Shift from the Clean Sea to the Bountiful Sea

Osaka Bay Fish Farm

Concept: Obayashi Project Team

III. The Osaka Bay Fish Farm Concept

We have deployed the Fish Farm in Osaka Bay.

Project Site

Osaka Bay was chosen as the project site for the following reasons:

① The area is well-known throughout Japan and it is close to major urban centers (plenty of consumers)
② The area was once known for its large fish catches, and the hope is that restoration of the marine environment will result in the eventual recovery of the local fishing industry
③ The area should not have a unique topography. It should be an area that can be planned for versatility and expandability to other areas

Osaka Bay is located at the eastern end of the Inland Sea separating Honshu (the largest of Japan's four home islands) and Shikoku (the smallest of the home islands). It is bounded on the west by the Harima Sea through the Akashi Straits and to the south by Kii Channel through the Kitan Straits. Osaka Bay has an area of approximately 1,450 square kilometers, with an average depth of less than 28 meters and a gentle seabed slope.

In earlier times, Osaka Bay was known as the Chinu Sea as well as the Naniwa Sea. These waters were famous for excellent catches of black sea bream (known as chinu west of the Osaka region). As the Chinese characters for naniwa (魚庭) suggest, Osaka Bay was for centuries a veritable "garden of fish".

However, fish catches in Osaka Bay have been declining since peaking at approximately 114,000 tons in 1982. By 2020 catches had fallen to about 15,000 tons. To deal with this challenging situation, fishermen in Osaka are cultivating akou (yellowtail grouper), which at one time was a rare fish, and have selected the fish caught in Osaka Bay as a "Pride Fish" (*2). The Osaka Bay fishery is being promoted in a number of other ways through industry-academic-government collaborations and other sophisticated approaches to improving the fishing ground environment. Also, Osaka Prefecture certifies and promotes agricultural, forestry, and fishery products which are produced and processed in the area under "Product of Osaka (Osaka-mon)" brand.

*2 Pride Fish: An initiative of the JF Group (Fishermen's Federation and Fishermen's Cooperative) since 2014 to promote the consumption of fish. Fishermen across Japan choose seasonal fish they most want consumers to eat.

Source : National Association of Fisheries Co-operative Associations
Source : Osaka Prefecture

Spotlighted Fish Varieties

Since ancient times, the Japanese have been familiar with "groundfish" that live among reefs, seaweed, and in rapids, but the numbers of groundfish have long been in decline. The focus of the Fish Farm project is on these groundfish, species which have a high degree of cultural significance in Japan. The aquaculture facility aims to increase production of a variety of fish species that are highly prized in Osaka but whose numbers are declining, such as kijihata grouper, conger eel, and dogtooth vole, a species of sole, which is expected to become a specialty of Osaka Bay.

The project includes the construction of artificial reefs to be built around the aquaculture facilities with seaweed beds placed on them. The fry of sea urchins, abalone, sea cucumbers, and other species that feed on and live in the seaweed beds will be released into the environment. To foster the creation of a rich fishing ground, the same approach will be used for groundfish like red-spotted grouper, black rockfish, and dusky stingfish, species which prefer rocky reefs. Since black rockfish and dusky stingfish are difficult to raise using aquaculture techniques because it takes five to six years for these fish to grow to maturity, we decided to increase their numbers by releasing them into the natural environment.

Cultivated fisheries by release Aquaculture
Dusky stingfish ①
Abalone ②
Red-spotted grouper ③
Black rockfish ④
Sea urchin ⑤
Sea cucumber ⑥
Red-spotted grouper ③
Sole (robust tonguefish) ⑦
Conger eel ⑧
Red sea bream
①-⑥、⑧ ©Aflo/⑦ ©PIXTA

Plans for Each Facility

(1) In-Bay Farm

The facility that will form the basis of the in-bay farm is designed with ease of construction and stable management in mind and will consist of aquaculture ponds, eutrophic ponds, and seaweed beds having an area of 100 x 200m. The aquaculture ponds will raise yellowtail grouper and sole, among other species. The two aquaculture ponds are divided into eight sections each, allowing for the cultivation of a wide variety of fish. Leftover food and feces that accumulate at the bottom of the ponds are collected, converted into low molecular weight compounds by the water treatment system (biofiltration pool), and further decomposed into inorganic substances such as nitrogen and phosphorus. The products of the ponds are then sent to nutrient ponds, where it is effectively used for its nutritive value.

Bivalves like blood clams and Japanese littleneck clams will be raised in the nutrient-rich eutrophic pond using as food the phytoplankton grown through the combination of nutrients and sunlight. The shellfish are placed in baskets and suspended in the sea at a height that allows sunlight to reach them. Since the environment thus created is richer in nutrients than the surrounding sea, the shellfish is likely to grow faster than it would under normal conditions.

