Gokasho Bay Research Articles

Micro and Meso plastic debris on the beaches of Ago Bay and Gokasho Bay in Mie Prefecture

Micro and Meso plastic debris on the beaches of Ago Bay and Gokasho Bay in Mie Prefecture

A numerical modeling approach to support decision-making on design of integrated multitrophic aquaculture for efficiently mitigating aquatic waste

As a bio-mitigation strategy of aquafarming pollutants, the integrated multitrophic aquaculture (IMTA) system has been proposed and developed to mitigate the waste produced during co-cultivation. Although there have been considerable implementations of IMTA worldwide, its optimal design needs a further study. Through a numerical modeling approach, this study aims to facilitate decision-making when designing IMTA to more efficiently mitigate the negative change caused by aquaculture. An improved three-dimensional numerical model is adopted to simulate the dynamics of water current and organic waste in Gokasho Bay, Japan. Results show that (1) the maximum main surface water current velocity in Hazama-ura area is less than 10 cm s−1, where nutrient absorbers and/or suspended feeders can easily attach to a substratum; (2) southern-eastern direction water flow is dominant through the year suggesting that seaweeds (e.g., Laminaria japonica, Ulva ohnoi) or mussels (e.g., Mytilus edulis) are better arranged in the same direction; (3) horizontal distribution of organic waste on sea bottom indicates that when releasing sea cucumbers (e.g., Apostichopus japonicus), priority is given to the areas where the accumulated amount of particulate organic waste is high. Our case study provides a general idea on the optimal design of IMTA and can be scaled to the globe, especially regarding decision-making on locations for the extractive organisms. In conclusion, to improve the bio-mitigation efficiency of IMTA, location for nutrient absorbers and/or suspended feeders can be selected based on local water flow, and sea cucumbers need to be placed with a careful consideration of the waste distribution in local area.

Numerical analysis of particulate organic waste diffusion in an aquaculture area of Gokasho Bay, Japan

Particulate organic waste, originating from fish cultured in cages, often brings about negative effects on the structure of bottom sediments. These effects result in deterioration of the aquatic environment. In the present study, a three-dimensional numerical model was coupled with developed submodels of fish cage drag and aquaculture waste diffusion to simulate the hydrodynamic environment and distribution of particulate organic waste. Numerical simulation results showed that fish cages exerted an influence on both the velocity and direction of water current; the reduction of water current velocity was a maximum of approximately 38%. The contribution of fish fecal matter was dominant during winter (ca. 80%), whereas waste fish feed was the main source in other seasons. The distribution of organic waste near the sea bottom indicated that organic waste not only loaded mainly on the sea floor just beneath the fish cages but also diffused towards the coastlines.

Growth and food source of the sea cucumber Apostichopus japonicus cultured below fish cages — Potential for integrated multi-trophic aquaculture

In order to examine the efficiency of co-culturing fish with the Japanese common sea cucumber Apostichopus japonicus, field cultivation experiments were conducted in Gokasho Bay, central Japan. I cultured A. japonicus juveniles below a fish cage and at a control station for 238days, monitored the wet weight, and analyzed its stable carbon and nitrogen isotopes (δ13C, δ15N) together with potential food sources, and found that (1) juveniles cultured below fish cages exhibited high survivorship (96%) and significantly higher specific growth rate (1.9%) than those at the control site (1.2%), and (2) the juveniles had significantly reduced δ13C values (mean±SD=−19.1±0.3‰ vs. −17.5±0.4‰ at the control station), suggesting the incorporation of C3 plant material in fish feed through fish feces and settling organic matter. All sea cucumbers which were further cultured for additional 307days below fish cages grew to the marketable size (range and mean wet weight=142–181g, 160g, n=9). In order to evaluate the effect of density on growth, 6 culture vessels, in which 1, 5, 10, 15, 20 and 25 juveniles of A. japonicus were stocked, were maintained below the fish cage over 118days. The results showed that the final mean weight of A. japonicus decreased from 6.68g to 0.94g as the density increased with a large variation of weight at the end of the experiment (coefficient of variation=52.1–62.2%), suggesting that there was competition between individuals for a limited food supply and there were intraspecific effects on the growth. This study shows the possibility of integrated multi-trophic aquaculture in which A. japonicus is cultured in the water column below fish cages, because the survival and growth of the sea cucumbers were enhanced due to the ability to avoid predator interactions and adverse environmental conditions as well as nutritional feed supply from the fish cage.

