The Trawl Tracks from Space

Kind of sounds like the title of a really bad sci-fi movie, doesn't it? Unfortunately, it's not. A couple of years back satellite photos supposedly revealed the “devastation” due to sediment trails kicked up by shrimp trawlers. A staggered line of shrimp boats were shown, each preceding a plume of cloudier water, and Daniel Pauly, University of British Columbia researcher and one of the chief crepe hangers of the Pew funded “let’s blame it all on commercial fishing” campaign, was quoted widely as saying "these images of trawler mudtrails confirm that this mode of fishing is terrible. Think of the story about China’s Great Wall being the only human artefacts (sic) visible from space. Now we can add the mudtrails of trawlers.” In a rejoinder I wrote:

The satellite pictures are at a level of resolution that is available to billions of us (for free) on the web’s Google Earth, as it was when Dr. Pauly was waxing catastrophic on trawling. Now I’ve spent a reasonable amount of time messing with Google Earth, and while I’m not an expert in satellite photo interpretation, using it I can clearly distinguish the three skylights on my house, each of which is perhaps two feet wide and four feet long. The Great Wall of China is, I think, somewhat larger. So it appears that we can add to Dr. Pauly’s Great Wall of China and the mudtrails of trawlers perhaps a billion or so other human artifacts (including my skylights) visible from space. But hey, why should anyone, even if he is a respected scientist, be held accountable for everything that he is quoted as saying to make his “doom and gloom” predictions more worrisome?

But back to the shrimpers and their “catastrophic” sediment plumes. They were fishing on the Yangtze River delta. The Yangtze is one of the world’s largest rivers, until the recent construction of a series of dams it transported an average of 472 million tons of sediment to the sea each year. Needless to say, most of this sediment ended up being deposited on the delta. The finer sediment layer (these are the sediments that would be resuspended by trawling) ranges from about 130 feet thick at 60 to 100 foot water depths to 3 to 6 feet in thickness at 300 feet. It’s estimated that this “mud wedge” contains 500 billion tons of sediment.

While I’m certainly not the fisheries scientist that Dr. Pauly is, I’d be willing to bet that the critters in and around the Yangtze delta are pretty much adapted to suspended – or resuspended – sediment in the water. Any bottom dweller that can’t deal with suspended sediments shouldn’t be hanging out on the Yangtze (or Mississippi, Nile, Amazon, Mekong, or you name it) delta, and the way that nature works, they definitely aren’t. In spite of this, Dr. Pauly convinced a bevy of media people that trawling was dooming us to yet another ocean Armageddon. Quintessential sound bite science.

Catastrophic mud plumes? Not hardly. This was - and still is - mud that's being kicked off the bottom in areas that are regularly subjected to great amounts of naturally suspended sediment. That sediment is what builds river deltas, and it's what the organisms that live there have evolved to successfully deal with.