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Seaweek 2010: Oceans of Life - ours to explore; ours to restore

So what's wrong with fish?
by David Ogilvie

Technology opens oceans to trawling

In the past, trawlers fished in areas that were easy to access, avoiding rough bottoms, more remote areas and the deeper waters. With rapid advances in fishing technologies, including global positioning systems, depth sounders and fish-finding equipment, there are very few places left around the world with commercially valuable fishery resources that have not been trawled or dredged. Targeted species plus many other species caught and killed incidentally, are caught in depths and in areas previously avoided.

New, more powerful fishing equipment, such as rockhopper and streetsweeper trawls, now allow fishing even on rough bottoms, such as rocky reefs, that in the past were free of trawling. Large rollers were added along the bottom edge of the net that allowed hauling over rocky or coral-crusted areas of seabed without snagging. Consequently, bottoms with many "hangs," such as coral reefs, are pulverized or overturned during each tow of the trawl. Until recently, rough bottoms, where fish tend to congregate, had served as de facto refuges from this type of fishing. Fishers were reluctant to risk expensive gear damage, limiting the extent of trawl impacts. Now, few limits remain; trawling can occur almost anywhere.

Trawling destroys vital habitat

Trawling destroys fundamental components of habitat necessary for marine fisheries and other wildlife, eliminating the basic structures many species need to survive.

Recovery of this habitat can take decades. In the short term, trawling re-suspends plumes of sediment, clouding the water and potentially affecting critical natural processes such as photosynthesis and feeding.

The structural complexity of rocky reefs, boulders, cobbles and gravels is necessary for the survival of many marine species, including juvenile fish. Studies show that trawling reduces structural complexity and eliminates nursery habitats. A diverse habitat structure is vital to a wide variety of marine life because it provides surfaces for feeding and hiding places from predators. Natural features provide cover for commercially important species such as cod and lobster, but also for their prey which includes crabs, small crustaceans, marine worms, and sea urchins.

"It's not a sustainable practice to turn the coral to rubble, take the fish and leave," Mark Powell, director of fish conservation for the Ocean Conservancy, states.

A survey of fishermen, scientists and other experts published in May 2003 by the Marine Conservation Biology Institute found consistent agreement that bottom trawling was the most harmful commercial fishing method, with damage to the seabed judged worse than the damage to by-catch.

The report, called "Shifting Gears", online at, noted that 98 percent of marine species lived in, on or just above the sea floor, and are therefore vulnerable to trawling.

According to Dr Sylvia Earle, renowned marine scientist and explorer, deep-sea bottom trawling should have no place in 21st-century ocean use and management. "Bottom trawling is simply not sustainable," she said "The trawl nets are stripping the seabed of life, trashing ancient corals and destroying entire ecosystems. There is much that we are still to learn about life in the oceans. Sadly, much of it will be gone before we get the chance if we don't act now."

Recent improvements in design

Until recently, every innovation was aimed at catching more fish, with little regard for the ecological consequences. That began to change after biologists started tallying the loss of seabed ecosystems crushed by repeated towing and the vast unintended toll of sea turtles and unwanted fish swept into the gaping bags.

Fishermen, too, began to recognise that in capturing fish of all sizes they were undermining the health of the resource.

Since then, under tightening laws, fleets in North American and European waters have begun to shift to designs and practices that curb the by-catch and ecological effects.

For a number of years now the United States has required that trawled shrimp, whether imported or caught in American waters, must only come from nets equipped with special grates called turtle exclusion devices that let shrimp in but divert sea turtles. American shrimp fleets are also increasingly installing grates and escape holes that cut the unwanted fish harvest in shrimp nets.

But it is clear from history and the continuing spread of technology that trawling will be around as long as humans continue to harvest fish in the wild. Improvements in design may reduce ecological damage but will never prevent it entirely.

For that reason, Dr. Callum Roberts, a fisheries scientist at the University of York in Britain, recently urged in a report to the European Union that the only way to continue to trawl and still have something worthwhile catching was to put substantial parts of the oceans off limits.

"Yes, we should have the underwater equivalent of intensive agriculture - the muddy seabed plowed by trawls," Dr. Roberts wrote. "But give us wilderness, too, the nature reserves and national parks." Without them, he concluded, "the seas will become a sorry shadow of their former abundance and the giants that we once hauled from them creatures of imagination alone."

Fisheries in crisis
By-catch - an incredible waste
Case study: Tuna fishing and dolphins
The impacts of fishing on other animals
Commercial fishing
  Purse seine nets
  Drift nets
  Longline fishing
  Bottom trawling
Technology opens oceans to trawling
Trawling destroys vital habitat
Recent improvements in design
Fish Farming
Recreational fishing
   VNPA research review
   CSIRO Port Phillip Bay Study 1999
Fish and pain
Fish and intelligence
Fish and human health
   Omega-3 Essential Fatty Acids
  A lot of fat and cholesterol, and no fibre
  Bio-accumulation of toxic chemicals
  Recent research on mercury in the USA
   PCBs (Polychlorinated Biphenyls)
   Food poisoning risk
Recent fishing issues (news articles)
Links of Interest
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Marine and Atmospheric Research


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