Bioindicator and biofilter function of Ulva spp. (Chlorophyta) for dissolved inorganic nitrogen discharged from a coastal fish farm — potential role in integrated multi-trophic aquaculture

In order to examine the efficiency of Ulva spp. (Chlorophyta) as a bioindicator and biofilter for dissolved inorganic nitrogen discharged from coastal fish farms, field surveys and field cultivation experiments were conducted in Gokasho Bay, central Japan. Naturally occurring green algae Ulva spp. were collected in and around a fish farm in February, May and July, and their δ 15N and tissue N contents were analyzed. Values of these parameters obtained in May and July decreased with the distance from the fish cages, suggesting that Ulva spp. occurring in close vicinity to the fish farm assimilated the fish-excreted N. To evaluate this uptake, we cultured round cut pieces of thalli of Ulva ohnoi in transparent chambers beside a fish cage and at a control station for 2 weeks in May, July and September. The specific growth rate and N fixation rate of the thalli cultivated beside the fish cage showed 12.8–23.6% and 4.2–13.9 mg N g DW − 1 day − 1 , respectively, which were significantly higher than those at the control station and found to be among the highest levels known for seaweed biofilters. The high growth rates during the warm season and easy cultivation indicate the suitability of Ulva ohnoi as a biofilter.

Algicidal Activity of a Karenia mikimotoi-killing Bacterium Isolated from Gokasho Bay, Japan

Algicidal Activity of a Karenia mikimotoi-killing Bacterium Isolated from Gokasho Bay, Japan

Seasonal and Spatial Variation of Bacterial Community Structure in River-Mouth Areas of Gokasho Bay, Japan

Seasonal and Spatial Variation of Bacterial Community Structure in River-Mouth Areas of Gokasho Bay, Japan

Seasonal and Spatial Variation of Bacterial Community Structure in River-Mouth Areas of Gokasho Bay, Japan

This study investigated seasonal and spatial dynamics of the bacterial community in Gokasho bay with denaturing gradient gel electrophoresis (DGGE) profiles of PCR-amplified 16S rRNA gene fragments. The community structure was related to physico-chemical water conditions in the area examined. The bacterial community clustered into three groups: bacteria collected during January-May; those collected from water at the surface in July and September; and those collected from water at the bottom in July and September and from both depths in November. Canonical correspondence analyses indicated that the seasonal variability in bacterial community was associated with water temperature succession. On the other hand, concentrations of particulate organic matter and nitrite plus nitrate were related to the vertical change in community structure in summer and autumn as well as HNF abundance, suggesting that both top-down and bottom-up control affected the community. The influence of salinity was insignificant though bacterial production was related to salinity. No relationship was observed between the variation in community structure and that in hydrolytic enzyme activity. The results indicate that changes in bacterial activity are not coupled with variation in community structure.

Variation in food sources of the macrobenthos along a land–sea transect: a stable isotope study

To evaluate the relative importance for the macrobenthos of possible food sources, including riverine particulate organic matter, reeds, benthic microalgae, seaweeds, coastal phyto- plankton and aquaculture-derived matter, over 194 macrofaunal species were collected from 9 inter- tidal and subtidal stations along the axis of Gokasho Bay, Japan, and their isotopic compositions ana- lyzed. The δ 13 C values for terrestrial organic matter were very different from those of the consumers (>5‰), suggesting a negligible trophic role for this source. Differences in δ 15 N between seaweeds and primary consumers were small, suggesting a minor contribution of seaweeds. The δ 13 C values of the majority of consumers fell between coastal phytoplankton and benthic microalgae, allowing the calculation of the relative contribution of these microalgae to the diets of consumers. The estimated contribution ratios and the between-site comparison of the δ 13 C value for the same species along the bay axis suggest a shift of the main food source from benthic microalgae on the mudflat to coastal phytoplankton at the subtidal seaward stations. Several consumers collected from the fish-farm area had enriched δ 15 N and depleted δ 13 C values relative to samples of the same species that occurred out- side of the farm area. The δ 13 C values of the consumers at the fish farm were significantly more depleted than those at the other stations. These findings suggest that the consumers inhabiting the farming area incorporate 15 N-enriched fish meal and 13 C-reduced cereals that are major constituents

五ヶ所湾における密度流拡散装置の環境修復効果

Gokasho Bay in Mie Prefecture is a typical enclosed sea, and has some problems with water pollution, such as red tide, blue tide and hypoxic water at the bottom layer. The Marino-Forum 21 installed a density current generator in 1997, to restore the water quality by the improvement of vertical water exchange. The purpose of this study is to clarify a long-term environmental restoration effect of the apparatus by surveying sea bottom quality and seaweed bed distribution. The results of the field surveys showed that the conditions of both bottom quality and seaweed habitat in the test area installed with the apparatus are much better than those in the neighboring area surveyed for reference. This suggests that the apparatus works effectively for the long-term restoration of marine environments.

