Sanctuaries are the best solution (Image: Gary Bell/Taxi/Getty)

POPULATION crashes in many species of reef fish may be linked to an excess of males brought about by fishing – and imposing quotas won’t remedy the situation.

In many species, particularly those where individuals can change their sex, each fish produces fewer young as the population density drops. “It’s perplexing,” says Stefan Walker of James Cook University in Townsville, Queensland, Australia, “because as population density drops, more resources should be available and populations should increase.”

To find out why this happens, Walker tagged 232 cylindrical sandperches (Parapercis cylindrica) in a Great …