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Subject: Re: Deep Threats for High Seas
From: "Trevor J. Kenchington" <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Wed, 9 Aug 2006 13:11:23 -0300

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> I'm not a professional, and I'm certainly willing to be educated.  
> But isn't
> it generally accepted that a single pass removes an average of  
> twenty percent
> of benthic life in the track?

What is "generally accepted" depends on the community whose general  
acceptance is being evaluated. Over the past five years or so, a  
broad swath of public opinion has been shaped by an extremely  
effective PR campaign operated by opponents of trawling. I won't  
speculate as to the motivations (others have done enough of that) and  
I can only tip my hat to the skill and determination with which the  
campaign has been carried through, whatever I think of its ends. My  
concern is only for the scientific support for the claims being made  
in that campaign and, in my judgement, that support is weak.

If you mean "generally accepted" within the scientific community, you  
would still not be able to draw much of a judgement as most  
scientists have no specialized knowledge of the topic. With some 1200  
papers and reports published, there are probably a few dozen or maybe  
a few hundred people with that specialized knowledge. Such a small  
group is subject to changing opinions, short-lived fads and so forth.  
(There was a complete reversal of received wisdom on this topic  
around 1990. I think we are seeing another shift now, though such  
things can really only be perceived in retrospect.)

In short, "general acceptance" isn't much of a guide. You need to  
look at the hard evidence. In the case of the seabed-impacts  
literature, regretfully, that means a paper-by-paper review, not only  
of published conclusions but of whether they could be supported by  
the data available to the authors. It isn't straightforward.

As to your 20% figure: I do not think that any one number can be  
taken as a useful summary. Removal by a single pass ranges from zero  
(deep-living clam passed by an otter trawl) to 95% (coral tree in  
path of otter trawl). The number depends on the animal, the gear type  
and the bottom on which the animal lives -- if not other factors  

Personally, I suspect that your 20% figure is way too high, if you  
mean to average across numbers of animals or numbers of species. The  
small infauna in abundant, speciose and not much impacted. As an  
average weighted by biomass, 20% might be closer though I suspect  
(without much, if any, evidence) that it is still too high. But that  
is me. A recent paper by Hiddink et al. put the removal rate on sand  
and gravel bottoms at around 50% per pass. I don't think that that is  
supportable for the size classes they were dealing with (I would  
question whether 5% was supportable) but, since they didn't provide  
details of how they arrived at the number, it is a bit hard to refute  

> And that in areas repeatedly being trawled, up to
> ninety percent is removed?

90% of what? The only science-based reports that I can immediately  
recall of such high losses (and higher: essentially 100%) concern  
coral trees, large erect sponges and, in one special case, oyster  
reefs. (One case in the sense of only one is supported by scientific  
observations. I think I can point to similar losses of reefs in the  
19th century but that depends on inference and anecdote.)

> This would seem to me to be a terrible price to
> pay in bycatch. After all, such bycatch does provide food for  
> target aniimals.

Not sure that it is helpful to stretch the meaning of "bycatch" to  
cover impacts on the benthos.

Whether the net effect of the seabed impacts of trawling is to  
increase or reduce food supply for exploited species is much argued  
and likely depends on where and what exploited species is considered.  
Some may gain by having large, old benthic animals broken up, so as  
to free production to pass into small, short-lived species which are  
available to be eaten by small-mouthed fish.

The bigger issue is likely to be one of altering the structure of the  
seabed, to the disadvantage of species that like patch reefs and so  
forth. Still, even that only means that the production flows  
elsewhere, which may be to the advantage of some fisheries.

> I may have put the argument too offensively for your taste. But  
> what I was
> attempting to convey was that it might be useful to first study these
> environments before we chew them up.

In the case of the deep-sea environments, I will guess that a lot of  
people agree. I certainly do. However, your argument as stated was  
not confined to such environments and was equally applicable to areas  
long trawled, where if there was ever a stage of chewing up, it is  
past and gone.

> Why must these fisheries run the risk of collapse first, before  
> anyone takes
> the issue seriously enough to put a hold on trawling until we know  
> a bit more
> about what's going on there? Once we have determined what sort of  
> methods are
> sustainable, then we can have the rest of time to farm these unique  
> marine
> environments.

If by "there" and "unique marine environments" you mean places like  
the seamounts, I think that there is no reason not to take the issue  
seriously today. However, broadcasting propaganda against mobile-gear  
fishing, without focusing the case on those special environments,  
just builds the opposition and confuses the issue.

> Sorry about the "junk" and "garbage". I must have struck a nerve.

Of course you did. The anti-trawling PR campaign has, as I noted  
above, been very effective and it is far beyond the ability of mere  
scientists to combat. However, we do have a duty to counter such  
propaganda when it is introduced into scientific fora.

Trevor Kenchington

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