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Gear selectivity is crucial in fisheries management because it influences the size and species of fish caught, with traditional methods often basing optimal mesh size on the mature length of a target species. However, to ensure sustainable fisheries, both biological aspects of resource protection and fishing operation efficiency must be considered. This study aimed to determine the ideal mesh size for trap fishing of marbled flounder (Pseudopleuronectes yokohamae) in Gyeongsangnam‐do, South Korea, by using biological and economic factors for sustainable fisheries management. We sampled fish caught in traps of varying mesh sizes (35, 50, 65, 80, and 100 mm) and developed two models based on fish maturity, bycatch ratio, and CPUE. Model 1 used a maturity criterion of 17.8 cm, and Model 2 used the legal minimum length limit of 20 cm. Net selectivity curves indicated that the optimal mesh size ranged 50–65 mm based only on a minimum mature body length of 17.8 cm. In contrast, the optimal mesh size based on both biological and economic factors ranged 57.5–90 mm. Our findings could guide policymakers in enhancing marble flounder population management strategies through policy adjustments in mesh size and gear restrictions. [ABSTRACT FROM AUTHOR]

The hypervariable region (HVR) in the control region of the mitochondrial DNA has frequently been used for population genetics and phylogeographic studies because of its highly variable nature. Although the HVR is beneficial for evaluating recent evolutionary history, including population demography, recent studies have implied the incidence of homoplasy in this region. To assess the accuracy of relying solely on the HVR for population genetics studies, molecular evolutionary analysis of the HVR, NADH-dehydrogenase subunit 2 (ND2), and cytochrome b genes were performed using 120 individuals of marbled flounder Pseudopleuronectes yokohamae. The HVR exhibited the highest genetic variability among the three regions, with sites showing high site-specific substitution rates. Considering the reticulate haplotype network structure and evolutionary linkages between regions, homoplastic mutations were indicated in the HVR in addition to ND2, underestimating genetic diversity. We found that homoplasy was less likely to affect coalescent-based demographic inferences in the population; however, there is still a potential risk of misinterpretation of population demography when solely using the HVR owing to its hypervariable nature. Collectively, we suggest analyzing other regions in addition to the HVR in fish population genetic research to improve accuracy and eliminate biases caused by homoplasy. [ABSTRACT FROM AUTHOR]

The marbled flounder (Pseudopleuronectes yokohamae) is a demersal flatfish species, widely distributed in the northwestern Pacific region. In the present study, the mitochondrial DNA (mtDNA) control region was used to determine the genetic diversity and population genetic structure of this species. We obtained a 380 bp segment of the mtDNA control region after the alignment of 78 individual sequences from P. yokohamae collected from two locations in Korea (Biungdo and Mukho) and one location in Japan (Tohoku) and 103 individual sequences from a previous study (Yokjido and Namhae). The overall haplotype diversity and nucleotide diversity were 0.983 ± 0.003 and 0.016 ± 0.008, respectively. The genealogical relationships of the mtDNA control region did not exhibit any specific genealogical association according to sampling location. The pairwise FST value indicated that the Biungdo (west coast of Korea) and Tohoku (Japan) populations showed genetically differentiated structures (but the Tohoku and Mukho populations did not). However, there was no discernible difference between the Mukho population from Korea's eastern coast and the Yokjido and Namhae populations from the southern coast. The biological characteristics of P. yokohamae and oceanographic barriers may have contributed to producing genetically distinct populations. [ABSTRACT FROM AUTHOR]

The marbled flounder (Pseudopleuronectes yokohamae) is a demersal flatfish species, widely distributed in the northwestern Pacific region. In the present study, the mitochondrial DNA (mtDNA) control region was used to determine the genetic diversity and population genetic structure of this species. We obtained a 380 bp segment of the mtDNA control region after the alignment of 78 individual sequences from P. yokohamae collected from two locations in Korea (Biungdo and Mukho) and one location in Japan (Tohoku) and 103 individual sequences from a previous study (Yokjido and Namhae). The overall haplotype diversity and nucleotide diversity were 0.983 ± 0.003 and 0.016 ± 0.008, respectively. The genealogical relationships of the mtDNA control region did not exhibit any specific genealogical association according to sampling location. The pairwise FST value indicated that the Biungdo (west coast of Korea) and Tohoku (Japan) populations showed genetically differentiated structures (but the Tohoku and Mukho populations did not). However, there was no discernible difference between the Mukho population from Korea’s eastern coast and the Yokjido and Namhae populations from the southern coast. The biological characteristics of P. yokohamae and oceanographic barriers may have contributed to producing genetically distinct populations.

