Japanese Mantis Shrimp Oratosquilla Oratoria Research Articles

Time-Course and Tissue-Specific Molecular Responses to Acute Thermal Stress in Japanese Mantis Shrimp Oratosquilla oratoria.

Current understanding of adaptability to high temperatures is increasingly important as extreme weather events that can trigger immediate physiological stress in organisms have occurred more frequently. Here, we tracked transcriptomic responses of gills, hepatopancreas, and muscle to acute thermal exposure at 30 °C for 0.5, 6, and 12 h in an economically important crustacean, Oratosquilla oratoria, to gain a preliminary understanding of the tissue-specific and dynamic physiological regulation process under acute heat stress. The unique physiological responses of muscle, hepatopancreas, and gills to acute thermal stress were associated with protein degradation, lipid transport, and energy metabolism in O. oratoria, respectively. Functional enrichment analysis of differentially expressed transcripts and heat-responsive gene clusters revealed a biphasic protective responsiveness of O. oratoria developed from the early responses of signal transduction, immunity, and cytoskeleton reorganization to the responses dominated by protein turnover and energy metabolism at the mid-late stages under acute heat stress. Noteworthy, trend analysis revealed a consistently upregulated expression pattern of high molecular weight heat shock protein (HSP) family members (HSP60, HSP70, and HSP90) during the entire thermal exposure process, highlighting their importance for maintaining heat resistance in O. oratoria. Documenting the whole process of transcriptional responses at fine temporal resolution will contribute to a far-reaching comprehension of plastic responses to acute heat stress in crustaceans, which is critical in the context of a changing climate.

Cryptic diversity and population genetic structure of the mantis shrimp Oratosquilla oratoria in South Korea.

The Japanese mantis shrimp Oratosquilla oratoria has two genetic lineages in its Chinese and Japanese populations, which are considered cryptic species. However, the Korean O. oratoria populations have only been studied morphologically, and their genetic diversity pattern remains unknown. To examine the genetic diversity of the Korean O. oratoria populations to explore its cryptic diversity and underlying processes. We sequenced the mitochondrial cytochrome c oxidase I gene of 107 O. oratoria individuals from 8 sampling sites in South Korea. Their genetic diversity and haplotype data were compared to previously reported data from 16 Chinese and 6 Japanese populations. The haplotype network, analysis of molecular variance, FST values, and Bayesian skyline plot of the Korean O. oratoria populations confirmed that they were divided into two genetic lineages corresponding to the two previously identified cryptic species, which might be correlated to ocean currents and water temperature. In addition, the population genetic analysis indicated that both lineages had high genetic diversity. Although the Geoje region belongs to the southern group, it only contained lineage N, which could be related to the specific environmental characteristics of Geoje or the hybridization of the two lineages of O. oratoria. Korean O. oratoria populations have enough cryptic diversity to be described as cryptic species and are correlated with ocean currents and water temperatures. Lineage N dominance in Geoje might be related to environmental characteristics or hybridization.

Insights into adaptive divergence of Japanese mantis shrimp Oratosquilla oratoria inferred from comparative analysis of full-length transcriptomes

The heterogeneous seascapes in the northwestern Pacific (NWP) can be important selective forces driving adaptive divergence of marine coastal species distributed along the gradients. Here, we tested this hypothesis in Japanese mantis shrimp (Oratosquilla oratoria) with a wide distribution in the NWP and a significant north-south population structure. To this end, the full-length (FL) transcriptomes of northern and southern O. oratoria were firstly sequenced using PacBio single molecule real-time sequencing technology. Based on the FL transcriptome data, we captured large-scale FL transcripts of O. oratoria and predicted the FL transcriptome structure, including coding region, transcription factor and long noncoding RNA. To reveal the divergence between northern and southern O. oratoria, we identified 2,182 pairs of orthologous genes and inferred their sequence divergences. The average differences in coding, 5’ untranslated and 3’ untranslated region were 1.44%, 2.79% and 1.46%, respectively, providing additional support to previous proposition that northern and southern O. oratoria are two species. We provided further evolutionary context to our analysis by identifying positive selected genes (PSGs) between northern and southern O. oratoria. In total, 98 orthologs were found evolving under positive selection and involved several environmentally responsive genes associated with stress response, immunity and cytoskeletal organization, etc. Furthermore, we found PSGs also diverged in gene expression response of northern and southern O. oratoria to heat stress. These findings not only highlight the importance of genetic variation in these genes in adapting to environmental changes in O. oratoria, but also suggest that natural selection may act on the plasticity of gene expression to facilitate O. oratoria adaptation to environmental gradients. Overall, our work contributes to understanding how marine coastal species has evolved to adapt to heterogeneous seascapes in the NWP.