In Vitro Propagation of Two Perkinsus spp. Parasites from Japanese Manila Clams Venerupis philippinarum and Description of Perkinsus honshuensis n. sp.

Perkinsus species are destructive parasites of commercial Manila clams, Venerupis philippinarum, in Japan, Korea, and Spain. However, in vitro parasite cultures from this important host clam are not available. Tissues of Manila clams collected during April 2002 in Gokasho Bay, Japan harbored Perkinsus sp. parasites at a 97% prevalence (28/29) of moderate- and high-intensity infections. Perkinsus sp. cells in tissue samples were enlarged in alternative Ray's fluid thioglycollate medium, before propagation in DME:Ham's F-12 Perkinsus sp. culture medium. Enlarged parasite hypnospores zoosporulated at high frequencies to release motile zoospores, which gave rise to continuous schizogonic cell lines that also zoosporulated continuously at low frequencies. Four Perkinsus sp. in vitro isolates comprising two distinct morphotypes were cryopreserved, cloned, and archived for public distribution. For three isolates of one morphotype, nucleotide sequences of the ribosomal DNA internal transcribed spacer region, of the large subunit rRNA gene, and of actin genes, were consistent with those reported for P. olseni. Similar sequences from one morphologically unique isolate differed from those of all described Perkinsus species. These results show that at least two Perkinsus spp. infect Japanese Manila clams, and that one represents a new species, Perkinsus honshuensis n. sp.

Seasonal variation in ^|^delta;13C and ^|^delta;15N of epilithic microalgae in Gokasho Bay

IntroductionBenthic microalgae are an important component in theestuarine and coastal waters as one of major primary pro-ducers to fuel ecosystems. The term benthic microalgae in-cludes epiphyton growing attached to other plants, epipelongrowing on sediments, epilithon growing attached to rocksurfaces, etc. (Round 1971). Among them, epilithon(epilithic microalgae) have been shown to be a food sourcefor many herbivores inhabiting intertidal rock shores andsubtidal stony seabeds (e.g. Steneck & Watling 1982).Stable carbon and nitrogen analyses have been success-fully used to determine trophic links from primary produc-ers to higher trophic levels in various ecosystems (reviewedby Peterson & Fry 1987). The analyses, however, are possi-ble when only a few primary producers are important andthose sources have distinct isotopic signatures. In order totrace the flow of epilithic microalgal organic matter, it isnecessary to know the isotopic compositions of these algae.It has been reported that there are large seasonal or tem-poral variations in the d

Numerical and hydraulic simulations of the effect of Density Current Generator in a semi-enclosed tidal bay

A large dyke built in 1997 in Isahaya Bay (the spatial scale of about 10 km) in Ariake Bay (about 80 km) raises huge scientific and social discussions in Japan. It is said that, since the construction of the dyke, physical and ecosystem environments have changed and fishery has been damaged enormously. On the other hand, it is known that the Density Current Generator is successfully enhancing water quality in Hazama Inlet (the spatial scale of 3 km) of Gokasho Bay. In this study, we conducted numerical and physical model simulations on the hydrodynamics to investigate whether the apparatus is also applicable to Isahaya Bay in Ariake Bay. A laboratory-scale rotating hydraulic model and a three-dimensional numerical code called the MEC Ocean Model were adopted for the physical and numerical simulations, respectively. The MEC Ocean Model has a special feature, which is a hybrid of hydrostatic and nonhydrostatic models. Tests in a simple bay elucidated the importance of the nonhydrostatic model nested in the hydrostatic model, i.e. the former can simulate the appropriate intrusion depth of the mid-density water discharged in stratification. The numerical results for Ariake Bay suggested that the water discharged from the apparatus spreads in the head of the bay within a couple of weeks with nontrivial concentration of nutrients. This was also confirmed by the physical model test by using the luminance measurement of dye spread.

