Eastern Seto Inland Sea Research Articles

Evaluation of cryptic invasion in Japanese Undaria populations based on mitochondrial haplotypic analysis

Abstract To improve the quality and quantity of seaweed mariculture harvests, seedlings are frequently introduced from regions geographically apart from cultivation sites. The effects of such introduction have been insufficiently studied, despite increasing demands on seaweed cultivation as a measure against global warming. We here evaluated the degree of cryptic invasion occurring through mariculture using a brown algal species, Undaria pinnatifida. Cultivated materials used in the Seto Inland Sea, Japan, were introduced from northern Japan in the 1970s. Based on the clear genetic structure among Japanese Undaria populations, we compared mitochondrial haplotypes between wild populations and cultivated materials in the Seto Inland Sea. All cultivated materials analyzed had haplogroups native to northern Japan. Multiple haplogroups were observed in the wild populations, especially those in the eastern Seto Inland Sea, which is exceptional considering the reported genetic structure. Some northern Japan haplotypes were observed both in cultivated materials and wild individuals. A northern Japan haplogroup was predominant at several sites near Undaria-cultivation sites. These results strongly suggested that cryptic invasion through Undaria mariculture occurred here, although its ecological impact remains unclear. There is a clear risk of invasion by mariculture seedlings from other regions, even when native conspecific populations are present.

Temporal variations in the copepod community structure during the sand lance fishery season near the Akashi Strait, eastern Seto Inland Sea

Temporal variations in the copepod community structure during the sand lance fishery season near the Akashi Strait, eastern Seto Inland Sea

Long-term changes in the occurrence of the fish-killing raphidophyte <i>Chattonella</i> along with environmental factors in southern Harima-nada, eastern Seto Inland Sea, Japan, from 1984 to 2021

Long-term changes in the occurrence of the fish-killing raphidophyte <i>Chattonella</i> along with environmental factors in southern Harima-nada, eastern Seto Inland Sea, Japan, from 1984 to 2021

Seasonal copepod dynamics and biomass in Harima-Nada, eastern Seto Inland Sea, Japan

Seasonal copepod dynamics and biomass in Harima-Nada, eastern Seto Inland Sea, Japan

Toxification characteristics of Japanese cockles and ark shells with paralytic shellfish toxins in southwestern Harima-nada, eastern Seto Inland Sea, Japan

Toxification characteristics of Japanese cockles and ark shells with paralytic shellfish toxins in southwestern Harima-nada, eastern Seto Inland Sea, Japan

Possible dispersal of the coastal and subterranean carabid beetle Thalassoduvalius masidai (Coleoptera) by ocean currents

Abstract The spatial genetic structures of insects are reflected in their habitats. Coastal insects tend to have less genetic differentiation than subterranean insects. However, the general pattern of population structure of coastal species residing in subterranean habitats is unknown. Thalassoduvalius masidai (Coleoptera: Carabidae: Trechinae), endemic to Japan, is a coastal and subterranean beetle. Here, we hypothesized that T. masidai populations disperse among regions by ocean currents, floods and tsunamis, despite their habitat being underground. To test this hypothesis, we performed a phylogeographical analysis of T. masidai using single nucleotide polymorphism data from multiplexed inter-simple sequence repeat genotyping by sequencing (MIG-seq) and the mitochondrial COI gene. The phylogenetic network and tree and STRUCTURE analyses indicated that T. masidai populations were divided into three groups: the western Seto Inland Sea–Izu Peninsula coasts, the eastern Seto Inland Sea–Kii Peninsula coasts and the Japan Sea coasts. The genetic diversity of T. masidai was higher in sites with lower longitude and latitude. These results suggest that T. masidai migrated and dispersed from the south-western area of Japan by ocean currents, despite its underground habitat. The genetic analysis data from the present study would also be helpful for the conservation and taxonomic revision of this species.

