Biologist Rick Howard and his team looked at transgenic fish to see what happened when they mated. Earlier studies have focused on how genetic modification affects food products. This one focused on how a species is affected.
The results of the study showed that the heavier genetically modified males mated three times more and their offspring was less likely to make it to adulthood. This could lead to types of fish disappearing as generations of fish pass on.
Howard says "Though altering animals' genes can be good for humans in the short run, it may prove catastrophic for nature in the long run if not done with care. And we don't know just what kind of care is necessary yet, or how much." Source