# Seaweed Beds and Urchin Barrens in Numbers: Japan, Saga Prefecture and Karatsu

Created by: Karatsu Farm&Food (NPO), Karatsu Data Bank / HTML: https://karatsu-f-f.com/en/seaweed-beds-data.html / Published: 2026-10-27

- Seaweed and seagrass beds in Japan: 207,615 ha → 201,212 ha, 1978 → 1992 (Ministry of the Environment, 4th National Survey on the Natural Environment)
- Seaweed beds in Saga Prefecture: 1,005 ha, from satellite imagery in FY2018–2020 (Biodiversity Center of Japan)
- Karatsu City's grants for seaweed bed and fishing ground conservation groups: JPY 7,181,010 in FY2025. This is the city's 15% share; the national government (70%) and the prefecture (15%) pay directly

## Seaweed beds and urchin barrens, in numbers

Seaweed and seagrass beds (moba) are shallow coastal areas covered by marine plants, where fish and shellfish spawn and grow. This page lists, from national, prefectural and city documents, the area of seaweed beds in Japan, Saga Prefecture and Karatsu, when and why urchin barrens (isoyake, the loss of seaweed beds) appeared, the money spent on conservation, and blue carbon credits. Japan's seaweed and seagrass beds fell from 207,615 ha in 1978 to 201,212 ha in 1992, a loss of 6,403 ha ([Ministry of the Environment](https://www.env.go.jp/nature/koen_umi/umi02_3.pdf)).

This page is part of the Karatsu Data Bank by Karatsu Farm&Food. It gives no conclusions or evaluations, only the figures and statements found in the documents. Values we calculated are marked "our calculation". Values read from charts are not used. English renderings of Japanese documents are our translations; the original text takes precedence.

## Area of seaweed beds in Japan

| Scope | 1978 | 1992 | Change | Ratio |
|---|---|---|---|---|
| Japan (total) | 207,615 ha | 201,212 ha | −6,403 ha | 97% |
| Karatsu–Imari sea area | 6,373 ha | 6,366 ha | −7 ha | 100% |

Note: The 1978 value is the sum of existing and lost beds recorded in the 4th survey; the 1992 value is existing beds in the 4th survey. Source: Ministry of the Environment (data from the 4th National Survey on the Natural Environment)

## Area of seaweed beds in Saga Prefecture

| Category | Survey plots | Area |
|---|---|---|
| Existing beds | 58 | 1,268 ha |
| Lost beds | 1 | 1 ha |

Note: Survey of Saga Prefecture (Karatsu and Imari sea areas), mostly carried out in October–November 1989 and March 1990. The previous survey was around 1978. Source: [Biodiversity Center of Japan, Ministry of the Environment](https://www.biodic.go.jp/reports/4-12/r290.html)

| Survey | Saga | Fukuoka | Nagasaki | Japan |
|---|---|---|---|---|
| 5th survey (1998) | 648 ha | 3,909 ha | 8,394 ha | 145,865 ha |
| Seaweed bed survey (FY2018–2020) | 1,005 ha | 1,971 ha | 19,095 ha | 164,300 ha |

Note: FY2018–2020 values come from maps based on about 50 cm resolution satellite imagery checked by field surveys. Tokyo Bay, Ise Bay, the Seto Inland Sea, the Ariake Sea and Shimabara Bay, and the Yatsushiro Sea are excluded (Saga's value excludes its Ariake Sea side). The source states that the 5th survey used very different methods, such as interviews, and the two "cannot be simply compared". Source: [Biodiversity Center of Japan](https://www.biodic.go.jp/moba/files/02_11.pdf)

Saga Prefecture's fisheries infrastructure plan for the Genkai area states that the seaweed bed area "has been maintained at about the same level as about 10 years ago", while "localized urchin barrens" have appeared ([Saga Prefecture](https://www.pref.saga.lg.jp/kiji00380333/3_80333_245881_up_8rwcbmiy.pdf)).

## When and why urchin barrens appeared

- Kyushu: large-scale urchin barrens were reported in the 1990s
- Genkai sea area of Saga Prefecture: relatively small-scale urchin barrens began to appear in the 2000s
- Cause in the Genkai sea area: high densities of the sea urchins Diadema setosum and Heliocidaris crassispina grazing on seaweed
- Average sea surface temperature of the East China Sea: up 1.07°C over about 100 years to 2014
- The prefecture's Genkai Fisheries Research and Development Center began developing restoration methods in FY2011

Source: Saga Prefectural Genkai Fisheries Research and Development Center, Seaweed Bed Restoration Manual for the Genkai Sea Area of Saga Prefecture (2015)

In test plots where urchins were removed, large seaweeds tended to increase from about two years after removal. The manual also states that warmer water has increased Diadema setosum and lengthened the season when fish graze on seaweed ([Saga Prefecture](https://www.pref.saga.lg.jp/kiji00320179/3_20179_2_mob2015.pdf)). Monthly sea temperatures off Genkai are on [Genkai Landings and Sea Temperature](https://karatsu-f-f.com/en/genkai-landings.html).

## Seaweed and urchin monitoring by the prefecture

The Genkai Fisheries Research and Development Center counts seaweeds and urchins by diving at depths of 0–10 m in three districts (Yakataishi, Hado and Kariya) every year and publishes the results in its annual reports. The beds in the three districts are described as "mixed beds of Eisenia bicyclis, Ecklonia kurome and perennial Sargassum species" ([FY2021 report](https://www.pref.saga.lg.jp/kiji003116451/3_116451_370365_up_03wpwy1v.pdf), [FY2023 report](https://www.pref.saga.lg.jp/kiji00396228/3_96228_370148_up_gbr8wyzc.pdf)).