Evaluating Impacts of Trophic Interactions on the Effectiveness of Single-Species Fisheries Management

Single-species fisheries management (SSFM) is applied to many fisheries ecosystems around the world. The associated ecological impacts are usually not well understood due to the lack of considering trophic interactions among species in the ecosystem. This impedes the implementation of SSFM in an ecosystem context and reduces our ability to understand the possible ecological impacts of fishing activities. This study focuses on two economically important species in the Jiaozhou Bay, China: the short-lived, fast-growing, and relatively abundant Japanese mantis shrimp (Oratosquilla oratoria) and the long-lived, slow-growing, and less abundant Korean rockfish (Sebastes schlegelii). We evaluated how varying trophic interactions influenced O. oratoria and S. schlegelii (i.e., target-species) who were managed under constant fishing pressure. The increase of fishing pressure to other species (i.e., non-target species) was beneficial to O. oratoria and S. schlegelii. O. oratoria was more sensitive to the decrease of fishing pressure to other species. The predation mortality of age-0 O. oratoria increased with the increased fishing pressure to other species. The predation mortality of age-1 O. oratoria and age-0 S. schlegelii had negative relationships with the fishing pressure to other species. Age-1 S. schlegelii seemed not to be sensitive to the changes in trophic interactions. The predation mortality of O. oratoria and S. schlegelii had bigger changes than the starvation mortality after fishing changed. It suggested the prey-predator relationship had a bigger impact than the food competition. The increase of high-trophic-level fish Johnius belangerii fishery positively impacted O. oratoria, but negatively impacted S. schlegelii. S. schlegelii was more sensitive to the changes of the low-trophic-level fish Pholis fangi fishery. Given the complex dynamics of ecosystems, this study highlights the importance of species-specific responses of fishes to shifting trophic interactions in fisheries management.

Nuclear microsatellites reveal population genetic structuring and fine-scale pattern of hybridization in the Japanese mantis shrimp Oratosquilla oratoria.

The interplay between historical and contemporary processes can produce complex patterns of genetic differentiation in the marine realm. Recent mitochondrial and nuclear sequence analyses revealed cryptic speciation in the Japanese mantis shrimp Oratosquilla oratoria. Herein, we applied nuclear microsatellite markers to examine patterns and causes of genetic differentiation in this morphotaxon. Population structure analyses revealed two genetically divergent and geographically structured clades in O. oratoria, one dominating the temperate zone of the Northwestern (NW) Pacific and the other occurring in the subtropical and tropical waters where are influenced by the Kuroshio Current. Two sympatric zones, one around the Changjiang Estuary in China coast and the other in the northern Japan Sea, were demonstrated to be hybrid zones where introgressive hybridization occurred asymmetrically. The interaction between historical climate shifts and contemporary factors (e.g., freshwater discharge, temperature gradient and isolation by distance) may contribute to the present-day genetic architecture in the Japanese mantis shrimp. Range shift induced by climate changes and oceanographic factors may promote hybridization and gene flow between the O. oratoria complex. Our results provide insights into the interacting mechanisms that give rise to diversification and speciation of coastal species in the NW Pacific.

Molecular identification and phylogenetic relationship of Erugosquilla massavensis (Kossmann, 1880) from the Mediterranean Sea, Egypt

The Erythraean species of mantis shrimp, Erugosquilla massavensis, has been reported as the first Lessepsian migrant stomatopod to the Mediterranean Sea in 1933, off Alexandria, Egypt via the Suez Canal. Currently, it extended to the middle and western Mediterranean competing with the native spot-tail mantis shrimp Squilla mantis (Linnaeus, 1758). The continuous increase of invasive species to the Mediterranean claimed the necessity to update and revise the identification of those early migrants. The present study aimed to confirm the morphological identification of E. massavensis from the coast of the Egyptian Mediterranean Sea using the DNA barcoding gene cytochrome oxidase subunit I (COI) and conducting a phylogenetic analysis. Genomic DNA of samples collected from the Mediterranean Sea at Port Said in 2016 was extracted and COI was amplified and sequenced. Our phylogenetic analysis revealed that E. massavensis (MH447072 and MH447073) constitutes a single monophyletic clade and appeared more linked with E. woodmasoni than the Japanese mantis shrimp Oratosquilla oratoria (divergence value < 3%). The phylogenetic relationship between E. massavensis, E. woodmasoni, and O. oratoria was confirmed by the Neighbor-Joining tree. The present study confirmed the morphological identification and document, for the first time in GenBank/EMBL/ DDBJ genetic databases, the genetic status of E. massavensis from the Mediterranean of Egypt using COI.

