Sydney, Aug 25 - The tiniest fish on the Great Barrier Reef are warning of profound changes taking place in the reef's natural systems, thanks to human activity.

A little more than an inch long, the gobies are so small and cryptic they are often invisible to the casual visitor. But they make up almost half of all the fish life on the reef, says David Bellwood, professor at the ARC Centre of Excellence for Coral Reef Studies.

'These fish may be tiny, but they are very important. They are telling us that the world has changed, and in ways we do not understand. That we may not be able to manage things as well as we hoped,' ichthyologist Bellwood said.

'In 1998 there was a major coral bleaching event that affected corals across a huge area of the reef. After some years, quite a lot of the coral has recovered -- and looks more or less as it once did.'

'But the gobies have not come back. Something is not right if the fastest breeders of the reef are still missing.