Causes of Isoyake and Fixes: Urchin Grazing and Seaweed Bed Restoration

What is isoyake? The disappearance of the sea forest

Definition, and how it differs from normal variation

Isoyake is a phenomenon in which "mobaba" (seaweed beds) of large algae such as kelp, wakame and sargassum decline or disappear markedly, beyond seasonal swings and modest year-to-year change. Fisheries Agency materials describe it in this way. Seaweed beds serve as spawning grounds, nurseries and feeding grounds for fish, shellfish and crustaceans, so when they vanish, coastal fishery resources themselves dwindle.

Seaweed naturally changes with the seasons, growing in spring and dying back in summer, so a poor year alone is not isoyake. It is judged to be isoyake when seaweed does not return for years and the rock stays covered with whitish coralline algae.

Seaweed bed decline across Japan

Fisheries Agency materials summarize that 31 prefectures had declining seaweed beds as of 2021. It is reported everywhere, from kelp grounds in Hokkaido to seaweed and seagrass beds in Kyushu and the southwestern islands, so it is not a problem of one sea only.

Why it matters to our lives

• Fishery resources: abalone, turban shell, urchin, spiny lobster and juveniles of many fish depend on seaweed beds
• Water purification: seaweed absorbs nutrients from seawater
• Carbon storage: seaweed beds draw attention as blue carbon (blue carbon explained)

Causes of isoyake: urchins are not the only culprit

Three factors organized by Hokkaido fisheries experiment stations

Materials from Hokkaido fisheries experiment stations organize the factors behind isoyake into three groups. Of these, people can most readily control grazing by urchins and others and a shortage of seaweed seeds (spores).

How urchin grazing creates a vicious cycle

Urchins do not die quickly when food runs short. They survive in a thin state and promptly eat the tiny seaweed that sprouts. This fixes a vicious cycle: many urchins → seaweed cannot grow → isoyake persists.

Worse, urchins in food-poor areas have almost no meat (gonads) and cannot be sold. With no market value, fishers do not harvest them, and they remain.

Fish grazing west of Kanto and rising water temperature

Urchin grazing is a nationwide problem, while grazing by herbivorous fish such as rabbitfish (aigo) and parrotfish (budai) is described as a problem west of the Kanto region. Tokushima Prefecture materials also suggest that rising coastal temperatures made herbivorous fish and urchins feed more actively and for longer periods.

Identifying grazing in the field

Basics: feeding marks and the animals present

In the field, divers and underwater cameras check feeding marks on seaweed, urchin density and schools of herbivorous fish. Grazing intensifies when young seaweed sprouts, so it is important to match survey and control timing to the seaweed life cycle.

Season is the key

A Nagasaki Prefecture case shows that removing urchins (long-spined urchins) before autumn, when seaweed sprouts, tends to be effective. Reducing urchins beforehand leaves room for new shoots to grow without being eaten.

Why monitoring must continue

A Saga Prefecture manual stresses keeping the density of urchins and snails low and monitoring regularly. Even after recovery, urchins return if many remain nearby, so a system that does not end with a one-off effort is needed.

Urchin control in practice: removal, dispersal and protection

Removal is most reliable but costly

Urchin control is broadly divided into removal/dispersal and protection from urchins, and removal is described as the most reliable. Methods include trap fishing, collection from boats and removal by diving, and efficiency and labor costs are challenges.

• Traps and boat collection: cover a fairly wide area but are weak in deep water and rock crevices
• Diving removal: reliable but needs labor and time
• Urchin fences: enclose seaweed to keep urchins out (installed in Nagasaki Prefecture and elsewhere)

Effects can be temporary

Reports note that effects can be temporary and that continuous removal is needed when many urchins remain around. Clearing only a small plot is not enough, because urchins move back in. The balance of area and continuity matters.

Results from fishers' activities

In the Takahama district of Nagasaki City, young fishers of the youth division led urchin removal from 2010. According to reports, in the spring after removal began, barrens that had been dominated by coralline algae showed small seaweeds as well as large brown algae such as wakame and sargassum.

In Kochi Prefecture too, the effectiveness of urchin-removal plots run by fishers was demonstrated, and sites grew from the original 4 to 12.

Turning a nuisance urchin into a resource

Raising thin urchins: stock enhancement

Urchins in isoyake seas have little meat and do not sell. Efforts are under way in various places to raise removed urchins elsewhere with feed, improving their meat before shipping. In Ito City, Shizuoka Prefecture, a seafood company's challenge to turn nuisance urchins into "delicious urchins" was reported by local media.

If removal gains economic value, fishers find it easier to continue. The idea is to reconcile environmental conservation with income.

Moving urchins to seaweed-rich places

The Nagasaki Prefectural Fisheries Experiment Station is examining a model in which poor-quality urchins are moved to places with small seaweed, fed there and harvested in May. It reports that points remain to be clarified, such as the link to removal timing and transplant density.

Munakata City (Fukuoka) pilot project

As a fiscal 2024 pilot, Munakata City (Fukuoka) raised removed urchins on land using discarded food, and set fertilizer nets made from the urchin shells, with akamoku (Sargassum horneri) seedlings attached, at sea. It is reported to have also helped build a cooperative framework among the Munakata fisheries cooperative, Fukuoka Prefecture and a private company.

National frameworks and growing support

The Seaweed Bed and Tidal Flat Vision and isoyake guidelines

The Fisheries Agency published the "Seaweed Bed and Tidal Flat Vision" in January 2016, and visions based on it have been drawn up for 80 sea areas nationwide (as of December 2023, when it was revised). It also revised its isoyake countermeasure guidelines as the third edition in March 2021, covering everything from diagnosing causes to carrying out countermeasures.

Sharing cases at the national council

The Fisheries Agency holds a national council on isoyake countermeasures and shares cases from around the country. Materials are published on its website for local fishers and governments to consult.

Links to blue carbon

Restoring seaweed beds also attracts attention from the viewpoint of blue carbon, which takes CO2 into the sea. For how protecting seaweed beds can be recognized, see our explainer on J-Blue Credits.

What we can do: learn, eat, share

Deepen your understanding with related articles

• Isoyake from a global perspective: Why do kelp forests turn into urchin barrens?
• Predators and ecosystem balance: Why are sea otters a keystone species?
• Restoring seaweed beds: Restoring seaweed and seagrass beds, the nurseries of fish

Three everyday actions

• Learn about local sea efforts: check seaweed bed conservation by local fishery cooperatives and governments
• Choose local seafood: know and enjoy seafood supported by seaweed beds
• Look at climate change: rising water temperature is one factor affecting seaweed beds

Isoyake is not a problem of urchins alone, fish alone, or temperature alone. Identifying the cause, and fishers, researchers and governments sustaining effort over time is the shortest path to bringing the sea forest back.