Simple Summary: The marbled flounder (Pseudopleuronectes yokohamae) is a fish species with high commercial value in Korea and has been gaining attention as a new aquaculture target. To clarify the digestive physiology and feeding habits of the marbled flounder, this study conducted morphological, histological, and histochemical analyses of its digestive tract. In conclusion, the feeding habits of marbled flounder are close to those of carnivorous fish. In addition, we found that the marbled flounder has a stomach with a structure suitable for eating small amounts of hard food, and that most of its digestion takes place in the anterior intestine portion. Our study presents the histological and histochemical characteristics of the digestive tract of the marbled flounder, providing basic knowledge for both physiological and nutritional studies. This study investigated the morphological, histological, and histochemical characteristics of the digestive tract of the marbled flounder (Pseudopleuronectes yokohamae). The relative length of the gut of the marbled flounder digestive tract was 1.54 ± 0.10 (n = 20), and it had a simple stomach and 6–9 pyloric caeca. The mucosal folds of the marbled flounder digestive tract exhibited a general branched morphology. The thickness and mucosal fold length of the intestinal muscularis externa showed similar aspects in all areas. The thickness of the intestinal muscularis externa was the thickest in the posterior intestine portion, and the length of mucosal folds was the longest in the anterior intestine portion. It was indicated that food digested by gastric acid in the stomach moves to the anterior portion (including pyloric caeca) and mid portion of the intestine, ensuring effective stimulation of cholecystokinin (CCK)-producing cells. In addition, the distribution pattern of CCK-producing cells in the intestine was very similar to that of mucus-secreting goblet cells. The CCK-producing cells and goblet cells in the marbled flounder were well-adapted to promote optimal control of the digestive process. Based on the morphological and histochemical studies, it was concluded that the marbled flounder displays a digestive tract comparable to that of fish species with carnivorous habits. [ABSTRACT FROM AUTHOR]

Offshore manmade structures, termed artificial reefs (e.g., wind farms, oil platforms, and manmade islands), affect the movement and migration of wild migratory marine animals. Certain types of migratory pelagic and benthic fish are known to utilize artificial reefs as feeding areas, for spatial reference (landmarks for their navigation), or as meeting points. However, little is known about the impact of artificial reefs on the migratory behavior of demersal flatfish. The marbled flounder (Pseudopleuronectes yokohamae), an important commercial flatfish species, exhibits seasonal migration between shallow water for spawning in winter and deeper water to avoid high water temperatures in summer. In this study, 39 tagged marbled flounders were released in the shallow water of the inner Osaka Bay area in June 2016, and then actively tracked, predominantly in deeper waters (100–140 m), from a fishing boat every week until mid-September 2016. Our active tracking detected no tagged fish in the deeper waters, whereas 36% of the fish were detected in the shallow water (10–15 m) around an artificial reef (offshore manmade island) at a high water temperature (26 °C). We suggest that a portion of the marbled flounder population did not show seasonal migration but remained in the shallow water around the artificial reef, even though the water temperature was high in summer. We suggest that this is related to the abundant food supply associated with the artificial reef. [ABSTRACT FROM AUTHOR]

An 8-week feeding trial was conducted to determine the optimal dietary protein level for juvenile marbled flounder. Five semi-purified test diets were formulated to contain different protein levels (CP) including 42.7%, 47.4%, 53.3%, 58.8%, and 64.5% (dry matter), named as CP42.7, CP47.4, CP53.3, CP58.8, and CP64.5, respectively. Five hundred and twenty-five juveniles (6.0 ± 0.1 g) were randomly distributed into 15 tanks (300 L tanks), resulting in 35 fish per tank (n = 3 tanks). Fish were fed the test diets 5 times per day until satiation. The CP58.8 resulted in the highest gain in weight and the best efficiency in feed utilization among the tested protein levels (P

The marbled flounder (Pseudopleuronectes yokohamae) is highly esteemed for its exceptional nutritional value and delicious taste. However, this species has extremely limited transcriptome data, which can offer priceless information for disease protection. In the study, the skin transcriptomic sequencing of P. yokohamae revealed 7.72 GB of clean data using the Nanopore sequencing platform. The results revealed 30,498 transcripts of functional annotations in the P. yokohamae transcriptome. All transcripts were searched in eight functional databases. A total of 10,337 ORFs were obtained, of which 6081 complete ORFs accounted for 58.83% of all predicted CDS. Moreover, 10,195 SSRs were detected. Meanwhile, the non-pecific immunity pathways were investigated for better understanding of the immunological reaction in P. yokohamae, and seven innate immune pathways were identified. The innate-immune related genes were highly expressed in the NOD-like receptor signaling pathway, followed by the C-type lectin receptor signaling pathway, Toll-like receptor signaling pathway, RIG-I-like receptor signaling pathway and Cytosolic DNA-sensing pathway. In this study, four families of antimicrobial peptides (AMPs) in P. yokohamae were analysed for the first time , including piscidins, hepcidins, liver-expressed antimicrobial peptide and defensins. Seven AMPs, including Py pleurocidin-like WF3, Py pleurocidin-like WFX, Py hepcidin 1, Py hepcidin-like 1, Py LEAP-2, Py beta-defensin and Py beta-defensin-like 1, were further identified. The seven AMPs showed a highly identity in their cDNA and genomic structures and an inducible expression pattern preferable to skin in response to pathogens. The transcriptomic data and investigation of AMPs from P. yokohamae promote a deeper awareness of fish mucosal immunity and provide information in the prevention of fish diseases. • Full-length transcriptome of P. yokohamae from skin was generated using Nanopore sequencing technology. • A total of 30,498 transcripts of functional annotations were obtained. • Novel AMPs were identified and showed a highly identity in genomic structure. • The AMPs were expressed widely in tissues of P. yokohamae. • The AMPs were induced in skin against Vibrio splendidus infection. [ABSTRACT FROM AUTHOR]