Annual variation in condition factor of sand lance Ammodytes japonicus during the estivation period in Harima-Nada, eastern Seto Inland Sea, Japan

The sand lance Ammodytes japonicus has been one of the most important fisheries resources in the eastern Seto Inland Sea, but long-term variations in annual catch have shown a decreasing trend. This species estivates for about 5 -months and does not eat during the period in Harima-Nada, eastern Seto Inland Sea. Therefore, a sufficient condition factor is important to survive the estivation period and start maturity thereafter. In this study, we investigated the condition factor of age-0 fish, which account for most of the catch, during the estivation period (July–November) together with water temperature in Harima-Nada from 2010 to 2017. The mean condition factor of the age-0 fish in late July ranged from 3.76 to 4.55 and decreased each month to values in late November from 2.91 to 3.59. The reduction rate (%) of the condition factor varied from 15.5% to 26.6%. The slopes of the decreasing condition factor, which were calculated from the relationship between the monthly condition factor of individuals and Julian Days each year, showed no significant correlation with cumulative water temperature from the first sampling day in July to the last one in November. The condition factor of sand lance was generally maintained at the end of the estivation period if the condition factor was high at the beginning of the period. The results suggest that starting estivation with a high condition factor is the most important criterion to enable survival of sand lance in Harima-Nada.

Seasonal variations in phytoplankton productivity in a shallow cove in the eastern Seto Inland Sea, Japan

We examined phytoplankton productivity in Shido Bay, an active oyster farming area in the eastern Seto Inland Sea, Japan, by an in situ 13C method from March 2016 to January 2017. The depth-integrated daily phytoplankton production ranged from 0.13 to 1.61 g C m−2 day−1, and the estimated annual production was 218 g C m−2 year−1. The daily production peaked in the rainy season and autumn, when phytoplankton blooms were observed, and production during the bloom events largely contributed to the annual production. There was a strong correlation between daily production and phytoplankton biomass (chlorophyll a) throughout the study period. In the study area, due to the shallow depth (6 m) and moderate light attenuation coefficient of the water column (0.38 ± 0.07 m−1), the mean light intensity in the water column was maintained at high levels (36 ± 5% of incident photosynthetically active radiation) throughout the study period. In contrast, the dissolved inorganic nitrogen concentration of the water was persistently low (< 1.5 µM) from late winter to summer. As a consequence, an increase in nitrogen supply seemed to be the key factor causing the increases in daily production and directly triggering the phytoplankton blooms in the bay.

Ontogeny of robust tonguefish (&lt;i&gt;Cynoglossus robustus&lt;/i&gt;) larvae with initially a single, subsequently double-dorsal elongated fin rays in Osaka Bay, eastern Seto Inland Sea, Japan

Ontogeny of robust tonguefish (&lt;i&gt;Cynoglossus robustus&lt;/i&gt;) larvae with initially a single, subsequently double-dorsal elongated fin rays in Osaka Bay, eastern Seto Inland Sea, Japan

大阪湾における植物プランクトンの長期変動と 有毒渦鞭毛藻Alexandrium tamarense の大増殖

大阪湾における植物プランクトンの長期変動と 有毒渦鞭毛藻Alexandrium tamarense の大増殖

Population structure and growth of juvenile tri-spine horseshoe crabs released on the Kohnoshima tidal flat, Kasaoka, eastern Seto Inland Sea, Japan

Population structure and growth of juvenile tri-spine horseshoe crabs released on the Kohnoshima tidal flat, Kasaoka, eastern Seto Inland Sea, Japan

Annual and seasonal changes in the commercial catch of rabbitfish Siganus fuscescens in Harima-nada, eastern Seto Inland Sea, Japan

Annual and seasonal changes in the commercial catch of rabbitfish <I>Siganus fuscescens</I> in Harima-nada, eastern Seto Inland Sea, Japan

Age, growth and spawning period of rabbitfish Siganus fuscescens in the eastern Seto Inland Sea, Japan

Age, growth and spawning period of rabbitfish <i>Siganus fuscescens</i> in the eastern Seto Inland Sea, Japan

