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NOAA: Trawl Gear Modification - Preserving Fish Habitat in the Bering Sea

Scientist and fisherman working cooperatively
aboard the charter fishing vessel the
F/V Pacific Explorer on the set up and
attaching of a recapture net to
the main net. Photo Credit: Carwyn Hammond.
NOAA.gov July 26, 2012

Flatfish trawl gear isn’t what it used to be—and that’s a good thing. In the Bering Sea, traditional trawl gear had long sweeps, or cables running from the doors to the wings of the net, which moved across the bottom of the seafloor to herd flatfish into the center of the net. NOAA Fisheries scientists and partners like Alaska Seafood Cooperative and Bering Sea flatfish fishing industry members collaborated to modify the gear so it reduces the damage to important bottom habitat.

The team tweaked the gear by raising the sweeps off the seafloor at various spacings—2 to 4 inches—and studied the impact this had on catch rates and seafloor habitat. The results were positive; the raised sweeps successfully maintained flatfish catch rates, minimized negative effects on bottom habitats, and reduced crab mortality rates—a plus for Alaska’s crab industry. The new gear reduced seafloor contact by a whopping ninety percent, further protecting floor animals that are important habitat for fish and crabs.

Taken on the deck of a twin trawler F/V Cape Horn, the
sweeps on the left side of the picture are the
modified ones with the elevating disks. The sweeps
on the right side are the old conventional sweeps
without the elevating disks. Photo Credit: Carwyn Hammond.
Since 2011, fishermen for all Bering Sea flatfish vessels must use the modified Bering Sea flatfish trawl gear. The current trawl gear leaves less of an environmental imprint while improving catch of marketable fish. The modified trawl gear has reduced effects of the Bering Sea flatfish fishery on crab and habitat, and others are taking notice. These strong results led the North Pacific Fishery Management Council to recommend requiring modified sweeps with the same disc height and spacing parameters for the Central Gulf of Alaska flatfish fishery. Those modified trawl sweep requirements should be in place in 2013. Burgeoning interest on these modifications has come from both West Coast and Dutch flatfish fisheries as well.

Gearing Up for Gear Change

“Raising the sweeps was a simple idea, but those are the best ideas to test. It takes time to establish scientific evidence,” says John Gauvin, science director for the Alaska Seafood Cooperative. “Along with NOAA Fisheries scientists, we spent considerable time looking at the effects of this change on how many fish were caught and how it interacted with the seafloor.”

Scientific comparisons of catch rates were made using twin trawls—towing two separate trawls side-by-side on adjacent tracks—allowing scientists to link catch and habitat differences to differences in gear. Sonar and video technology helped researchers examine how the sandy, muddy seafloor responded after the passage of different height sweeps. After pinpointing which sweep spacing yielded the best results, the team spaced light weight discs 90 feet apart to raise sweep wires, greatly reducing seafloor contact.

What’s Good for Habitat is Good for Sea Critters

Changing gear is a powerful tool in achieving long term benefits. “By implementing gear change, you are improving things anywhere the fishery occurs,” says NOAA Fisheries scientist Craig Rose, “instead of potentially just displacing effects on habitat with closures.”

Not only did the new gear protect the seafloor, it had positive influences on both sea whips and crab mortality. The conventional gear showed 20 to 25 percent of sea whips—slender colonial octocorals that anchor vertically in soft sandy bottoms—laying down on the seafloor after the trawl passed through. With the new gear, one third to one half fewer sea whips were flattened after the trawl passed. The new gear also reduced crab mortalities substantially for all Bering Sea species.

While the trawl gear change was relatively simple given the usual complexity of fishing gear, it is one modification that will have lasting benefits for both fishermen and habitat. Implementing the new gear was of some cost to fishermen, but it will continue to pay off. That’s just the kind of innovation we can get geared up about.

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