Estimation of the assimilative capacity of fish-farm environments based on the current velocity measured by plaster balls

Current flow under fish-farm cages was investigated in eight net-pen fish farms in Gokasho Bay and Owase Bay, Pacific coast of central Japan, to assess the assimilative capacity of the water body. Time-averaged current velocities during neap- and spring-tide periods were determined using plaster balls and a concomitant current meter set 1 m above the seabed. An index of suitable location for fish farms (ISL), which is given by DV2, where D is the water depth (m) and V is the mean current velocity (m/s), is proposed. Field data on biotic and abiotic factors that have been obtained from the same stations were used to validate ISL. Values of biotic and abiotic factors changed along a gradient of ISL versus annual fish production. As ISL values decreased and fish-production levels increased, values of dissolved oxygen of the bottom water decreased, values of sediment total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD) and acid-volatile sulfide (AVS) increased, and values of biomass, density and number of species of the macrobenthos decreased. Three zones of fish-farm environments, i.e., healthy, cautionary and critical zones, which had been defined on the basis of the macrofauna and chemical factors, were also arranged in a gradient of ISL and fish production. The healthy zone was located at areas having large ISL values, whereas the critical zone was located at areas having small ISL values and a high level of fish production. As fish-production levels increased, the cautionary zone shifted to areas having larger ISL values. These findings suggest that bathymetry and water flow control waste dispersal and loading, and ISL is an effective indicator of the assimilative capacity. Plaster balls can be used as a pragmatic tool in determining time-averaged current velocities.

Rare mud shrimp genus Naushonia Kingsley (Decapoda: Thalassinidea: Laomediidae) from Japan : description of a new species and new record of N. lactoalbida Berggren

Two species of the rare mud shrimp genus Naushonia Kingsley, 1897 (Thalassinidea: Laomediidae) are reported from Japanese waters. Naushonia japonica, new species, described based on a single male specimen from Shimonoseki, southwestern Honshu, is similar to N. perrieri (Nobili, 1904) known only from the Red Sea. They are differentiated by the armature of the rostrum and anterolateral margin of the carapace, development of the second lateral carina on the gastric region of the carapace, armature of the antennal scale and shape of the dactylus of the first pereopod. Naushonia lactoalbida Berggren, 1992, originally described from Mozambique, East Africa, is recorded from Iriomote Island, southern Ryukyu Islands. The record of species of Naushonia in the western Pacific region has been represented only by planktonic zoeas of an unidentified species taken at Gokasho Bay, central Japan (Konishi, 2001). The occurrence of species of Naushonia in the region is now fully confirmed by the discovery of the adult specimens.

Calculation of Mass Transfer in Gokasho Bay using the Box or Reservoir Model

Calculation of Mass Transfer in Gokasho Bay using the Box or Reservoir Model

Environmental quality criteria for fish farms in Japan

Environmental deterioration around fish farms has been widespread in Japanese coastal areas. In order to prevent self-induced deterioration of the surrounding environment, the “Law to Ensure Sustainable Aquaculture Production” was enacted in 1999. Criteria based on three indicators, i.e., (1) dissolved oxygen content of water in fish cages, (2) acid volatile sulfide content (AVS-S) in the sediment and (3) the occurrence of macrofauna under the fish cages were determined to promote the “Aquaculture Ground Improvement Program” by applying this Law. The second criterion (AVS-S) is based on the assimilative capacity of the sediments to organic wastes from a fish farm. For applying this criterion to each farm, the maximum phase in the process of biological remineralization must be detected for a farm site when the benthic oxygen uptake (BOU) rates shows maximum. The peak of BOU, however, could not be determined during a survey in Gokasho Bay, suggesting a need for reexamination regarding the practical application of this criterion. In order to obtain data for refining the third criterion (macrofauna), a quantitative survey was conducted at 22 fish farms (red sea bream and yellowtail) along the Kumano-nada coast, central Japan. The biomass of the macrobenthos peaked in sediments containing 1.2 mg/g of total nitrogen, where the majority of aerobic mineralization of the loaded organic matter is supposed to occur. In summer, animals were scarcely found in sediments with AVS-S>1.7 mg/g, suggesting that this is a critical condition for the fish farm environment. An index embayment degree (ED), which represents the topographic conditions of a farm site, is proposed to discriminate artificial factors arising from fish farming activities from the natural factors related to the topography. Community parameters of the macrobenthos and environmental factors were significantly correlated with ED ( P<0.001). In shallow, semi-enclosed sites (larger ED values), environmental deterioration and decreases in the benthic biomass were more conspicuous in large-scale farms than in small-scale farms. Six assemblages of the macrobenthos were identified by cluster analysis and were classified into three groups, indicating conditions as healthy, cautionary and critical, respectively. As fish production increased, habitat of the assemblage in the cautionary zone shifted to the offshore, deeper areas (smaller ED values), indicating that both of aquacultural activities and topographic conditions affect the species composition. Macrofauna, sediment parameters and the index ED are concluded as useful to develop pragmatic guidelines for site selection of fish farms.

