How much is floating in Karatsu Bay?
Microplastics (plastic pieces of 5 mm or smaller) collected near the surface of Karatsu Bay by Japan's Ministry of the Environment on 6 November 2019 numbered 2.912 pieces per cubic metre of seawater. This was the highest value of the 18 survey lines in Tokyo Bay, Ishikari Bay and the Genkai Sea that year (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 4/5, in Japanese)).
This page is part of the Karatsu Data Bank by Karatsu Farm & Food, a non-profit in Karatsu, Saga, Japan. It lists counts of floating microplastics as published in government survey reports and a peer-reviewed paper, without conclusions or ratings.
In the survey, a neuston net with a 75 cm square mouth and 350 μm mesh was towed at 2 knots for 20 minutes, collecting pieces of 350 μm to 5 mm from roughly the top 35 cm of the sea. The count was divided by the volume of water that passed through the net. Each line was sampled once, at one point (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 4/5, in Japanese)). The ministry reported the 18 coastal values as 0.10 to 2.91 pieces per m³, within the maximum of its coastal surveys since FY2015 (64.15 pieces per m³) (Ministry of the Environment press release, in Japanese).
The six Genkai Sea lines
In the Genkai Sea, six lines were sampled from off Tsuyazaki to Karatsu Bay. The highest was Karatsu Bay (Genkai_6) at 2.912 per m³, next off Mise (Genkai_4) at 2.110, and the lowest off Tsuyazaki (Genkai_1) at 1.237. All six lines were above 1 per m³. The report notes that the ratio between the highest and lowest line (about 2) was the smallest of the three seas (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 4/5, in Japanese)).
| Line | Area | Date | Water filtered (m³) | Plastic | Foam | Fibres | Microbeads | Total (pieces) | Pieces per m³ |
|---|---|---|---|---|---|---|---|---|---|
| Genkai_1 | off Tsuyazaki | 2019-10-30 | 223.05 | 237 | 30 | 9 | 0 | 276 | 1.237 |
| Genkai_2 | off Uminonakamichi | 2019-10-30 | 229.25 | 379 | 8 | 18 | 2 | 407 | 1.775 |
| Genkai_3 | Hakata Bay | 2019-11-10 | 217.46 | 276 | 6 | 7 | 2 | 291 | 1.338 |
| Genkai_4 | off Mise | 2019-11-08 | 180.59 | 369 | 6 | 6 | 0 | 381 | 2.110 |
| Genkai_5 | off Dokizaki | 2019-11-06 | 217.46 | 119 | 151 | 0 | 0 | 270 | 1.242 |
| Genkai_6 | Karatsu Bay | 2019-11-06 | 190.56 | 553 | 0 | 2 | 0 | 555 | 2.912 |
| Genkai Sea total | 6 lines | - | 1,258.37 | 1,933 | 201 | 42 | 4 | 2,180 | 1.732 |
Of the 555 pieces from Karatsu Bay, 553 were plastic fragments and similar, and none were foam. Off Dokizaki (Genkai_5), 151 of 270 pieces were foam (polystyrene), 56% of the composition. On the same line, observers also saw more than ten clusters of small foam fragments floating at the surface, and the report suggests they may have come from the Matsuura River and other rivers after rain and gathered there with the wind and currents (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 3/5, in Japanese)).
Comparing the three seas
Measured in the same year with the same method, the combined values were 1.732 per m³ for the Genkai Sea, 1.420 for Tokyo Bay and 0.379 for Ishikari Bay (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 4/5, in Japanese)).
| Sea | Lines | Combined | Lowest line | Highest line |
|---|---|---|---|---|
| Genkai Sea | 6 | 1.732 | 1.237 (Genkai_1) | 2.912 (Genkai_6, Karatsu Bay) |
| Tokyo Bay | 6 | 1.420 | 0.643 (Tokyo_2) | 2.560 (Tokyo_5) |
| Ishikari Bay | 6 | 0.379 | 0.100 (Ishikari_5) | 1.271 (Ishikari_3) |
What they are made of, and their colours
Across the three seas, 68% of pieces were polyethylene and 20% polypropylene. In the Genkai Sea the shares were 73% polyethylene, 12% polypropylene and 15% other materials, including 10% polystyrene (the material of foam). White pieces made up 66% in the Genkai Sea, while blue (30%) was most common in Tokyo Bay and clear (29%) in Ishikari Bay. Microbeads were found at 0.009 per m³ on two Genkai Sea lines (off Uminonakamichi and in Hakata Bay) and not on the other four (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 4/5, in Japanese)).
