Trawling and dredging effects; an industry perspective

Nils E. Stolpe

Presented to the NAS/NRC Committee on Ecosystem Effects of Fishing: Phase 1

Effects of Bottom Trawling on Seafloor Habitats


New England Aquarium

February 6, 2001

Thank you very much for allowing me the opportunity to speak here today.

Iíve got to start off by saying that, because of the limited time I had to prepare, these remarks donít represent the opinion of anybody in the fishing industry other than mine. As Iím sure you are aware, however, this panelís understanding of trawling and dredging effects are of vital interest to a large part of the commercial fishing industry in New England, in the Mid-Atlantic, in the rest of the U.S. and throughout the world. Trawling and dredging account for a large part of the fish and shellfish harvested from the worldís oceans. While estimates are that about a third of the total is harvested with trawls or dredges, these two techniques are more important to the fishing industry than their share of the total indicates because of the relatively higher value of the species they harvest. Due to the interest in this Committeeís charge, Iíll be posting my remarks on the NJ Fishing website (, linked to the "Whatís New" page) and distributing them widely throughout the industry. Iíd like to ask that the panel allow them to be supplemented with any other industry input that you feel appropriate. As some of the researchers here can attest, commercial fishermen and women have an awful lot of knowledge to contribute on this subject and you would be seriously remiss if you didnít avail yourselves of it.

The trawling/dredging indictment

I assume that part of the reason that weíre all here today is because the subject of trawling and dredging effects has been elevated to a position of prominence through a flurry of press releases, articles, etc. that was spread about several years ago. The primary theme was that trawling and dredging were the aquatic equivalents of clear cutting (which, while not perceived as such by the public, is an acceptable forestry technique). In some of the most visible assaults on trawling and dredging this clear cutting analogy was then extended to large continental land mass-based comparisons. Thus we had " (trawling and dredging by commercial fishing boats is) an activity that each year disturbs an area of seabed as large as Brazil, the Congo and India combined...." by Drs. Eliot Norse and Les Wattling, "... the annual wordwide trawling of seabeds takes place over an area greater than the U.S. and Mexico combined " by actor Ted Danson, and "The practice and technology of bottom trawling and use of other mobile fishing gear on the seabed has increased to the point that an area of seabed twice the size of the contiguous United States is affected by these practices each year." by the Honorable Joel Helfy, Congressman from Colorado (in draft legislation).

Now the sea floor is a fairly foreign place for most of us, and the methods used to harvest fish and shellfish from it are as equally foreign. This accounts, I guess, for the terrestrial analogies. But Iíve found that the more "real" we can make complicated technical issues, the more likely that they will be understood by the technically unsophisticated. So I used the methodology adopted by Wattling and Norse when they came up with their "... disturbs an area of seabed as large as Brazil, the Congo and India combined," but I applied it to another situation, this one even more close to home.

There were 4.3 million cars, trucks and buses registered in New Jersey in 1997. We can safely - and conservatively - assume that the tire tread width on each of these vehicles was one foot. We can also assume, again safely and conservatively, that each of these vehicles is driven 6,000 miles in a year. Using these conservative figures, itís not too difficult to come to the conclusion that every square inch of New Jerseyís 7,500 square miles of surface area is crushed under the tires of a car, bus or truck at least 600 times a year. Or, in keeping with the large land mass comparisons, every square inch of the total area of the contiguous United States could fall victim to the wheels of New Jerseyís traffic almost twice each year.

But, a sophisticated person might claim, New Jerseyís traffic isnít evenly inflicted on every square inch of New Jerseyís - or the United Statesí - topography. In fact, itís concentrated on a relatively small area. In spite of all the prime-time commercials for sport utility vehicles, virtually all of the driving we do is on hard pavement (in New Jersey, thatís generally the Garden State Parkway, the New Jersey Turnpike or the closest mall parking lot).

Most of us arenít as sophisticated about the sea floor and fishing gear as we are about SUVs and the interstate highway system, so the idea of "clearcutting" the bottom communities covering large land mass equivalents seems much more reasonable than crushing an entire state twice a day, but is it?

In a word, no.

Areas swept by trawls and dredges

The large land mass comparisons, and much of the anti-dredging/trawling rhetoric, are based on the idea that trawling and dredging activities are distributed over the entire sea floor more or less evenly. They were arrived at by multiplying estimated dredge/trawl widths by estimated numbers of fishing vessels by estimated average trawling/dredging speeds, similar to what I did with the vehicles. But this methodology no more fits the case with fishing gear than it does with Dunlops, Michelins and Goodyears. Drivers drive where the pavement is and fishermen fish where the fish are, and in each case thatís not everywhere.

As any fisherman will tell you, neither trawling nor dredging is done in a random manner, at least by the successful fishermen. They have their favorite areas and their favorite tows, and they go back to them season after season, decade after decade, and sometimes generation after generation. As the reproduction of a scalloperís plotter output below (taken from R. Taylorís website shows, the fishing effort tends to be concentrated. Accordingly, some areas of the bottom are trawled or dredged many times every year while most others are ignored completely.

So, rather than two or three large land masses, perhaps Congressman Helfey et al should have have referred to several smaller ones.

But, coupled with this is the fact that large areas of the sea floor are also inaccessible to dredge or trawl gear for other reasons as well. Among these are:

Closed areas

(click for a larger image) 

Rather than several medium sized land masses, maybe a large island or two might be more appropriate?

While the data doesnít exist to allow an easy estimation of the amount of seafloor that is actually trawled or dredged each year, fishing patterns, gear limitations and geographic restrictions are each at work to constrain the fishing effort to much smaller areas than are immediately apparent. As far as the comparisons with multiple large land masses, while they are demonstrably effective at grabbing a bit of attention, they donít seem to have added much to any consideration of the actual effects of trawls and dredges.

