Millions of people visit Hanauma Bay in East Honolulu every year but one marine biologist says those outer reefs are healthier than the ones on Oahu's North Shore where fewer people swim. 

During a dive in Hanauma Bay three weeks ago, the researcher noticed full and healthy corals, surrounded by schools of fish. One week later on the outer reefs at Pipeline Beach on the North Shore, bare and some even look corroded. 

Marine biologist Terry Lilley claims bad coral health correlates with erosion. He says dead corals are lower in height, so the energy from waves go over the reef instead of breaking on it, making it easier to wash away the shoreline. 

"Our coral reefs are important for a lot of reasons but we're really gonna start seeing it in erosions along our coastlines if we don't do something about it," Lilley said. 

He says more research is needed to determine why the reefs are dying, but says once a cause is determined, they can take action to the help the coral thrive again.