Effect of salinity fluctuation on the transcriptome of the Japanese mantis shrimp Oratosquilla oratoria

Salinity fluctuation may detrimentally affect the composition and biological processes of crustaceans. As a euryhaline crustacean, Oratosquilla oratoria can survive at salinities ranging from 20 psu to 40 psu. Therefore, we designed five salinity gradients (20, 25, 30, 35, and 40 psu) and 66.39 Gb clean transcriptome data were obtained after O. oratorias were exposed to each gradient for 24 h. All clean data were spliced into 50,482 unigenes and 17,035 unigenes were annotated in at least one database. Compared with 30 psu, 1010, 851, 1733 and 2188 differentially expressed genes were obtained at 20, 25, 35 and 40 psu, respectively. Results also showed that the osmoregulation of O. oratoria is primarily regulated by lipid and amino acid metabolism, amongst others. No significant up-regulated pathways were enriched at 25 psu and 35 psu, although more significant down-regulated pathways were obtained at 35 psu. Therefore, we assumed that the optimum survival salinity of O. oratoria may range from 25 psu to 35 psu. However, 35 psu may be more suitable for O. oratoria. In addition, 55 unigenes that encode putative inorganic ion exchanges were identified. This study aims to provide fundamental information for understanding the osmoregulation mechanisms of crustaceans.

Ultrastructure of spermiogenesis and the distribution of spermatozoal nuclear histones in the Japanese mantis shrimp, Oratosquilla oratoria (Crustacea: Stomatopoda).

The Japanese mantis shrimp Oratosquilla oratoria (Stomatopoda; Crustacea) is one of the most economically important aquatic species of Pacific shrimp and it is distributed from Japan to the coast of China, the Philippines, the Malay Peninsula, and the Hawaiian Islands. Early studies described certain characteristics of spermatogenesis and the sperm ultrastructure in Stomatopoda, but the composition of sperm basic nuclear proteins (SBNPs) remains completely unknown. We studied the sperm ultrastructure of O. oratoria using transmission electron microscopy and the histone composition using immunofluorescence and immunoelectron microscopy. We found that the spherical nucleus is adjacent to the electron translucent external coat, which occurs in early spermatids. The acrosomal structure begins to form at the junction of the nucleus and the external coat. At the mid‐spermatid stage, part of the chromatin appears to be more electron‐dense than the external coat side. The aflagellate sperm of O. oratoria, are rounded or slightly ovoid in shape and have a consistent granular nucleus, an acrosome structure of pushpin shape and a spherical vesicular body in which faintly granular material is scattered. The acrosome consists of an acrosomal vesicle, perforatorium, and subacrosomal material. The sperm contains histones H2A, H2B, H3, H4, H3.3, H2AX, and H2AZ as well as some histone modifications, that is, H3K9me3, H3K4me2, H3S10ph, H4Kac, and H2A + H4S1ph. Histones are localized not only in the nucleus of the sperm but also in other structures outside the nucleus. The results may provide new perspectives for systematic studies of crustaceans and their sperm chromatin components. These findings extend the study of the sperm structure of Stomatopoda and provide basic data to elucidate the epigenetic mechanism of fertilization.

Isolation and characterization of microsatellite markers for exploring introgressive hybridization between the Oratosquilla oratoria complex.