## Money Karatsu City spends on seaweed beds and fishing grounds

| Program (department) | FY2025 settled amount | Description in the document |
|---|---|---|
| Fishing ground productivity and multifunctional fisheries program (Fisheries) | JPY 7,181,010 | Removal of urchins such as Diadema setosum and of drifting and stranded debris by fishers' groups. City share 15%; national 70% and prefecture 15% paid directly. JPY 8,629,000 carried over to FY2026 |
| Remote island fisheries revitalization grants (Fisheries) | JPY 28,218,000 | Seed release, urchin removal, intermediate rearing of red sea urchins, trial seaweed farming by fishing communities on seven islands |
| Carbon credit creation program (Environment) | JPY 4,598,000 | Plan for credits from marine ecosystems. FY2024: literature review and diving survey of seaweed beds; FY2025: draft local action plan |
| Local SDGs creation program (Environment) | JPY 351,940 | Nature positive symposium fee JPY 101,160; study trip on blue carbon JPY 250,780 |

Source: Karatsu City, Report on Results of Major Policies, FY2025

The city's FY2025 grants to 11 conservation groups totaled JPY 7,181,010, its 15% share; the national government (70%) and Saga Prefecture (15%) paid their shares directly ([Karatsu City](https://www.city.karatsu.lg.jp/uploaded/attachment/27438.pdf)). If the city's amount is 15% of the total, the combined amount is about JPY 47.87 million (our calculation). Grants to fishing communities on seven islands totaled JPY 28,168,000 (JPY 28,218,000 including office costs).

## Blue carbon and credits

| Item | Value | Time | Source |
|---|---|---|---|
| Removals by seaweed/seagrass beds and mangroves reported by Japan | about 340,000 t-CO2 | FY2023 | [Ministry of the Environment](https://www.env.go.jp/content/000371388.pdf) |
| J-Blue Credits certified in FY2022 | 21 projects, 3,733.1 t-CO2 | FY2022 | [MLIT](https://www.mlit.go.jp/sogoseisaku/environment/content/001717241.pdf) |
| of which Kushiura, Chinzei, Karatsu | 41.1 t-CO2 | FY2022 | same |
| Wakame farming education project, Karatsu Bay | 0.4 t-CO2 | certified 10 Jan 2025 | [JBE credit registry](https://www.blueeconomy.jp/wp-content/uploads/jbc2025/20251219MB-JBC.pdf) |
| Price of the Karatsu Bay wakame credits | JPY 55,000 per 0.1 t-CO2 (tax incl.) | Jan 2025 | [Japan Airlines](https://press.jal.co.jp/ja/release/202501/008587.html) |

If all 0.4 t-CO2 were sold at this price, the proceeds would be JPY 220,000 (our calculation). Income and costs of nature credits are compared on [Nature Credits in Numbers](https://karatsu-f-f.com/en/nature-credits.html).

## Donations for seaweed bed restoration

| Campaign | Period | Target | Raised | Donors |
|---|---|---|---|---|
| Turning CO2 uptake of restored beds into credits (survey and application) | 17 Jan – 16 Apr 2025 | JPY 1,000,000 | JPY 202,000 (20.2%) | 15 |
| Urchin removal and mother-seaweed rafts | 9 May – 6 Aug 2025 | JPY 2,000,000 | JPY 550,000 (27.5%) | 23 |

Note: Both were hometown tax donation campaigns of a Karatsu-based NPO under Saga Prefecture's NPO support scheme. Source: Furusato Choice ([1](https://www.furusato-tax.jp/gcf/3806), [2](https://www.furusato-tax.jp/gcf/3983))

The second campaign's JPY 2,000,000 budget was JPY 500,000 for urchin removal and rafts, JPY 700,000 for project management, JPY 500,000 for thank-you gifts and shipping, and JPY 300,000 for the prefecture's fee. It raised JPY 550,000.

## FAQ

### Q1. What causes urchin barrens off Genkai?

The prefecture's manual names grazing by high densities of Diadema setosum and Heliocidaris crassispina, and notes that warmer water has increased Diadema setosum and lengthened the grazing season of fish.

### Q2. How much have Japan's seaweed beds declined?

In the 4th National Survey, from 207,615 ha in 1978 to 201,212 ha in 1992, a loss of 6,403 ha.

### Q3. How large are Saga Prefecture's seaweed beds?

1,005 ha in the FY2018–2020 satellite survey, excluding the Ariake Sea side.

### Q4. How much does Karatsu City spend on seaweed bed conservation?

In FY2025: JPY 7,181,010 in grants to fishers' conservation groups (city's 15% share), JPY 28,218,000 for remote island fishing communities, and JPY 4,598,000 for a marine carbon credit plan.

## Not covered on this page

Evaluations of individual organizations or projects, values read from charts in presentations, figures we could not confirm in primary sources, and links to offshore development such as offshore wind.

## Citing this data, joint research and site visits

For citations of the figures on this page, joint research, or site visits in Karatsu, please contact us via our [contact page](https://karatsu-f-f.com/en/contact.html) or [info@karatsu-f-f.com](mailto:info@karatsu-f-f.com).

## About this page

Author: Hiroaki Kojima (Vice Director, [Karatsu Farm&Food](https://karatsu-f-f.com/en/)) / Published: 2026-10-27 / Markdown: [seaweed-beds-data.md](https://karatsu-f-f.com/en/seaweed-beds-data.md)

## Related

▶ [Karatsu Data Bank](https://karatsu-f-f.com/en/data-bank.html) ▶ [Genkai Landings and Sea Temperature](https://karatsu-f-f.com/en/genkai-landings.html) ▶ [Nature Credits in Numbers](https://karatsu-f-f.com/en/nature-credits.html) ▶ [日本語版](https://karatsu-f-f.com/seaweed-beds-data.html)