The unit is surrounded by a gently sloping reef consisting of lightweight blocks that allows the passage of seawater, and transplanted paddle weed and sargassum (a genus of brown macroalgae) are used to build up seaweed beds. Nutrient-rich water from the eutrophic pond is supplied to the seaweed beds to promote the growth of seaweed. In the richly cultivated seaweed beds, young fish and shellfish raised at the onshore facility are released and ultimately harvested when they are mature (e.g., sea cucumbers that also feed on leftover food, sea urchins, abalone, and groundfish that prefer rocky seaweed beds).

The outflow of excessive nutrients into the environment is prevented by the fact that nitrogen, phosphorus, and other elements are absorbed by the seaweed beds.

Units can be interconnected and can accommodate various sizes of fish. The Osaka Bay Fish Farm consists of 10 interconnected units (5 x 2). A catenary mooring(*3). method is used. This method is typically used in waters shallower than 50 m and can easily cope with unit movement induced by tides and waves.

*3 catenary mooring: a mooring system in which a chain is used to flexibly connect a floating body to an anchor, etc. on the sea bottom(catenary=suspension line)

[In-Bay Fish Farm Layout (10 interconnected units)]

[In-Bay Fish Farm Floor Plan (1 unit)]

[In-Bay Fish Farm A-A' cross sectional view]

[In-Bay Fish Farm detailed cross sectional view]

(2) Out-Bay Farm

In contrast with the in-bay farm, the out-bay farm can disperse and purify the water of residual fish feed and nutrients released from aquaculture more than the in-bay facility due to the out-bay's proximity to the open sea. There is no need to close the aquaculture facility because measures to prevent excessive nutrient release into the ocean are rendered unnecessary. Despite being an open-type facility, we have developed a system to supply nutrient-laden water from the aquaculture area to the seaweed beds.

The out-bay farm is situated in waters that are 50-60 meters or more in depth, making it less susceptible to the effects of sea surface temperature fluctuations. Since seawater temperatures at greater depths are relatively stable, this facility raises fish that prefer colder temperatures. The out-bay farm can also be expanded to serve as a refuge when seawater temperatures rise.

The structure of the facility essentially consists of a submergible fish cage, a water treatment system (biological filtration pool), six surrounding seaweed beds, and a control vessel. The Osaka Bay Fish Farm has a five-by-two arrangement of this basic structure.

At the bottom of the 50 m diameter aquaculture fish cages is a sedimentation basin. A pump conveys residual feed, excrement, and other material to a water treatment facility where the material undergoes biological treatment. Water containing the rich nutrients yielded by biological treatment is supplied to the seaweed beds formed on reefs constructed of lightweight blocks. Although the area outside the bay has the disadvantage of low nutrient content in seawater and low biological density, the nutrient salts obtained by this processing can be used to grow the seaweed beds to serve as a rich fishing ground, even outside of the bay. The seaweed beds are cultivated at depths within the range of sunlight penetration.

The out-bay farm is more susceptible to forces of nature like typhoons than the in-bay facility. During storms, the breeding ponds will be submerged to avoid the effects of rough surface seawater, and the control vessel will be evacuated to the bay or other location for safety.

The control vessel, which manages the operations of five units, is equipped with various control devices, a small wind turbine generator for power, solar panels serving as an auxiliary power source, and storage batteries. This vessel will be permanently stationed at sea, positioned near the top of the water treatment plant to provide the energy needed by the water treatment plant and other out-bay facilities.

The tension leg platform (TLP) method(*4), which uses the tensile force generated by the structure's buoyancy to stabilize its position and depth in the sea, is used to moor this semi-submerged floating structure.

*4 Tension leg platform: A system in which a floating structure is vertically anchored to a sea bottom foundation.

[Out-Bay Fish Farm Layout (10 interconnected units)]

[Out-Bay Fish Farm Floor Plan (1 unit)]

[Out-Bay Fish Farm B-B' cross sectional view]

The issue this article appears

No.63 "Fishery"

Our country is surrounded by the sea. The surrounding area is one of the world's best fishing grounds for a variety of fish and shellfish, and has also cultivated rich food culture. In recent years, however, Japan's fisheries industry has been facing a crisis due to climate change and other factors that have led to a decline in the amount of fish caught in adjacent waters, as well as the diversification of people's dietary habits.
In this issue, we examine the present and future of the fisheries industry with the hope of passing on Japan's unique marine bounty to the next generation. The Obayashi Project envisioned a sustainable fishing ground with low environmental impact, named "Osaka Bay Fish Farm".
(Published in 2024)

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