that's what they do, they eat frog spawn, but our native frogs should be save as they do not just lay their eggs and bugger off, they look after their offspring and the introduced ones, too many everywhere anyway the bigger problem nowadays with importing amphibians is chytrid-fungus which can kill whole populations in a short time if the amphibians are already 'stressed' and 'weakened' because of other enviromental influences such as raising temperatures or lack of rain as examples.