The Japanese mantis shrimp Oratosquilla oratoria is one of the most dominant stomatopod species found in coastal waters of the northwestern Pacific. We previously reported cryptic speciation and hybridization in this taxon by integrating mitochondrial and nuclear evidence. The present study aims at developing potential useful microsatellite markers for the O. oratoria complex through transcriptome sequencing, with a view to reveal the occurrence of hybridization. Of the 100 tested microsatellites, 55 were experimentally validated. 24 of these microsatellites were transferable across different species of the O. oratoria complex and showed polymorphic among individuals. The average number of alleles, observed and expected heterozygosity per locus was 6.125, 0.446 and 0.577 for the temperate species, and 6.083, 0.444 and 0.578 for the subtropical and tropical species. We also explore genetic differentiation and hybridization between O. oratoria cryptic species using these 24 microsatellite loci. The pairwise FST values and phylogenetic tree indicated a strong genetic differentiation between the two cryptic species. In addition, Bayesian analysis provided evidence for the presence of hybridization between the O. oratoria complex. These markers provide valuable genomic resources for exploring introgressive hybridization and expanding understanding of evolution in the O. oratoria complex.

De novo transcriptome analysis and differentially expressed genes in the ovary and testis of the Japanese mantis shrimp Oratosquilla oratoria by RNA-Seq

The mantis shrimp Oratosquilla oratoria is a widely distributed, commercially important crustacean species. Although its conservation and the development of successful artificial breeding technologies have recently received considerable attention, there are currently no available data regarding the molecular mechanisms in controlling reproduction. In this study, we performed transcriptome sequencing of the testis, ovary, female and male eyestalks and the androgenic gland of O. oratoria, and compared the expression pattern of transcripts from the testis and ovary libraries to identify genes involved in gonadal development. A total of 147,130,937 clean reads were retrieved after removing the adapters in reads and filtering out low-quality data. All the reads were assembled into 94,990 unigenes (23,133 in testis and ovary) with an average length of 783 base pairs (bp) and N50 of 1502 bp. A search of all-unigenes against COG, GO, KEGG, KOG, Pfam, Swiss-Prot and Nr databases resulted in a total of 19,404 annotated unigenes. Comparison of the sequences in the ovary and testis libraries revealed that 1188 unigenes were up-regulated in the ovary and 2732 were up-regulated in the testis. Twenty ovary-up-regulated and 21 testis-up-regulated unigenes were confirmed by quantitative real-time PCR. Additionally, 13,437 simple sequence repeats (SSRs) and 275,799 putative single nucleotide polymorphisms (SNPs) were identified. The important functional genes and pathways identified here provide a valuable dataset for understanding the molecular mechanisms controlling gonad development in O. oratoria, and the numerous (13,437 SSRs and 275,799 SNPs) molecular markers obtained here will provide fundamental basis for functional genomic and population genetic studies of O. oratoria.

Cryptic diversity in the Japanese mantis shrimp Oratosquilla oratoria (Crustacea: Squillidae): Allopatric diversification, secondary contact and hybridization

Mounting evidence of cryptic species in the marine realm emphasizes the necessity to thoroughly revise our current perceptions of marine biodiversity and species distributions. Here, we used mitochondrial cytochrome oxidase subunit I (mtDNA COI) and nuclear ribosomal internal transcribed spacer (nrDNA ITS) to investigate cryptic diversity and potential hybridization in the Japanese mantis shrimp Oratosquilla oratoria in the Northwestern (NW) Pacific. Both mitochondrial and nuclear gene genealogies revealed two cryptic species in this morphotaxon, which was further confirmed by extensive population-level analyses. One cryptic species is restricted to cold waters with a distribution range corresponding to temperate affinities, while the other dwelled warm waters influenced by the Kuroshio Current. Their divergence was postulated to be attributable to the vicariant event which resulted from the isolation of the Sea of Japan during the middle Pliocene (c. 3.85 Mya, 95% HPD 2.23–6.07 Mya). Allopatric speciation was maintained by limited genetic exchange due to their habitat preferences. Furthermore, the observation of recombinant nrDNA ITS sequence and intra-individual ITS polymorphism suggested recent hybridization event of the two cryptic species occurred in sympatric areas. Our study also illustrated that the Changjiang River outflow might act as an oceanic barrier to gene flow and promoted allopatric diversification in O. oratoria species complex.

