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Subject: Re: The Good Depletion
From: Patrick Cordue <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Thu, 23 Nov 2006 09:30:52 +1300
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Dear Mark,

The old deterministic concepts of MSY are not nearly as prevalent in
fisheries management as they used to be. However, to sensibly exploit a fish
population without, on average, depleting the population would be, in
practice, a bit tricky to say the least.

I find your attack on Dr Ray Hilborn quite inappropriate. He knows a great
deal more about fisheries management, stock assessment, stock assessment
methods, and fisheries modelling, than almost all of the readers of this
forum.

Regards
Patrick Cordue

-----Original Message-----
From: Scientific forum on fish and fisheries
[mailto:[log in to unmask]]On Behalf Of Mark Powell
Sent: Thursday, November 23, 2006 5:49 AM
To: [log in to unmask]
Subject: The Good Depletion


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The Good Depletion

Fish depletion is not a problem, it's the goal of fishery management. So
says Ray Hilborn in "Re-interpreting the Fisheries crisis." So don't
worry about disappearing fish, it's part of the grand management plan.

Thus we have The Good Depletion; it allows us to maximize our fish
catches. Or so say the equations. The Good Depletion has us liquidating
the big fish, shrinking population size by 60-80%, and thus increasing
the "productivity" of exploited fish populations. Mr. Hilborn says the
decline shown in the figure is not a problem, because we're still
catching plenty of fish.

The Good Depletion was an advance in the mid 1900's, and it has some
practical value. But it's time to give it a gold watch, a rocking chair,
and a graceful retirement. Unfortunately, Mr. Hilborn and others want to
keep it working in it's dotage.

We need a new paradigm for 21st century fisheries, and here's what I
think it needs. It should maximize the probability of good reproduction
years by valuing big fish (because of high reproductive value) and life
history diversity (e.g. wide range of spawning times and places). It
should maintain fish populations' geographic and age distribution. In
brief, it should emphasize the value of what's in the ocean, not what
comes out. Fishing should "make hay when the sun shines" by fishing hard
during fish population booms, and switching to other species during lean
times when reproduction is weak. To me this would be good
ecosystem-based fishery management.

This whole fight reminds me of the transition in managing public
old-growth forests of the Pacific northwest. When I came of age, we were
clearcutting old-growth forests to produce maximum sustained yield,
following the rationale of The Good Depletion (forestry version). It
failed for many reasons, such as the forests that didn't regrow well in
hot southern Oregon, or the wildlife that went missing in massive tree
farms.

Science has undermined the assumptions of The Good Depletion (fisheries
version). Equilibrium doesn't exist, all spawners are not equal, life
history diversity is important, the ecosystem context matters, etc.,
etc.

Interestingly, Mr. Hilborn says this within the family, just not in
public when The Good Depletion is threatened by outsiders. According to
Mr. Hilborn and colleagues: "For some years the concept of maximum
sustained yield (MSY) guided efforts at fisheries management. There is
now widespread agreement that this concept was unfortunate," and
"Distrust claims of sustainability. Because past resource exploitation
has seldom been sustainable, any new plan that involves claims of
sustainability should be suspect."

Circling the wagons around The Good Depletion won't save it. I suppose
the testiness of its defenders (watch the video link) is evidence of the
coming paradigm shift. Fishery scientists would do well to help
fisheries make the transition, rather than propping up The Good
Depletion until it's really too late and everything falls with a great
crash.

Mark Powell
live links to sources at
http://blogfishx.blogspot.com/2006/11/good-depletion_22.html


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