Goldfish gone wild.

• Position: THE BIG PICTURE

Most goldfish weigh just a few ounces. This one, found recently in a river in Australia, tipped the scales at more than 4 pounds. Fish like this are part of a growing problem: People who no longer want their goldfish release them into streams and lakes, where the fish have no natural predators. Outside their fishbowls, they become an invasive species: non-native organisms that harm focal plants and animals. A recent Australian study found that once in the wild, goldfish can grow to more than a foot...