Energy reserves and gonad steroid levels during the reproductive cycle of the Japanese mantis shrimp Oratosquilla oratoria De Haan, 1844 (Stomatopoda: Squillidae) in Pikou Bay, Dalian, China

Energy reserves and gonad steroid levels during the reproductive cycle of the Japanese mantis shrimp Oratosquilla oratoria De Haan, 1844 (Stomatopoda: Squillidae) in Pikou Bay, Dalian, China

Reproductive biology of the Japanese mantis shrimp Oratosquilla oratoria (Crustacea Stomatopoda): Annual cycle of gonadal development and copulation

We investigated the reproductive biology of the stomatopod Oratosquilla oratoria for both males and females of different size classes. Samples were collected from Tokyo Bay, Japan, by monthly bottom trawl surveys from May 2004 to October 2005. Histological examinations of testis, vas deferens, and penis for males and ovary and seminal receptacle for females were conducted. Males attained sexual maturity at a smaller body size (between 4 and 5 cm body length [BL]) than did females (BL > 7 cm). Although there were significant changes in the developmental stage of the testis between months, medium- to large-sized males (BL ≥ 7 cm) stored sperm in the vas deferens and penis throughout the year, indicating year-round functional maturity of the males. On the other hand, females showed clear seasonal patterns in ovarian maturation. Mature females were found between March and September. The early (May–June) and late (July–September) spawning peaks involved large (BL ≥ 10 cm) and medium (7 ≤ BL < 10 cm) size classes of females, respectively, confirming a similar trend shown by previous studies for size-related seasonal patterns in ovarian maturation. In addition, sperm was found in the seminal receptacle concurrent with ovarian maturation, suggesting that the timing of the onset of copulation is regulated by the sexual receptivity of females. This study is the first to reveal this size-related reproductive pattern in both sexes in the Stomatopoda.

Interannual variation in quantitative relationships among egg production and densities of larvae and juveniles of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay, Japan

To explore which lifestages affect the stock size of young-of-the-year mantis shrimp Oratosquilla oratoria in Tokyo Bay, Japan, we investigated interannual variations in the quantitative relationships among egg production, larval density, and juvenile density. We collected adult females, larvae, and juveniles during monthly field surveys from 2004 to 2007. The interannual trend for the juvenile density index differed from those for egg production and larval density; although indices of both egg production and larval density were high in 2004 and 2007, the juvenile density index was high only in 2007, suggesting high mortality during the pelagic larval stage or the early phase of the postsettlement juvenile stage in 2004. We found that larval settlement started at the end of August and peaked in October, although larvae from the early spawning season (May–June) should have settled in August or earlier. Juveniles were found throughout the bay except in areas where bottom hypoxia occurred, suggesting that hypoxia restricts the spatial distribution of juveniles. Our results suggest that mortality during the early life history fluctuates among years, probably because of changes in environmental conditions in the bay, resulting in interannual variation in the stock size of young-of-the-year juvenile O. oratoria.

Relationship between body length, processed-meat length and seasonal change in net processed-meat yield of Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay

There is concern regarding a possible decrease in the minimum exploitable size of the mantis shrimp Oratosquilla oratoria in Tokyo Bay, coincident with a decline in stock size. To assess this problem, the condition factor, the relationship between body length and processed-meat length, and the estimated body length for each market size category were investigated and compared monthly. The condition factor was lowest in spring and peaked in winter. The difference between body length and meat length was higher in summer and winter than in spring and autumn, suggesting that a high condition factor does not result in an increased net yield of processed meat. The annual mean minimum exploitable size in the present low-stock-size condition did not differ from the minimum exploitable size measured during a high-stock-size period (11-cm body length). However, the minimum exploitable size changed seasonally, which coincided with seasonal change in the net processedmeat yield. Exploitation during the season when net meat yield is high would result in increase of the large-sized meat products of high price.

Changes in reproductive patterns in relation to decline in stock abundance of the Japanese mantis shrimp Oratosquilla oratoria in Tokyo Bay

We investigated changes in the reproductive patterns of mantis shrimp Oratosquilla oratoria concurrent with stock-abundance decline in Tokyo Bay, Japan. Stock abundance was high in the mid to late 1980s but decreased abruptly in the early 1990s. The yearly change in annual mean larval abundance was similar to that of stock abundance. Mantis shrimp in the bay have two spawning seasons, an early season (May–June) for ≥1-year-old individuals and a late season (July–September) for 0–1-year-old individuals. This general reproductive pattern does not differ among different stock-abundance levels. However, the monthly pattern in larval abundance has changed with stock-abundance decline; larval abundance from the early spawning season was highest in the high-stock-abundance period, and it decreased significantly in the low-stock-abundance period, probably as a result of decreased spawning-stock abundance of large female mantis shrimp ≥1-year-old. Correlation analysis on the egg production index and larval abundance suggested that during this low-stock-abundance period the population is supported mostly by late-hatched larvae spawned by small, 0–1-year-old female mantis shrimp. Considering the reproductive pattern and the present status of the fishery, the stock of small female mantis shrimp should be conserved to enhance reproduction of the population for stock recovery.

