Sonar might help
with state fish census

Identifying species by acoustic
signature could help the state
shape restricted areas

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By Helen Altonn
haltonn@starbulletin.com

Fishery scientists someday may be able to identify different species of bottom fish in the ocean without catching them.

They are trying to develop a method of telling the fish apart by their acoustical signals.

"The only way we can assess the reserves is to go in there and get a really good sample year after year," said Chris Kelley, Hawaii Undersea Research Laboratory chief biologist and University of Hawaii Institute of Marine Biology researcher.

"To remove fish and get a sample size defeats the purpose, so we have not assessed the reserves in that way."

This is the last year of a five-year program led by Kelley and funded by the UH and state Department of Land and Natural Resources to study bottom-fish populations and habitats in Hawaii waters.

He is building up data to help the department improve management of 19 bottom-fish reserves. Ehu (red snapper) and onaga (long-tailed red snapper) are endangered bottom fish, and opakapaka (pink snapper) is nearing that status.

Aquatic biologist Walter Ikehara worked with fishermen, scientists and others for 2 1/2 years to develop the bottom-fish management plan, adopted in 1998.

Kelley has collected a lot of information the past four years with surveys, topographic mapping and sonar scans that was not available when the restricted fishing areas were designated, Ikehara said.

Considering what is known now about the reserves, he said: "I think some are hits and some misses. But I view this as an evolutionary plan. As we learn more, we will adjust it to make it work better."

Kelley agreed: "When five years are up and they want to review the plan, I think it would be prudent to consider the possibility of altering some of the boundaries."

Identifying the fish in deep water is one of the biggest problems, Ikehara said, explaining echo sounders can tell where a school is but not what the fish are. With expertise and experience, "fishermen can make some pretty good educated guesses about what the fish are," he said.

Fishery scientists want to take the guesswork out of it.

Whitlow Au, chief scientist for the Marine Mammal Research Program at the Hawaii Institute of Marine Biology, Coconut Island, and graduate student Kelly Benoit-Bird are looking at the acoustic signature of various bottom fish.

"We're doing some really basic work with bottom fish that has never been done before," Au said. "The state has a whole bunch of reserves but no way of monitoring what's going on in the reserves. ... Is the stock recovering? That's one big question."

Another is whether most of the bottom fish are in the reserve or outside, he said. "What is the range of habitat of some of these bottom fishes? We have no idea."

One way of monitoring the prized commercial fish is to use an acoustic sonar-type technique, similar to fish-finding sonar, Au said.

But the Hawaii Institute of Marine Biology researchers want to go a little further and see if there is a way to tell a school of opakapaka from a school of onaga by how they reflect energy, he said.

The biggest reflector in the fish is their swim bladder, he said. X-rays of fish done at the Queen's Medical Center showed different shapes of swim bladders in different species, he said. The scientists are trying to relate the different shapes to the acoustic signatures of the fish, he said.

Au, who works with dolphins, said he is using a simulated dolphin echolocation signal as one kind of signal to get detailed mapping of echoes that come back from bottom fish.

"All the acoustic work done so far seems to be lined up with what we see in X-rays, maybe different shapes and different volumes."

Au said he is seeking to lease a sonar system developed by a company that uses acoustic features of echoes to distinguish different fishes on the mainland. "Whether it's applicable for bottom fish, we need to find out."

Besides acoustics, the bottom-fish program includes genetic research, habitat surveys and efforts to rear the fish in captive conditions.

"Genetics is a powerful tool to differentiate species," Kelley said. "We are able to identify new species of ehu previously unknown."

It was believed fish catches were separate stocks in the three zones of the bottom-fish fishery from the main Hawaiian Islands to the Northwestern Hawaiian Islands, with onaga and ehu reported overfished, Kelley said.

Genetic analysis proved another theory, that they are single stocks with larvae transported between the islands, he said.

The Western Pacific Fisheries Council used the data to delete ehu and onaga from the overfished list and instead consider them a local depletion problem, Kelley said. If this had not been done, the council probably would have closed Penguin Bank to fishing, he said.

Penguin Bank is a long, submerged shelf that extends from West Molokai.

The ocean survey is aimed at "trying to find all bottom-fish habitats in and out of restricted areas to provide the Department of Land and Natural Resources with another management tool," Kelley said.

He has compiled more than 3,000 fishery records around the islands, identifying sites with information and photographic images of what the sea floor looks like and if the sediment is hard or soft.

He has used a variety of methods to gather information, including the Hawaii Undersea Laboratory's submersible, surface vessels, the Geographic Information System, U.S. Geological Survey multibeam mapping system and a University of Mississippi towfish with side-scan sonar.

"We're acquiring equipment to put together a good system, going for habitat images in restricted areas," Kelley said.