Feeding of the bivalve Theora lubrica on benthic microalgae: isotopic evidence

A stable isotope study was conducted to identify the sources of nutrition for the semelid bivalve Theora lubrica living in estuarine subtidal sediments. Along an estuarine gradient in Gokasho Bay, central Japan, δ 13 C and δ 15 N were determined both in tissues of T. lubrica and its potential food sources, i.e. particulate organic matter (POM) from riverine and marine sites, sedimentary organic matter (SOM) and microalgae on an intertidal mudflat. Differences existed among δ 13 C and δ 15 N in riverine POM (yearly mean value: -26.5‰ and 0.5 ‰, respectively), marine POM (-20.4‰, 6.3‰) and benthic microalgae (-14.7 ‰, 4.9‰). SOM exhib- ited a gradual isotopic enrichment both in 13 C and 15 N from the estuarine station adjacent to the mudflats to the more sea- ward stations, indicating a progressive mixing of terrestrial and marine sources. However, the isotopic composition of T. lubrica did not follow this trend: as the sampling station was shifted seaward, δ 15 N increased from 7.5 to 9.4‰, whereas

Effects of water mass exchange on bacterial communities in an aquaculture area during summer

Bacterial abundance (BA) and bacterial production (BP) and their relations with chlorophyll a, dissolved organic carbon (DOC) and dissolved organic (DON) and dissolved inorganic nitrogen (DIN) concentrations, were measured in the eutrophic fish farming ground of the Hasama-ura inlet of Gokasho Bay in the summer of 1996. The BP in the aquaculture area was 172±154 μgC/l/day (mean±standard deviation; n=24) and was higher in the aquaculture area than in the mouth of the bay, especially at the lower chlorophyll a concentrations of the mid and bottom waters. It correlated well with the concentrations of DON, but not with chlorophyll a or DOC. The BP per unit chlorophyll a was higher in the aquaculture area than in the bay mouth, especially at the lower chlorophyll a concentrations of the middle and bottom waters. Our findings suggested that the allochthonous organic matter added by farmed fish supported the bacterial activity in the aquaculture area. An occasional water exchange by intrusion of oceanic water into the Hasama-ura inlet did not cause a significant reduction of BA or BP. However, the correlation coefficient between BP and the DON concentration increased significantly after the water exchange, indicating that the water exchange caused a change in the organic substrate supporting the bacterial community.

Impact of fish and pearl farming on the benthic environments in Gokasho Bay: Evaluation from seasonal fluctuations of the macrobenthos

In order to clarify the influence of mariculture on the benthic fauna, samples of the macrobenthos were collected from Gokasho Bay, where intensive fish culture and pearl oyster culture have been carried out. Monthly samples collected from the fish farm and pearl farm sites during June 1995 to July 1996 revealed that the community structure of the two sites showed distinct differences with seasonal fluctuations. At the fish farm site, azoic conditions were found from July to November; after December, the diversity increased markedly through successive recruitments of small-sized species such as the polychaetes Capitella sp. and Pseudopolydora paucibranchiata, and the amphipods Aoroides spp.; macrofaunal density, biomass and species richness peaked from March to April. At the pearl farm site, a higher diversity, including larger-sized species, and no clear seasonal fluctuations in abundance was found, and the community structure was similar to that at the control site. These results show the large impact by fish farming on the macrofauna, whereas pearl farming causes less effect on the benthic fauna. It is suggested that the difference in the level of organic input between the two sites results in the differences in the dissolved oxygen content of the bottom water, sulfide content of the sediments and, subsequently, the macrobenthic assemblages.