NEW ORLEANS — Consumers around the nation can’t be sure what kind of shrimp they’re buying if they simply look at the label or menu at supermarkets, grocers and restaurants, an advocacy group says. Oceana did a DNA-based survey of shrimp sold at outlets in New York City; Washington, D.C.; Portland, Oregon; and various spots around the Gulf of Mexico. The group said it found about 30 percent of 143 shrimp products bought from 111 vendors were not what the label said.