Hawai'i scientists are joining a worldwide effort to tally everything living on the reefs — a part of the Census of Marine Life, which is trying to count life throughout the oceans.

One issue is that reefs are in trouble around the world, and scientists don't even have a wild guess at how much life is there, and how much is being lost. Estimates run from a million to 9 million species.

"Reef decline worldwide is troubling, just within the last three decades, declines of 8 percent in coral cover have been reported for Caribbean reefs, and even apparently healthy reefs have suffered measurable degradation," said Russell Brainard, head of NOAA's coral reef ecosystem division in Honolulu, in a printed statement.

In conducting the global reef census, NOAA's Pacific Islands Fisheries Science Center will join the Scripps Institution of Oceanography, and the Australian Institute of Marine Science. Theirs is one of 17 projects of the Census of Marine Life — a project scheduled to take 10 years and to involve 1,700 scientists from 73 countries. It seeks to know what's in the ocean, how much of it there is, and how it is distributed.

The things already known about reefs are mainly based on the topics on which previous researchers have been interested in working, and that's mainly corals, fish and some molluscs. As an example, marine algae — seaweeds — have not been that popular as a research topic, and as a result, even today new species are regularly being described as scientists head into remote places like the Northwestern Hawaiian Islands.

With reefs in distress from warming oceans and the associated coral bleaching, mud and fertilizer running off the land; development; and other threats, some species could be lost before science knows they are there, said Julian Caley of the Australian Institute of Marine Science.

"We do not have any clear understanding of how many reef-associated species can survive various levels of reef degradation," Caley said in a written statement.

The project, to be known as CReefs, in addition to identifying and counting species, will try to establish what is needed to preserve the biodiversity of the reefs.

Nancy Knowlton of Scripps said the scientists will use the most modern techniques, including DNA-based assessments, to help detect new species.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.