Yellow-eyed penguins (YEP) are dying in droves on Stewart Island, and scientists are at a loss to explain why.About 70 percent of the penguin chicks have died over the past 6 years.Researchers for the Yellow-Eyed Penguin Trust (YEPT) along with Otago and Massey university scientists and the Department of Conservation cannot find a cause."We don't know if the penguins are starving and then succumbing to disease or if they're diseased and then starving," said trust spokeswoman Sue Murray.Researchers have already found dead chicks from this year's  generation hatched over the past weeks.Massey University's associate professor Maurice Alley of the NZ Wildlife Health Centre said blood samples showed cases of diphtheria or leucocytozoon, a malaria-like parasite spread by sandflies that causes anemia and weakness. Diptheria causes lesions in the mouth, which prevent the chicks from eating."It was very bad last year ; all the nests at the top part of the island were affected. There are no vaccines at the moment, but we're trying to get to the bottom of it," Alley said.DoC (Department of Conservation) figures show differing survival rates for penguins in different areas of the island."There's a new theory that oyster dredging in Foveaux Strait may be impacting on foraging grounds for some penguin colonies, but then there are other colonies that don't seem affected. We really don't know what the implications are at this stage," DoC southern islands area manager Andy Roberts said."There's a huge collaborative effort going on to try and work out what is killing these chicks, with avian vets working alongside the trust and DoC."