The first study ever done of the effects of bottom trawling on mud seafloors off the West Coast of North America – a huge area that comprises thousands of square miles – suggests that trawling not only reduces fish numbers, but also severely alters communities of organisms inhabiting these deep-sea habitats.

The research compared trawled to untrawled areas 600 to 1,200 feet deep off the southern Oregon coast, and found nearly 20 percent fewer fish overall in the trawled areas, and 30 percent fewer fish species. Certain seafloor dwellers in this type of marine terrain, including sea pens and crabs, were six times more abundant in areas that had not been trawled.

In trawled areas, numerous scavenging species largely replaced the marine life common on undisturbed seafloors.

The study, made by direct observation from small two-person submarine, was just published by scientists from Oregon State University and Washington State University in the Journal of Experimental Marine Biology and Ecology.

This report is the first of its type, scientists say, to examine the effects of a common fishing practice on a rather ordinary but vast ocean floor ecosystem off Washington, Oregon, and California – the mud flats that dominate more than 75 percent of the outer continental shelf.

Bottom trawling, in which large nets are dragged by ships along the seafloor and scoop up pretty much everything in their path, has been done in much of this area. This is one common source, among others, of the sole, lingcod, rockfish and other fish that are common seafood staples in grocery stores and restaurants. In recent years, 94 percent of the continental shelf and slope off Oregon and Washington that was trawled was swept less than once per year, and 6 percent more than once annually, according to a report by the National Academy of Sciences.

Regulatory approaches, including gear modifications and closed areas, have actually steered trawl fisheries toward the mud seafloors, keeping them out of rock or coral areas, because trawls cause less environmental damage on mud. But the long-term implications of fishing with this technology over such a broad area are a concern, according to study authors Mark Hixon, a professor of zoology at OSU, and Brian Tissot, a professor of environmental science at WSU.

“This ecosystem shows striking differences between trawled and untrawled areas,” said Tissot, an expert in seafloor organisms. “Areas that had obviously not been trawled were covered by forests of sea pens and other marine life, and the trawled areas looked like a desert, crisscrossed with trawl tracks.”

Untrawled mud seafloors were dominated by slow-growing but long-lived sea pens in forest-like stands inhabited by crabs, other invertebrates, and 27 species of fish, including soles, poachers, ratfish, and sablefish. In trawled areas, a very different range of scavenger species dominated the disrupted environment – including hermit crabs, sea stars, eelpout, hagfish, and others. These scavenger species may have been attracted to burrowing prey exposed by trawling, Tissot suggested.

“Past overfishing has already led to large and costly cutbacks in the trawl fishing industry,” said Hixon, an expert in marine fishes. “Some have compared bottom trawling to hunting elk by bulldozing forests. It’s very tough on seafloor habitats and larger organisms.

“We really don’t know much about how these systems work, how much trawling they can take, and how resilient they are to this type of damage,” he said. “Mud seafloor ecosystems of the continental shelf may not seem that important, and in the past they have been completely off the radar screen.

“But a question that must be asked is whether we want to sacrifice these ecological communities, not even knowing what the long-term effects of bottom trawling might be, or whether some mud areas on the continental shelf deserve permanent protection.”

The very deep ocean beyond the continental shelf is already protected from bottom trawling, as are certain rocky areas on the shelf. There are also temporary trawl closures on the shelf for rebuilding populations of over-fished rockfishes.

Hook and line, and trap fisheries are far less destructive to seafloor habitat, Hixon noted, but all fishing gears have their pluses and minuses. Bottom trawling has been an efficient means of gathering a huge amount of sea life off the ocean floor, but at the cost of seafloor alteration and wasted bycatch, or discarded, non-seafood species.

Among the species most directly reduced by trawling on deep mud seafloors were sea pens, the research found. Also known as sea whips, these are soft-bodied, erect organisms that anchor in the seafloor and project upwards as much as 3 feet, forming forest-like stands. Sea pens, which can live up to 50 years, were nearly absent on trawled bottoms.

Off Oregon and most of the rest of the West Coast, the continental shelf extends up to 30 miles offshore – a comparatively shallow area before the ocean drops into very deep waters – and is subject to state or federal regulations. Most of the continental shelf is dominated by mud seafloor, with smaller parts covered by rock or sand. A very small number of studies previously had looked at the effects of trawling in other types of marine terrain off Alaska and California, but none studied deep mud floors.

Numerous studies of bottom trawling done elsewhere in the world have concluded that this fishing practice often reduces habitat complexity, alters seafloor communities, reduces productivity, and has particular impact on fragile species that inhabit the still waters of the deep sea. Although the findings of the Oregon research are based on comparisons of limited scope that are less definitive than experimental studies, the observed patterns are entirely consistent with broader studies worldwide, Hixon said.

This study was funded by the U.S. Minerals Management Service, as part of a regional survey to see what type of sea life was living on rocky seafloors of the outer continental shelf of Oregon. Some of the submarine transects, by coincidence, ran onto mud seafloor areas instead of rocky areas.