of the empirically determined size spectra were between 1.25 and 2.42 times steeper than the theoretical slopes. The percentage of the theoretical distribution represented by the empirical size structure ranged between 1.1 and 29.4%, with the closest and furthest estimates associated with the smallest- and largest-sized fishes, respectively, and strongly influenced by estimates of trophic efficiency and primary productivity. Regional variation was also observed, with southern regions reaching 1.3-32.3% of the theoretical biomass density and the northernmost region reaching only 0-8.3%. Importantly, the descending slopes varied depending on fish size, with biomass density of larger sizes decreasing faster than that of smaller sizes. Variations among sub-regions and fish guilds were also observed. These analyses provide a means to derive potential ecosystem targets and indicators through which recovery of fish communities can be monitored and assessed.

Spatio-temporal variability in the distribution pattern of anglerfish species in the Mediterranean Sea

The Mediterranean distributions of two species of anglerfish, the blackbellied anglerfish (Lophius budegassa) and the white anglerfish (Lophius piscatorius), were analysed from trawl survey data (MEDITS project – Spain, France, Italy and Greece) from 2006 to 2015 implementing a Delta model approach with residuals autocovariate boosted regression trees. Sea bottom temperature (SBT), sea bottom salinity (SBS), bathymetry, slope of the seabed and distance to the coast were considered possible predictors. The results show that the locations with a higher presence, abundance and biomass of L. budegassa are those with a depth range between 150 to 300 m, with an SBT range between 17.5 and 18.5°C, and SBS of 37-38 PSU. Similarly, L. piscatorius shows a higher probability of presence, abundance and biomass in location with a bathymetry range of 200-400 m, an SBT of 17.5°C to 18.5°C and an SBS of 36.5 to 37.5. Our results identify preference habitats for the anglerfishes in the Mediterranean Sea such as the Aegean Sea, the Gulf of Lions, south and southeast Spain and the northwestern Ionian Sea. In general terms, these findings enhance our understanding of the differences in the spatio-temporal distribution of these two species, providing useful information that can help their fisheries management and conservation.

Spatial and temporal trend in the abundance and distribution of gurnards (Pisces: Triglidae) in the northern Mediterranean Sea

In this study we investigated the spatio-temporal distribution of gurnards (8 species of Triglidae and one species of Peristediidae) in the northern Mediterranean Sea using 22 years of MEDITS bottom trawl survey data (1994-2015). Gurnards showed significant differences in terms of abundance, dominance and composition among geographical sub-areas and ecoregions, with the highest relative biomass (BIy) being found in Malta, eastern Corsica, the Balearic Islands and the eastern Ionian Sea. The lowest gurnards BIy were observed in the highly exploited areas of the western Mediterranean and the Adriatic Sea, where the largest number of species with a negative linear trend in BIy was also found. The temporal trends in species abundances highlighted a general decrease for the coastal species (C. lucerna, C. lastoviza, C. obscurus) as compared with the species inhabiting the deep continental shelf and slope (T. lyra, P. cataphractum). The results provide for the first time an overview of the spatiotemporal trend in the abundance of gurnards over the wide spatial scale of the northern Mediterranean Sea, also suggesting the possible use of these species as indicators for monitoring the impact of fishing pressure on demersal fish assemblages.

Climate change, interspecific competition, and poleward vs. depth distribution shifts: Spatial analyses of the eastern Bering Sea snow and Tanner crab (Chionoecetes opilio and C. bairdi)

Snow crab (Chionoecetes opilio), an arctic species, and Tanner crab (C. bairdi), a subarctic species, support large-scale industrial fisheries in the eastern Bering Sea and are spatially overlapping congeneric species at the opposite boundaries of their distributional ranges in the eastern North Pacific. Using summer bottom trawl survey data from 1982 to 2017, spatial analyses and geostatistical regressions were conducted for each species for three population categories (immature females, oldshell mature females, and oldshell large males) to explore intra and interspecies differences in habitat and spatial distributions as well as interspecific spatial co-occurrence patterns. Variability in depth distributions rather than poleward distributional shifts may be a Tanner crab response to ocean warming, the apparent converse of snow crab. Both temperature and depth have strong effects on the Tanner crab distributions and 2 °C is an approximate minimum temperature threshold for Tanner crab habitat. 2 °C is an important maximum temperature threshold for immature female snow crab but temperature effects are weaker for adult snow crab. Snow crab ontogenetic movement results in loose coupling between settlement and adult habitat and in significant spatial overlap between older snow crab and Tanner crab. The similarity in spatial distributions and habitat of older snow crab with juvenile Tanner crab habitat and the predatory nature of snow suggest a possible regulatory role of snow crab on the northern extent of spatial distributions of Tanner crab, along with other possible factors such as groundfish predation. Snow crab distributions will most probably continue to shift northwards with warming ocean temperatures but a poleward shift for Tanner crab appears more uncertain. The utilities of comparing spatial and non-spatial models and accounting for spatial confounding in spatial regression modeling are also addressed.