Previous studies have shown an increase in otolith Mn caused by exposure to hypoxic water masses. The mechanism leading to the increases in otolith Mn is still unclear, but might possibly be due to the larger amount of available Mn left in the water column under hypoxia. Thus, this study aimed to examine the relationship between hypoxia, water Mn and otolith Mn through marbled flounder (Pseudopleuronectes yokohamae, Günther) captured from Tokyo Bay and reared under different water Mn at laboratory. Otoliths from the Bay showed a higher (Mn/Ca)otolith than outside, together with a seasonal trend of high (Mn/Ca)otolith at the start of translucent zones (which form in the summer), supporting the occurrence of summer hypoxia in Tokyo Bay. Nonetheless, juveniles reared under control (Mn 0.50 μmol l−1), middle (Mn 6.94 μmol l−1) and high (Mn 10.4 μmol l−1) treatments of water Mn concentrations showed a disproportional smaller increase in (Mn/Ca)otolith. Comparing the laboratory experiment with the field data, (Mn/Ca)water under hypoxia in Tokyo Bay could reach a low level similar to control treatment, yet (Mn/Ca)otolith of the Bay showed a higher value than the high treatment. These results revealed an elevated (Mn/Ca)otolith towards hypoxia, but also suggested that changes in water Mn might not be directly recorded by otolith Mn. [ABSTRACT FROM AUTHOR]

The marbled flounder (Pseudopleuronectes yokohamae) is a widely distributed, economically important demersal species in Japan. A study into the growth of marbled flounder in six sampling areas around Japan also found significant differences in otolith nucleus size (diameter of otolith opaque core area) among areas, warm areas showed the bigger nucleus size of otolith, and cold areas showed the smaller size. Based on the physical environmental factors, it was suggested that water temperature affected the nucleus size among these sampling areas. To investigate the factors that influenced the formation of opaque and translucent zones in otolith nucleus, we conducted a rearing experiment using juvenile fish at 122 days post hatch (dph). Fish were reared at salinity of 33? and different water temperatures (15, 20, 25 °C) from 138 to 213 dph. The otolith growth was correlated positively to water temperature. We also measured the relative greyscale values of the opaque and translucent zones during the period of rearing experiment in each otolith, which showed a negative correlation with water temperature. The results showed that there was a significant relationship between water temperature and otolith formation. The otolith of the marbled flounder will grow larger and have a higher matrix density at higher water temperatures. [ABSTRACT FROM AUTHOR]

The bycatch issue is a major concern in small-scale accordion-shaped trap fisheries in the Yellow Sea of China because of the poor species and size selectivity of fishing gear and the co-occurrence of multispecies. Two threatened flatfish species assessed by the IUCN, marbled flounder (Pseudopleuronectes yokohamae) and stone flounder (Platichthys bicoloratus), are the two main bycatches in accordion-shaped trap fisheries. This fishery also captures a large proportion of juvenile target species, resulting in both recruitment and growth overfishing of these species. In this study, we introduced a new concept of a combined sorting grid and escape vent (CSEV), a selection device aimed at reducing the bycatch of flatfish and undersized target species in mixed-species trap fisheries. We used a catch comparison method to assess the performance of the CSEV in field experiments, with conventional traps as the baseline. Our results demonstrated that the CSEV significantly improved the species and size selectivity of the traps. Test traps mounted with the CSEV significantly reduced the catch rates of marbled flounder and stone flounder by 70% and 74% throughout the length class, respectively, compared with conventional traps. The catch of undersized target species, namely, fat greenling (Hexagrammos otakii), black rockfish (Sebastes schlegelii), and Asian paddle crab (Charybdis japonica), was reduced by 79%, 75%, and 80%, respectively, while the catch rates of target-sized individuals were maintained or even improved. Given these promising results, the CSEV may have great potential for application in accordion-shaped trap fisheries and similar fisheries. [ABSTRACT FROM AUTHOR]

Highlights: [•] A novel interpretation of biochemical indices were conducted using the GLM approach. [•] The study is to evaluate the biochemical effects on coastal fish by chemical pollutant. [•] Endogenous morphometric indices were defined in this model. [•] The GLM approach allowed the improvement in interpretation of the biochemical indices. [Copyright &y& Elsevier]