Seabed litter in Karatsu Bay from the same survey
The same survey counted litter that bottom-trawl fishers caught in their nets during normal fishing. In western Karatsu Bay (with the Karatsu branch of the Saga Genkai Fisheries Cooperative), fishing from 16 to 29 October 2019 brought up 2,802 man-made items weighing 137.55 kg (1,484.93 litres), of which 2,547 items were plastic (90.9%, our calculation). The density, 749.2 items per km² trawled, was the highest of the three Genkai Sea areas (Ministry of the Environment, FY2019 coastal drifting and seabed litter survey report (part 2/5, in Japanese)).
| Area | Fishery cooperative | Fishing period (2019) | Items | of which plastic | Weight (kg) | Volume (L) | Items per km² |
|---|---|---|---|---|---|---|---|
| Western Karatsu Bay | Saga Genkai FCA, Karatsu branch | 16-29 Oct | 2,802 | 2,547 | 137.55 | 1,484.93 | 749.2 |
| Eastern Karatsu Bay | Itoshima FCA, Kafuri branch | 15 Nov-3 Dec | 982 | 867 | 33.85 | 420.18 | 319.9 |
| Fukuoka Bay | Fukuoka FCA (three branches) | - | 3,915 | 3,378 | 190.53 | 3,820.58 | 265.4 |
Among the seas around Japan
Observations in East Asian seas around Japan in summer 2014 by researchers from Kyushu University, Tokyo University of Marine Science and Technology and others estimated 1,720,000 microplastic pieces per km² of sea surface, 16 times the North Pacific and 27 times the world ocean. The paper considers the small fragments to have originated in the Yellow Sea and East China Sea, southwest of the study area (Isobe et al. (2015), Marine Pollution Bulletin).
By FY2022 the ministry's coastal surveys covered 239 points in 15 sea areas, and Tokyo Bay had the highest area average. A 2024 report lists Tokyo Bay, Kagoshima Bay, Ise Bay, Wakasa Bay and the Genkai Sea under "density" in its table of criteria for choosing survey sites, and gives past averages of 16.62 per m³ for Tokyo Bay, 0.90 for Ise Bay, 0.10 for Osaka Bay, 0.24 for the Seto Inland Sea and 1.17 for Kagoshima Bay (Ministry of the Environment, coastal microplastic survey report, March 2024, in Japanese). For offshore waters, the ministry reports higher concentrations in the northern Sea of Japan, off western San'in, along the Pacific coasts of Kyushu and Shikoku and from the Tsugaru Strait to off Sanriku (Ministry of the Environment press release, in Japanese).
The larger plastics that wash ashore
Some microplastics form when plastic items on beaches or at sea break into small pieces. In beach litter surveys by prefectures at 78 sites in 39 prefectures in FY2022 (the Saga site was Kotari Beach in Karatsu), 110,532 of 118,762 man-made items (93%) were plastic. By count, the top items were oyster-farming spacer tubes (19,673, 15.9%), bottle caps and lids (16,429, 13.3%) and fishing nets (13,676, 11.0%) (Ministry of the Environment, FY2022 beach litter composition survey summary, in Japanese). Our Precious Plastic Karatsu project makes products from collected bottle caps and other plastics. Figures on bottle cap collection are on PET Bottle Caps in Numbers.
FAQ
Q1. How many microplastics are there in Karatsu Bay?
The Ministry of the Environment sample of 6 November 2019 found 2.912 pieces per cubic metre of seawater, the highest of 18 lines in Tokyo Bay, Ishikari Bay and the Genkai Sea. It is one sample at one point.
Q2. Does the Genkai Sea have more than Tokyo Bay?
In the FY2019 survey using the same method, the combined values were 1.732 per m³ for the Genkai Sea, 1.420 for Tokyo Bay and 0.379 for Ishikari Bay. Including other years, Tokyo Bay averaged 16.62 per m³; values vary widely by year and point.
Q3. What are the microplastics in Karatsu Bay made of?
