Activists say measures to protect Maui's dolphins are insufficient, and predict the dolphins -- the rarest in the world -- could be extinct within 20 years. The New Zealand government has proposed expanding a protective zone for the black-and-white Maui's dolphins, which are being threatened by fishing and disease, but researchers say the actions don't go far enough, the BBC reported Friday. Maui's dolphins are found only along the west coast of New Zealand's North Island. A 2012 survey by New Zealand's Department of Conservation found there were about 55 adult Maui's dolphins living in waters up to a depth of about 328 feet. The government recently announced new restrictions on fishing, extending by about 131 square miles a ban on the use of set nets. Conservation Minister Nick Smith said the change would help reduce the biggest threat to the dolphins. "We are taking a cautious approach by banning set netting where there is clear evidence the Maui's dolphins go while not unnecessarily banning fishing where they are not," Smith said. Critics said the proposals amount to a "death sentence" for dolphins, because more than 75 percent of habitat would remain unprotected from set netting and trawling, the BBC said. "These new measures will do nothing to stop the dolphins' decline," Elizabeth Slooten, a University of Otago professor who researched Maui's dolphins for years, said. The International Union for the Conservation of Nature has declared Maui's critically endangered.