Native fish in the U.S. are in sharp decline due to the introduction of nonnative species, according to a survey published in the journal Science. In the past, many different fish species existed in every state, but now, fish populations are blending into one group of common fish. Three fish — largemouth bass, black crappie, and the common carp — are found in all lower 48 states. Some of the nonnatives escape from aquariums, while others are poured from bait buckets. And experts say the intruders often benefit from habitat changes that harm natives, such as wetlands draining and dam building. “It’s a one-two punch,” says Peter Moyle, a fisheries biologist at the University of California at Davis. “You change the habitat, then you send in the exotics, and the poor native fish don’t stand a chance.” Removal of nonnative species is both difficult and costly.