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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 11:25:26 -0300

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After some 15 years of reasonably-intensive studies of the seabed  
impacts of fishing gears (some 800 of the 1200 items having been  
published in that time), it is beginning to look like the whole  
effort to date has been a bit tangential. That is, we have mostly  
been trying to test the hypothesis that trawling changes the benthos  
which, in retrospect was probably so obvious that it didn't need  
testing. What we need is quantitative information on rates of removal  
or death of different components of the benthos, on different  
habitats when impacted by different intensities of fishing with  
different kinds of gear but there have only been a handful of studies  
that have aimed at that sort of quantification, so we really don't  
have much in the way of hard numbers and are left gleaning what we  
can from studies that were never really designed to provide such  

By "low rate of loss", I meant anything less than, say, 10% removal  
per pass of the gear. For a lot of gears and  lot of species, the  
loss rate will be much less than that (deep-living clams and much of  
the meiofauna may be effectively invulnerable to otter trawling, for  
example). At the opposite end of the spectrum, removal rates above  
90% (per pass of a trawl) have been reported for coral trees and  
above 80% for large sponges.

Not sure what you mean by "How is that quantified". How it is  
estimated is usually to tow a trawl one or more times and compare  
densities of organisms in the track of the gear with those outside.  
The units in which I am quantifying the rate, in these messages, is  
percentage removal of the members of some benthic species that were  
in the path of the gear. "Removal" may be a euphemism but I am trying  
for a word that covers a bunch of pathways by which an animal  
previously there is so no longer: capture in the net, immediate  
destruction on the seabed, wounding followed by death (or even  
sublethal reductions in future growth), perhaps aided by disease or  
predation, displacement of infauna exposing them to predators, and more.

What was it compared with? By "low" in my previous posting, I meant  
low relative to the notion that a trawl "scoops up everything in its  
path". Hence, 10% per pass of the gear is "low" in that sense. More  
generally, loss rates of a lot of the benthos are low relative to  
natural variability, which is a big part of why it is so difficult  
and so expensive to get statistically-valid evidence of a significant  
effect of trawling over scales larger than the track of a single tow.  
[I do realize that there are a lot of published claims of  
significance. However, once you set aside the pseudo-replicated  
experiments and the ones that lacked randomization, then make the  
required Bonferroni adjustments for multiple hypothesis tests, the  
number of statistically-valid significant results becomes sadly few.  
That doesn't prove that the impacts of trawling are equally few, just  
that sufficient sampling to detect them in the face of natural  
variability is very expensive.]

How the effects of multiple passes of the gear relate to those of a  
single pass is one of (is maybe _the_) biggest question in gear- 
impacts work. I don't think that it has yet been answered for even  
one combination of gear type and impacted habitat. My best guess, for  
typical components of typical habitats (i.e. _not_ coral trees) is  
that there is a negative exponential relationship but that is likely  
only because I am familiar with the way mortality is modeled in  
fisheries dynamics. Even if my guess was to prove a reasonable  
approximation, it begs all sorts of questions about spatial and  
temporal scales of impacts: a single experimental tow in a closed  
area does not have the same effect in the track of the net as would  
opening that area to a commercial fishery that, collectively, swept  
an area equal to the seabed area. The track of a single tow can be  
swiftly re-colonized from adjacent bottom in a way that won't happen  
with the broader impact of a commercial fishery.

It is sobering that so much research has been done on a topic of so  
much public interest and yet we know so little. At the same time, we  
do know enough to discredit a lot of the silly propaganda that is  
fueling the public interest.

Trevor Kenchington

You wrote:

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> Trevor,
> I don't want to get caught in the argument, but I am curious about  
> your
> comment that you have literature that shows that, "while many studies
> have shown that most of the benthos (most of the benthos being  
> composed
> of small animals) in most habitats suffers only a low rate of loss per
> pass of a trawl."  What exactly is a "low rate of loss"?  How is that
> quantified and what was it compared with?  If that is on a per trawl
> pass basis, what happens when multiple passes occur in the same area?
> Would multiple trawl passes have a linear, logistic, or synergistic
> effect on benthos/habitat loss?
> Cheers!
> Justin C Johnston
> Biologist

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