Correlation of changes in seasonal distribution and catch of red sea bream Pagrus major with winter temperature in the eastern Seto Inland Sea, Japan (1972–2010)

AbstractRed sea bream Pagrus major is a commercially important fish in Japan. In eastern Seto Inland Sea (SIS), the catch has increased from 297 tons in 1972 to 2,039 tons in 2010. We examined the relationship, 1972–2010, between increase in catch and winter temperature, based on the catch in February and March and the lowest water temperature at 10 m depth. In 1972–1986, the lowest water temperatures in the inner SIS areas (Osaka Bay, Harima‐nada, and Bisan‐seto) were &lt;8°C, which is physiologically unfavorable for red sea bream. However, in 1987–2010 temperatures were generally ≥8°C. In the inner areas, the catch during winter had been minimal until the early 1980s, presumably because most red sea breams moved to the Kuroshio‐influenced (warmer) Kii Channel area. However, the winter catch in the inner areas of SIS increased from the late 1980s with warm winters. In addition, the catch between April and June, the spawning season, increased in the inner areas from the 1990s, and the catch rate of the inner areas was more than twice higher in the 2000s than in the 1980s. The results suggest that expansion of the distribution area during winter due to warm winter and increase in egg production in the inner areas greatly contributes to the increasing in catch in the eastern SIS.

Interannual decrease in condition factor of the western sand lance Ammodytes japonicus in Japan in the last decade: Evidence for food‐limited decline of the catch

AbstractAnnual catch of the western sand lance Ammodytes japonicus in the eastern Seto Inland Sea, Japan, has shown a decreasing trend since the 1990s. To examine whether food shortage was the main cause for the catch decrease, we investigated the condition factor of the age‐0 fish at the beginning of the estivation period (late July) in Harima‐Nada, eastern Seto Inland Sea, for 10 years from 2008 to 2017. The mean abundance of copepods as food for the age‐0 fish during the fish growth period (from February to June) around the estivation area was also determined. The condition factor showed a significant decrease, and values for 2011 and later years were mostly lower than the known minimum threshold (4.2) for maturation. In the recent 4 years from 2013 to 2016, the mean copepod abundance was much lower than values for the other years. The condition factor showed a significant positive correlation with the copepod abundance. These results indicate that decline of western sand lance catch after 1990 was caused mainly by food shortage.

Development of a new cultivar with high yield and high-temperature tolerance by crossbreeding of Undaria pinnatifida (Laminariales, Phaeophyta)

Recent production of the marine crop Undaria pinnatifida has become unstable due to rising temperatures caused by global climate change. The cultivar HGU-1 was bred in a previous study to achieve early cultivation. A new cultivar, NW-1, was bred for improved yield and stress tolerance. This was achieved by crossbreeding using male and female gametophytes, each isolated from a single zoospore. To evaluate whether the cultivar NW-1 is superior to the previous cultivar HGU-1, we compared their growth characteristics in an Undaria farm, in the eastern Seto Inland Sea of Japan. Despite the early start of nursery cultivation, due to a high seawater temperature of 24.5 °C, many young sporophytes of NW-1 grew on strings wound around the collector, whereas most of the HGU-1 sporophytes fell from the strings. After nursery cultivation, seawater temperature during rope cultivation was remarkably higher than usual. Nevertheless, the mean total length and weight of NW-1 were 115.5 cm and 140.1 g, respectively, at the end of December. In contrast, the mean total length and weight of HGU-1, at the corresponding time, were 85.4 cm and 58.1 g, respectively. Moreover, the growth and weight of the NW-1 sporophytes were significantly higher than those of HGU-1 throughout experimental cultivation. These results demonstrate that NW-1 is an excellent cultivar with a high yield and high-temperature tolerance.

