Disappearing seaweed in Japan
Japan, a voracious consumer of fish and other marine products, has discovered that dwindling fish stocks could be linked to the disappearance of tengusa seaweed colonies. Between 6-10,000 hectares of seaweed have been lost from Japan over the last 10 years.
The island of Hachijojima, almost 500 km south of Tokyo, has long been a key area for abalone fishing. But now the abalone catch has fallen from 20 tonnes per year to one tonne. And the sea surrounding the island is virtually devoid of tengusa, which is an important food source for various marine organisms. Until about five years ago, there were lush tengusa colonies on the island’s rocks.
Scientists have been trying to discover the cause of the problem. Rising sea temperatures may be partly to blame, as the sea temperature in some areas of Japan has risen by 2-3 °C. Another theory is that coastal development has led to the release of toxic effluents into the sea, which might change the level and nutrient content of shallow waters. Higher nitrate concentrations may also be adversely affecting the eco-system and causing algal death.
Sea urchins are voracious eaters of tengusa, so yet another possibility is to reduce the sea urchin population.
Meanwhile, various options for increasing the growth of tengusa are being studied. One method is by increasing the concentration of ferrous iron in the seabed, which is done by burying iron and steel slag and wood chips in the sea bed. New Zealand research has shoed that this increases the growth of algae and edible seaweed. Another approach is to drop seaweed “seedlings” are dropped into the sea.
