A comprehensive search for a link between pollutants and the appearance of 'feminized' male fish is being considered by Japan's National Institute for Environmental Sciences (NIES). A decision is expected later this month.

Female empowerment: egg cells developing within the testis of a 'feminized' male fish. Credit: S. HASHIMOTO

Recent research has shown that certain male fish in the seas around Japan have physiological properties usually found in females. For example, some males produce high levels of vitellogenin, an egg-yolk protein, and others even make oocytes — precursor egg cells — in their testes.

Some researchers suspect that these problems are due to chemical 'endocrine disrupters', which at low doses can interfere with hormones. But most of the evidence for this comes from apparent correlations between the presence of feminized fish and heavily polluted areas. Little progress has been made in identifying the molecular mechanisms and chemicals that are to blame.

Some scientists — as well as chemical manufacturers — dispute the existence of the endocrine-disrupter effect, regarding it as an excuse to regulate hundreds of chemicals without evidence that they cause harm.

The fact that some fish naturally switch sex further confounds the problem. “We need caution before jumping to a conclusion that something is abnormal,” says John Sumpter of Brunel University, UK, who pioneered a study of intersexed roach. “The evidence linking cause and effect is slim.”

But over the past couple of years, Japanese studies have indicated that the effect is widespread. “We are seeing an effect in a variety of fish,” says Shinya Hashimoto, an environmental chemist at Shizuoka University.

Researchers say that a comprehensive study could also help to establish a database for pollution monitoring. “It could be a valuable measure of relative pollution over the areas,” says Takahiro Matsubara of the Fisheries Research Agency in Hokkaido.

The NIES is scheduled to complete a feasibility study this month to determine how best to carry out the proposed investigation, but budget constraints may prevent a more extensive study, officials say.