Age estimation of the wild population of Japanese mantis shrimp Oratosquilla oratoria (Crustacea: Stomatopoda) in Tokyo Bay, Japan, using lipofuscin as an age marker

In this study, the age composition of the Japanese mantis shrimp, Oratosquilla oratoria, in Tokyo Bay, central Japan, was investigated using lipofuscin, an autofluorescent pigment, as an age marker. Lipofuscin in a histological section of protocerebral bridge cell mass (PBCM) in the brain was identified by confocal microscopy, and its concentration was quantified by image analysis. Modal analysis of the lipofuscin concentration showed four regularly-spaced modes that could each be regarded as a distinct age group. This implied a constant lipofuscin accumulation in PBCM at a 6.5×10−2% volume fraction per year; it also implied the existence of individuals that are at least 4 years old. The lipofuscin concentration was found to be a more appropriate index than body length for estimating the age of O. oratoria, because the modal analysis on the body-length histogram failed to detect apparent age groups. This was probably a result of the declining growth rate with age and the individual variations in growth through molting leading to considerable overlap in the size between different age groups. The lipofuscin analysis suggested that fast-growing individuals in each cohort are recruited to the fishery, and most individuals attain the exploitable size between 2 and 3 years of age.

Reproductive biology of the female Japanese mantis shrimp Oratosquilla oratoria (Stomatopoda) in relation to changes in the seasonal pattern of larval occurrence in Tokyo Bay, Japan

: The reproductive traits and the monthly larval abundance of the mantis shrimp Oratosquilla oratoria were investigated in Tokyo Bay, Japan, in 2002. The goal of the study was to elucidate the cause of changes in the monthly pattern of larval abundance from the 1980s to the 1990s as these changes relate to variation in the stock size of the adult shrimp. Oogenesis was divided into 10 stages by histological observation. The developmental stage of oocytes in an individual's ovary was synchronous, suggesting that almost all the oocytes in an ovary are spawned at the same time. The size at first maturity was estimated to be 7 ≤ body length (BL) < 8 cm. Fecundity was expressed as a function of BL, ranging from 19 300 eggs for 8 cm BL to 92 100 eggs for 14 cm BL. Small female shrimps (<10 cm BL) spawned around August. Most large female shrimps (≥10 cm BL) spawned around May, and some large female shrimps also spawned until September. Although most large female shrimps spawned in spring, the larval abundance was low before July and high from August onwards. The results suggest that a substantial decrease in the stock size of large individuals causes the low larval abundance before July.

石狩湾におけるシャコの産卵期

The spawning season of the Japanese mantis shrimp Oratosquilla oratoria (De Haan) was examined in Ishikari Bay. The spawning season was estimated based on the temporal changes of female gonad somatic index (GSI) and sex ratio (number of female individuals/total number of individuals×100(%)) obtained from data between 1989 and 1991. The female GSI gradually increased from April to June, decreased in July, and began to increase again after October until November. However, GSI in November did not attain the value in the next spring. The sex ratio was low between mid June and July and high in May and November. Changes of sex ratio show the difference between male and female activities outside their burrows. The extreme low sex ratio is caused by the egg nursing behavior of females in the burrow. The spawning season of O. oratoria in Ishikari Bay was between June and July and once a year.

東京湾の小型底曳網によるシャコの海上投棄量

In Tokyo Bay small-scale trawlers discard considerable percentages of their catch of the Japanese mantis shrimp Oratosquilla oratoria, but no quantitative survey has so far been carried out. We aimed at making clear the actual fishing mortality, performed a monthly survey on discard percentages from April 1989 to April 1990 to discover seasonal changes, and estimated mortality rate after discarding through simultaneous on board experiments. The results revealed that both the discard percentage and the mortality of the discarded individuals are highest in summer, which was considered to be caused mainly by high air temperature. In total, individuals 2.4 times as large as those landed were discarded and died in a year.