Analysis of Long-Term Phytoplankton Dynamics with Environmental Factors in Harima-Nada, Eastern Seto Inland Sea, Japan

Analysis of Long-Term Phytoplankton Dynamics with Environmental Factors in Harima-Nada, Eastern Seto Inland Sea, Japan

High-resolution surface salinity maps in coastal oceans based on geostationary ocean color images: quantitative analysis of river plume dynamics

Sea surface salinity (SSS) in coastal oceans is a direct indicator of riverine plumes and provides essential information about the ocean environment and ecosystem, which affects coastal fisheries, aquaculture, and marine harvests. However, to accurately capture SSS patterns in coastal oceans, high temporal and spatial resolutions are required. This paper introduces a methodology to produce high-resolution (~ 500 m) SSS maps for analysis of river plumes in coastal oceans based on hourly chromophoric dissolved organic matter data collected by the Geostationary Ocean Color Imager. Osaka Bay, located in the eastern Seto Inland Sea, was selected as a pilot region. A comparison between the initial estimates and calibrated SSS data showed a substantial decrease in estimation error, by up to 71%, over a wide range of salinity (20–34) using in situ SSS data collected through an automated observation system. Calculating the salinity anomaly based on the SSS map to identify plume areas, we evaluated the impact of a large runoff event induced by a super typhoon on the river plumes. After the plume formed in the estuary, it extended southward to the bay mouth along the southeastern coast. The plume area during the post-typhoon period covered half of the bay, approximately 1.5 times the area during the pre-typhoon period. The post-typhoon, low-SSS period continued for approximately 2 weeks. Our approach can be of practical use for analyzing the dynamics of river plumes in coastal oceans, leading to the development of coastal ocean prediction models related to operational oceanography.

Spatial variation of carbon and nitrogen stable isotope ratios in Japanese anchovy, Engraulis japonicus, in the eastern Seto Inland Sea, Japan

Spatial variation of carbon and nitrogen stable isotope ratios in Japanese anchovy, Engraulis japonicus, in the eastern Seto Inland Sea, Japan

Sea-level variations during Marine Isotope Stage 7 and coastal tectonics in the eastern Seto Inland Sea area, western Japan

Relative sea-level (RSL) observations for Marine Isotope Stage (MIS) 7 were obtained from three sites (Higashinada, Kakogawa and Mitsu) of the eastern Seto Inland Sea area, western Japan. We evaluated the magnitude of sea-level highstands during MIS 7 and the local tectonics based on RSL records of this coastal area. For this purpose, we analyzed diatom assemblages and sedimentary sulfur, and carried out tephra analysis. We also constructed a linear age model to determine the depositional ages of the sediments using altitudes of astronomical age control points obtained mainly from the present diatom data. At the Mitsu site, RSLs were estimated to be −22.87 ± 0.50 m at the MIS 7.5 highstand peak (234.6 ka), below −20.17 m at 211.2 ka, and below −20.00 m at 209.9 ka. With a tectonic subsidence rate of 0.11 ± 0.02 m/ka since the Last Interglacial Maximum at the Mitsu site, the elevation of the tectonically corrected MIS 7.5 highstand peak may have been +2.94 ± 5.19 m. The tectonically corrected sea-level elevations during MIS 7.3 are inferred to have been below +3.06 ± 4.72 m at 211.2 ka and below +3.09 ± 4.70 m at 209.9 ka. Comparisons between the diatom-inferred RSLs and the isotopically derived sea-level curve indicate that the MIS 7.3 highstand peak was above −18 m and may have been lower than the MIS 7.5 and 7.1 highstands. Based on the isotopically derived sea-level curve and the RSL data from Higashinada and Kakogawa, a tectonic subsidence rate of 0.26–0.32 m/ka was derived for Higashinada and a tectonic uplift rate of >0.17 m/ka and <0.29 m/ka was derived for Kakogawa. A relative uplift rate of >0.43 m/ka and <0.61 m/ka was derived along the traverse from Higashinada to Kakogawa. The uplift rate derived for Kakogawa relative to Mitsu is > 0.26 m/ka and <0.42 m/ka. These values indicate the intensity of crustal movements associated with active faulting of the Rokko–Awaji fault system along this tectonically active coast.