Stress of too many female companions causes smaller offspring

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A study has found that female damselfish surrounded by too many other females produce smaller fry.

The study by Dr Mark McCormick of James Cook University showed that the stress caused by too many female companions caused a significant amount of smaller fry to be produced, suggesting that stress is a major factor in the health of fish populations.

The study, which was undertaken on the damselfish Pomacentrus amboinensis on the Great Barrier Reef, indicates that the number of other females interacting with pregnant females has a direct effect on fry size.

McCormick from the Australian Research Council Centre of Excellence for Coral Reef Studies said that females surrounded by other females during pregnancy and breeding resulted in smaller offspring, which were less likely to survive to adulthood.

The study was conducted by introducing female damselfish into an area where there was already a breeding pair and measuring the size of the offspring produced compared to normal and unstressed conditions.

McCormick told The Age: "We found that the more females present, the higher the levels of aggression between them - and the higher the levels of the stress hormone, cortisol, in their ovaries."

McCormick says that Damselfish were the perfect case study, as they live in a small fixed area and share the same life history characteristics of important commercial fish.

The fact that stress has now been found to have a clearly defined effect on the health of fish can provide information of the ability of a population to renew itself under stressed conditions.

For more information see the paper: McCormick MI (2006) - Mothers matter: crowding leads to stressed mothers and smaller offspring in marine fishes. Ecology, Vol. 87, No. 5, pp. 1104-1109.