Pribilofs survey maps crab habitat

Posted: Friday, July 03, 2009

ANCHORAGE -- Scientists are breaking new ground in the Pribilof Islands, mapping habitat essential to crab using multi-beam sonar to survey the depths and texture of the seafloor.

Using a federal fisheries research vessel specially designed to do seafloor mapping, scientists are conducting the survey primarily to test the system for mapping habitat important for king crab, said Michelle Ridgway, an independent fisheries ecologist in charge of the project.

They are looking for crushed shell, or "shell hash," deposits around the Pribilof Islands. Shell hash is thought to be important for the survival of young crab.

Ridgway spoke via telephone June 17 from aboard the 231-foot hydrographic research vessel R/V Mount Mitchell. The shell hash mapping pilot project is a relatively small-scale survey that tiers off of a larger proposal Ridgway made to NOAA to map and clean up derelict crab pots from crab habitat areas.

The team of 25 people aboard the Mount Mitchell includes 15 people to run the vessel and a survey crew of 10.

The National Oceanic and Atmospheric Administration hasn't yet announced which projects to fund under federal economic stimulus bill. Ridgway said she opted to proceed with a pilot study that will refine methods to use in the larger-scale effort -- if the $2.8 million study is funded.

The focus of the project is to map in the vicinity of the Pribilofs to delineate seafloor features known to be the most important nursery grounds for juvenile crab, she said.

Many stocks of red and blue king crab are in decline, a major concern to biologists, as well as those involved in the multi-million dollar commercial crab fisheries.

Stock assessments for crab have been based traditionally on pot and trawl surveys. Alaska Department of Fish and Game biologists have attempted to record habitat where they find and do not find crab, but they have just a handful of samples, Ridgway said.

Habitat issues were not prioritized until essential fish habitat was incorporated into the Magnuson-Stevens Fishery Conservation and Management Act, reauthorized in 1996.

This pilot project will test the effectiveness of multi-beam sonar to map these habitats, which are also designated essential fish habitat for more than 27 other species.

"At the same time, we are collecting data at the resolution and to the very stringent standards required by NOAA's National Ocean Service for nautical charting," Ridgway said.

Doing the underwater work is challenging and expensive, Ridgway said. "It costs in excess of $3,000 an hour to use the system we are testing right now."

Showing that these two missions can be accomplished in a single expedition could prove huge cost savings to future marine habitat mapping efforts, she said.

Ridgway said the new methodology is being tested -- this effort with a focus on juvenile and breeding crab -- because it appears that assessing stock by counting crab alone is not providing a good understanding of what they need.

"Many of those stocks of red and blue king crab are in decline," she said.

Ridgway said scientists have now successfully completed the field survey portion of the pilot project.

"A highlight of the expedition was transiting survey lines in front of St. George Island and having over a dozen community members radio the ship and express their excitement over the fact that their 'front yard' is being surveyed for crab habitat and improved navigational mapping," Ridgway said in an e-mail sent from the ship.

Ridgway said she will now compare the sonar bottom imagery to seafloor samples, side scan sonar imagery and remotely operated vehicle video taken in the past to help interpret what acoustic backscatter images represent. Backscatter is a physics term used to explain the deflection of nuclear particles or of radiation in a scattering process through an angle greater than 90 degrees.

"The data will be fully processed and map mosaics will be available to the communities in the coming months," she said.

Ridgway also noted in an e-mail that she would like to do some addition "groundtruthing" of the area surveyed, but does not have yet the funding to do that.

"We will use the findings of this survey to conduct the larger scale Pribilof Island king crab habitat investigation and restoration project we have proposed to NOAA under the stimulus package funding opportunity," she said. "That larger project is half mapping, plus crab surveys and removing derelict fishing ear from the crab nursery grounds."

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