Trawling/dredging effects compared to natural processes

Many areas on the sea floor, particularly those at depths that are normally fished with mobile gear, can be considered as high energy environments. In these areas, storms or currents are constantly or intermittently at work on the bottom sediments and the organisms on or in them. Going back to an introductory oceanography text (The Oceans, Their Physics, Biology and Chemistry, Sverdrup, Johnson & Fleming, 1942), I found that a wave with a 10 meter height, a size not all that rare off the coast of New England or the Mid-Atlantic, can induce particle velocities of 40 centimeters per second 100 meters under the sea surface. That doesnít seem all that significant until it is compared to the 49 centimeters per second velocity two meters under a wave with a 1 meter height. A lot of rocks, stones, pebbles, sand and silt can be moved around on the bottom six feet under a three foot wave. Again in practical terms, in Effects of Fishing Gear on the Seafloor off New England (Conservation Law Foundation, 1998), Editor Ellie Dorsey describes the shallower areas of Georges Bank as being made up of ridges and dunes composed of medium-to-coarse sand and migrating " variable rates, up to 60 meters in three months...." This level of "natural" bottom perturbation would seem to be a characteristic of shallower waters, at least those underlain with mud, sand or gravel, on any exposed coast or bank, and would also seem to greatly overshadow the impacts of a towed trawl or dredge. And the predominant bottom sediments in the shallower waters, the waters most heavily fished, off our coasts are mud, sand and gravel.

Effects on the bottom communities

What do we know about the effects of trawling and dredging where these methods are utilized intensively? Most obviously, we know that they donít interfere with the production of the species being sought - at least in those areas that are being trawled or dredged. Fishermen keep coming back to those areas and making those tows because the fish, the clams or the scallops that were caught there once will be there again. In some situations with some fisheries, trawling and dredging actually increase the production of particular species in particular areas through reducing the ocurence of "competing" species, increasing the abundance of prey species and replacing larger individuals with those that are smaller and grow more rapidly. This phenomena is evident in the sea scallop fishery on Georges Bank and the Mid-Atlantic, and has been documented in several of the European flatfish fisheries.

The popular literature is replete with descriptions of how bottom tending fishing gear will reduce the complexity of bottom communities. There are seemingly countless "before and after" photographs supposedly illustrating how trawls or dredges have wreaked their "clear cutting" havoc on the attached plants and animals and the fish and shellfish that shelter in, around and under them. But while the descriptions of the havoc wrought tend to leave no doubt as to the wide ranging impacts of whatever gear was employed, quantitative measures of the degree of that havoc are generally lacking. The sediments might be more homogeneous, the flocculent matter might be less thick, the ripples in the sand less pronounced, or the rocks less covered with organisms, but Iíve found very little discussion about what it all means to either the bottom communities or to the species that are supposedly so dramatically affected. The direct impacts are often described, they are not so often quantified, and their "downstream" effects even less so.

Another perspective

There seems to be a feeling in some circles that anything that interferes with the "natural order" in the oceans is unacceptable. However, the very act of removing fish or shellfish for human consumption (or some other use) is a form of interference, with the level of interference varying with the method of removal and the amount removed. When trawling and/or dredging is referred to as "clear cutting" it is meant as an indictment and thatís the way itís perceived by many people. But isnít "clear cutting " what has allowed us our agriculture industry? At some point werenít natural forests or grasslands turned into cropland, areas with much less complexity and diversity? They were, but only for the tradeoff of increased productivity. Competing organisms were removed, predators were removed, and to the greatest extent possible the habitat was manipulated to maximize the production of whatever crops were being cultivated. Of course when most of our cropland was created we had little or no appreciation of or regard for the value of unique habitats or for the necessity to preserve and protect viable populations of the native species of plants and animals. If we had an undeveloped continent today, and if we were to develop an agriculture industry on it, we would probably do a much better job then we did a century or two ago. But I donít think anyone would question that we do it. Weíve got too many hungry mouths to feed.

How different is this from how we should be looking at the issue of fishing effects? There are without doubt particular areas that should be protected from particular actions. But at this point we donít know what effects which activities in what areas will have at some point down the line. We know that trawling or dredging over some areas will have visible effects on the organisms and the substrate in that area. We know that sometimes, in some locations, those effects will be more pronounced and more persistent than natural processes like wave action, ice scouring and currents. Sometimes they wonít.

But there are a multitude of questions that need to be answered before we can start moving towards a rational policy regarding fishing gear (or, for that matter, fishing) effects. These questions all seem to revolve around which areas should be considered valuable enough to warrant protection from all (possible) anthropogenic impacts, what the "downstream" impacts are of the observable trawling/dredging effects, and what level of disturbance of the sea floor - or, more specifically, particular areas of the sea floor - and concomitant "downstream" effects are acceptable for what level production.

As you might have noticed, there is what I consider an unconscionable amount of exaggeration and misrepresentation - or perhaps misreporting - of what trawls and dredges are doing to our oceans. Important questions have been raised that demand answers supported by solid science, not wild - or even tame - exaggerations (as an example of what we are dealing with, I have attached an article attacking the Georges Bank closed area scallop fishery and the response I wrote that was published in the New Bedford Standard Times). As I said when I began, an awful lot of attention is and will be focused on this panel. While itís probably not necessary for me to say it, please keep this in mind throughout your deliberations.

In the interim, members of the fishing industry will continue to work on techniques to lessen the impact their activities have on the ocean environment, not only because itís the right thing to do, but also because unnecessarily disturbing the bottom means more wear on the gear, decreased fuel efficiency, decreased quality of the catch and wasted time spent culling.

Thank you

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