Trawling and the Damage Done

31 August 2008

Interview with 

Les Watling, University of Hawaii


Les - Trawling is a method of fishing that started in Britain in the 14th century where someone found out that if you took a net and held it open in some way and hauled it behind a boat you could get a lot of fish.  You also get a lot of stuff.  I've actually been looking at effects of fishing for about fifteen years and I've been in submarines and I've had cameras on remote vehicles.  What I generally see is that where an area's been trawled there's not much living on the surface of the ocean floor.  We know that trawlers also dig in to the sediment.  They disrupt all kinds of things.  Generally if you go to an area that's trawled it's really noticeable.

The fish catch from trawling netsMeera - When the boats are going by what effects is the trawling having on the sea bed?

Les - Usually what happens is anything that's standing up from the bottom - anything like a sponge or coral that's growing up from the bottom - that's usually bent over, broken or removed.  If it's a muddy bottom then the gear digs into the bottom.  The important thing to realise is that most of the animals that live in the muddy bottom live in the upper 3 or 4 cm.  You don't have to dig in very far before you've disrupted the burrows and tubes of all these small things.

Meera - They've been disrupted but what impact does that have?

Les - It depends.  Some animals recover from this disruption okay, which means they can make a new burrow or two, but a lot of animals invest a huge amount of energy into making the burrow.  In fact in some cases some of the marine worms, for example, they've lost the ability to remake a tube.  Then that's it. They're laying on the surface and they could be eaten by a fish or any other thing that comes along.  They've lost their protection, as it were.  Other animals raise their young in their burrows and tubes.  If that's destroyed then the babies may not be able to burrow their way out of this mud that's been disturbed.  We tend to see species in these trawled areas that have really high reproduction.  They're weeds in the best sense of the word.  They have high reproduction, they can re-colonise.  They're capable of getting their house blown down, if you want to think of it that way.  They'll rebuild it real fast: all that sort of stuff.  You tend to lose the things that have a longer, more stable lifestyle.  Especially if it's an area that's trawled repeatedly.  If a person drags a trawler over an area once then a lot of things will survive that.  Maybe half of the things that live there will survive that.  There will be a certain amount of re-colonisation that can occur in two years.  A lot of times trawling occurs over and over again.  Fishermen have their particular favourite spots.  When you go to those areas you find that the whole bottom community has really changed to these weedy type species.  This from a fish perspective might not be so bad.  Fish can eat those weedy species to so you could get flat fishes, for example.  It's been shown in the North Sea that if you re-trawl areas a lot you can get flat fishes but you might not get other kinds of fish because their food is missing.  You create a completely different bottom community and a new ecosystem, as it were.  One parallel that I like to use is what happens when you go in and, as happened in North America for example, colonists came.  The cut down all the forest and they turned it into pasture land.  So we lost all the birds that nested in the trees.  It's a very similar kind of thing.  The community's still productive for growing sheep or cattle or whatever but you have lost the outliers there.

Meera - How much of the world's marine ecosystem is being affected by trawling?

Les - That's a hard estimate to make.  We looked at the number of fishing vessels and where they were around the world about ten years ago.  We figured that about half of the continental shelves of the world get trawled each year.  That number is obviously fluctuating because fisheries have collapsed in a lot of these areas.  It may be the collapse of some of these continental shelf fisheries that will allow some of this biodiversity to recover.

Meera - What do you think the solution is?  What can we now aim to do knowing this?

A diagram of trawlingLes - This is going to be a little controversial but you know people have been proposing for a number of years to set aside and really protect the areas that you don't go into with any kind of gear.  These areas have generally been proposed to be about 20-30% of the continental shelf area.  We should be looking at it the other way around.  We should protect most of the sea bottom and only allow trawls into a very small percentage.

Meera - I guess if that does happen do you think that the effects that trawling has had so far is reversible?

Les - It's reversible.  The time scales are going to be long the deeper you go in the sea.  I have a project that has just finished looking at an area that had been closed to trawling for six years.  It doesn't look at all anything like the areas that have never been trawled.  That's six years on.  If you go into really deep water we know on the sea mounts, for example, that corals recruit at extremely low rates.  We had a study where we were looking for coral recruits on the sea mounts of the North Atlantic and we found one on a block that had been put out for a year and a half.

Meera - So I guess fishing can be reduced but the outlook is quite promising.  It's not something that's just going to be left disturbed.

Les - Promising in the long run.  Yeah.  For the continental shelves, for areas that have rocky-stony-marl bottoms those we're looking at 25-50 years probably for recovery time: before the big things like sponges start to re-grow.  The deeper you go though the longer I'm certain that sea mounts that have had all the corals removed from the top will not have corals again for a century or two centuries.  This is a really long time.


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