Delineating management units for Pacific cod (Gadus macrocephalus) in the Sea of Japan

Advances in population genetics have brought about the use of management units (MUs) for the assessment and management of marine resources. Some marine species with an apparently continuous distribution actually have local genetically distinct populations across their range, so careful consideration is required in the delineation of MUs. To delineate MUs for Pacific cod (Gadus macrocephalus) in the coastal area of the Sea of Japan, we studied the geographic distribution and genetic variation of the species. Two geographically separate groups were identified based on the analysis of trawl survey data. A spatially explicit clustering analysis of 309 sequences of the mitochondrial NADH dehydrogenase 2 gene identified two groups of samples: one in the central to northern Sea of Japan (NJ) and another in the westernmost shelf area around the Tsushima Strait (WJ). The population structure was further confirmed by analysis of molecular variance and a significant pairwise Φst value between the two groups. The NJ population had experienced a post-glacial population expansion, whereas the WJ population size remained stable, probably reflecting different oceanographic conditions in the two regions. The two populations diverged from each other during the last glacial period, likely due to habitat discontinuity. Finally, based on the species’ genetic population structure and the geographic distributions, we parsimoniously propose that a single boundary be set along the coast of western Honshu Island to divide MUs in the Sea of Japan.

Decadal variations in the community status of economically important invertebrates in the Bohai Sea

With the decline of fish stocks, the proportions of economically important invertebrates like crustaceans and cephalopods have increased in the Bohai Sea. The community structure and trophic level of economically important invertebrates were analyzed using the bottom trawl survey data collected by the Yellow Sea Fisheries Research Institute in the Bohai Sea in May and August of 1958–1959, 1982, 1992–1993, 2004, 2009 and 2015. A total of 37 species of economically important invertebrates, belonging to 5 orders, 24 families, were captured. The biomass densities of economically important invertebrates in the Bohai Sea displayed an overall downward trend from 1982 to 2015. Oratosquilla oratoria and Loligo spp. were the most dominant species in the past 30 years, the biomass proportion of O. oratoria increased gradually in both May and August from 1982 to 2015. Moreover, biodiversity indices of economically important invertebrates in the Bohai Sea appeared to decline from 1982 to 2004 and then increased in 2015. Similarly, the mean trophic level of economically important invertebrates declined from 1982 to 2004 and increased slightly in 2015. Overall, although the proportions of invertebrates have increased, the biomass densities in the Bohai Sea have displayed an overall downward trend from 1982 to 2015. The increases in the biodiversity and trophic level of economically important invertebrates after the 2000s, possibly benefit from stock enhancement projects implemented by governments at different levels and national fishery management measures such as the “double-control” of the total number and engine power of fishing vessels and summer moratorium of fishing.

Roles of fishing and climate change in long-term fish species succession and population dynamics in the outer Beibu Gulf, South China Sea

A prevailing, controversial hypothesis is that fishing pressure has played a greater role than climatic and environmental drivers, in changing fish species succession and biomass fluctuation in the South China Sea (SCS). Based on otter trawl survey data from 1959 to 2010 in the outer Beibu Gulf (OBG), northern SCS, large seasonal and interannual variation is reported for fish species composition, the proportional abundances of dominant taxa, and fish biomass. Generalized additive models are developed to quantify relationships between fish biomass and the external factors of fishing pressure and climate change. Fishing pressure proved to be the main driver of sharp declines in demersal fish stocks, with high-value species being replaced by low-value ones over time. Abrupt decreases in fish biomass during the years of 1993 and 1998 correspond to El Nino events, with climate change possibly the main driver of proportional representation of pelagic species in fisheries trawl catch. The need to differentiate impacts of fishing and environmental drivers on fish species with different life history strategies is stressed to better understand fish community dynamics.