Sydney  - Not watching their weight can be deadly for gobie fish who live on the Great Barrier Reef on Australia's east coast, researchers have found.

The team from James Cook University in Townsville discovered that within shoals of gobie there was a hierarchy based on weight.

Smaller fish risked being expelled from the group if they eat too much and stood to move up the pecking order because of their increased bulk. Fish thrown out of the pack face certain death from their predators.

Team leader Philip Munday found that smaller fish would not touch extra food given to them for fear of gaining weight.

"We were really surprised," Munday told reporters. "We were thrilled when we saw that these things can make a decision. They will stop feeding if it's not in their best interest."

Munday said that in gobies, where only the largest male and female had mating rights within the group, subordinate fish were consistently 5 to 10 per cent smaller than the next largest.

"If the second in the queue gets too big then things get nasty and the first in the queue will actually try and boot out the second," he said. (dpa)