The report does not give a material breakdown for the Karatsu Bay line. For the six Genkai Sea lines combined, 73% were polyethylene, 12% polypropylene and 15% other materials (10% polystyrene). None of the 555 pieces from Karatsu Bay were foam.
Q4. Are the seas around Japan higher than elsewhere?
A paper based on observations in summer 2014 estimated 1.72 million pieces per km² in East Asian seas around Japan, 16 times the North Pacific and 27 times the world ocean.
Q5. Are there figures for the Ariake Sea?
We could not find values for the Ariake Sea in the published materials of the ministry's coastal surveys used on this page.
Sources
| Document | Publisher | Date | URL | Accessed |
|---|---|---|---|---|
| FY2019 marine litter survey results (press release) | Ministry of the Environment | 2021-03-30 | https://www.env.go.jp/press/108800.html | 2026-11-10 |
| Attachment 1: FY2019 marine litter survey results | Ministry of the Environment | 2021-03-30 | https://www.env.go.jp/content/900517319.pdf | 2026-11-10 |
| FY2019 coastal drifting and seabed litter survey report (index of parts) | Ministry of the Environment | 2020-03 | https://www.env.go.jp/water/marine_litter/post_107.html | 2026-11-10 |
| Same report, part 1/5 (aims and methods) | Ministry of the Environment (Sanyo Techno Marine) | 2020-03 | https://www.env.go.jp/content/900539014.pdf | 2026-11-10 |
| Same report, part 2/5 (seabed litter) | Ministry of the Environment (Sanyo Techno Marine) | 2020-03 | https://www.env.go.jp/content/900539015.pdf | 2026-11-10 |
| Same report, part 3/5 (drifting litter) | Ministry of the Environment (Sanyo Techno Marine) | 2020-03 | https://www.env.go.jp/content/900539016.pdf | 2026-11-10 |
| Same report, part 4/5 (microplastics) | Ministry of the Environment (Sanyo Techno Marine) | 2020-03 | https://www.env.go.jp/content/900539017.pdf | 2026-11-10 |
| Survey of drifting litter including microplastics in coastal waters, report | Ministry of the Environment (Sanyo Techno Marine) | 2024-03 | https://www.env.go.jp/content/000311853.pdf | 2026-11-10 |
| East Asian seas: A hot spot of pelagic microplastics (Marine Pollution Bulletin 101) | Isobe, Uchida, Tokai, Iwasaki | 2015 | https://doi.org/10.1016/j.marpolbul.2015.10.042 | 2026-11-10 |
| FY2022 beach litter composition survey data: summary | Ministry of the Environment | 2024-03 | https://www.env.go.jp/content/000224793.pdf | 2026-11-10 |
Notes on reading the figures
The Karatsu Bay value of 2.912 per m³ is a single sample at one point on 6 November 2019. Microplastic counts change a great deal from day to day with wind, currents and river water after rain. The ministry's FY2020 to FY2022 surveys sampled three other seas (west Hokkaido, west Noto Peninsula and Enshunada) in each season and counted pieces under 1 mm and 1-5 mm separately, so they are not directly comparable with FY2019. The paper's 1.72 million is per km² of sea surface, a different unit from per m³. The Genkai Sea material and colour shares are for six lines combined, not Karatsu Bay alone. Seabed litter figures are items caught in fishing nets, not the total on the seabed. The 90.9% plastic share for western Karatsu Bay is our calculation from the item counts in the report.
What this page does not cover
This page does not cover effects of microplastics on human or animal health, microplastics in fish or shellfish, counts in beach sand, chemicals attached to microplastics, or the Ariake Sea. Estimates of plastic flowing from Japan to the sea are on Plastics in Numbers, and the cost of collecting and treating marine litter is on The Cost of Collecting Marine Litter.
Citing this page and enquiries
To cite the figures on this page or to ask about local data, please use the contact page or email info@karatsu-f-f.com.
About this page
By Hiroaki Kojima (Vice Director, Karatsu Farm & Food) / Contact: info@karatsu-f-f.com (contact page) / Published 10 November 2026 / Markdown: microplastic-data.mdThis page is made together with AI. See this data bank and AI for how.
Related links
▶ Karatsu Data Bank
▶ Plastics in Numbers
▶ The Cost of Collecting Marine Litter
▶ Beach Cleanups in Numbers
▶ PET Bottle Caps in Numbers
▶ マイクロプラスチックのデータ（日本語）