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Subject: Re: depensatory exploitation
From: Patrick Cordue <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Mon, 31 Jul 2006 09:13:38 +1200
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Hi Rom,

Yes, I see the difference. The problem is that you keep using the term 
"function" (and this has consequences in interpretation of results). In 
specific cases, exploitation rate will often be an  "empirical function" of 
abundance. However, suppose that the abundance tracks down and then recovers 
(and possibly the cycle repeats a number of times). Then for a given 
abundance there will be, in real fisheries, more than one exploitation rate - 
and the "empirical function" is not, strictly speaking, a function. 

This is just a point of terminology. I can see that it would be interesting 
to look at the shape of the relationships between exploitation rate and 
abundance for a number of fisheries. However, without an understanding of the 
underlying mechanisms (the real functional response - i.e., all the 
parameters, not just abundance) I am not sure what usefully conclusions could 
be reached.

But, thank you for an interesting discussion.

Regards
Patrick


On Monday 31 July 2006 03:28, you wrote:
> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
>
> If you reply to this message, it will go to all FISH-SCI members.
>
> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
>
> Patrick,
>
> The functional response of predators can be viewed in two ways,
> mechanistically and empirically. The mechanistic explanation deals
> with the factors that determine the "functional" response of
> predators to its prey and which therefore determine the form of the
> function. I believe that this is what you refer to when discussing
> the "functional" response. The other way to look at it is purely
> empirical whereby one simply attempts to determine the form of the
> function, whether linear or non-linear. From the empirical functional
> response of a predator guild, which includes other elements such as
> aggregation, one can determine the total proportional mortality of a
> particular prey species. The proportional mortality is analogous to
> exploitation rate in fisheries, and its form has major consequences
> for the likelihood that a fished population will persist, be driven
> to collapse, or recover.
>
> It is the latter, empirical, approach that I have taken with regard
> to depensatory exploitation, and which is what Dick has explained.
> So, in the empirical situation, the exploitation rate is merely
> calculated by the Catch (in numbers) during the fishing year divided
> by the initial population size (N) of fishable individuals at the
> start of the fishing year, resulting in the standard calculation of
> exploitation rate as u = C/N. Once this has been done for several
> years, the form of the empirical function can be determined.
>
> In the Chesapeake Bay blue crab case, Dick is correct that effort and
> catch controls have not regulated fishing effort effectively. The
> form of the exploitation rate function for the blue crab in
> Chesapeake Bay is a negative exponential curve, which indicates that
> exploitation rate is substantially higher (twofold) when the
> population is at low abundance than when it is at high abundance.
> This is what may be preventing the population from recovering in the
> Bay. If you are interested, the following paper describes the Bay
> blue crab population status, and our recent stock assessment
> indicates that the population has not recovered:
>
> Lipcius, R.N. and W.T. Stockhausen.  2002.  Concurrent decline of the
> spawning stock, recruitment, larval abundance, and size of the blue
> crab Callinectes sapidus in Chesapeake Bay.  Marine Ecology Progress
> Series 226: 45-61.
>
> If you wish additional information on the functional response, the
> following paper has reasonable explanations of the predator and guild
> functional responses:
>
> Seitz, R.D., R.N. Lipcius, A.H. Hines and D.B.
> Eggleston.  2001.  Density-dependent predation, habitat variation,
> and the persistence of marine bivalve prey.  Ecology 82: 2435-2451.
>
> Rom
>
> At 02:42 AM 7/30/2006, Patrick Cordue wrote:
> > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> >
> >If you reply to this message, it will go to all FISH-SCI members.
> >
> > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> >
> >Hi Dick,
> >
> >My comments apply to any fishery. Because there is a 1-many RELATIONSHIP
> >between abundance and fishing mortality (or exploitation rate) then, by
> >definition, fishing mortality is not a FUNCTION of abundance ALONE.
> > Therefore to define "depensatory exploitation" in terms of the shape of
> > the function cannot be generally applicable - because, in general, no
> > such FUNCTION exists.
> >
> >I agree with you entirely, that understanding how fishermen respond to
> >changes in abundance is critical (for a number of applications).
> >
> >Regards
> >Patrick
> >
> >On Sunday 30 July 2006 15:32, you wrote:
> > > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> > >
> > > If you reply to this message, it will go to all FISH-SCI members.
> > >
> > > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> > >
> > > Patrick,
> > >
> > > You assumed a managed fishery.  There are no effective controls on
> > > either fishery inputs or outputs in the U.S. fishery for American
> > > lobster, for example.  The same may be true for the Chesapeake blue
> > > crab fishery that prompted Rom's inquiry.  In those cases, knowledge of
> > > the functional response of the fisherman to changes in abundance of the
> > > target species is critical.  In southern New England we are now moving
> > > into a regulatory framework that will have the theoretical ability to
> > > control lobster trap numbers.  The functional response of the fishermen
> > > will be a big factor in the effectiveness of the effort control
> > > program.
> > >
> > > Dick Allen
> > > Fisheries Dept. Graduate School
> > > Univ. of Rhode Island
> > > (401) 874-7153
> > > [log in to unmask]
> > >
> > > -----Original Message-----
> > > From: Scientific forum on fish and fisheries
> > > [mailto:[log in to unmask]]On Behalf Of Patrick Cordue
> > > Sent: Saturday, July 29, 2006 6:19 PM
> > > To: [log in to unmask]
> > > Subject: Re: depensatory exploitation
> > >
> > > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> > >
> > > If you reply to this message, it will go to all FISH-SCI members.
> > >
> > > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> > >
> > > Hi Rom,
> > >
> > > It was good to get the long definition, as the short definition was a
> > > bit unclear. However, there appears to be a conceptual problem. You
> > > talk of functional responses and fishing mortality as a function of
> > > abundance. In some deterministic systems this will be true, but, in
> > > general, fishing mortality is not a function of abundance (there is no
> > > 1-1, or many-1 relationship).  Consider a managed fishery. In year y1
> > > biomass is B and the catch is C1, in year y2 biomass is also B, but the
> > > TAC (for example) is set at a different level (for whatever reason) and
> > > the catch is C2. There is a 1-many relationship (not a function, not a
> > > functional repsonse).
> > >
> > > Regards
> > > Patrick Cordue
> > >
> > > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> > >
> > >        To leave the Fish-Sci list, Send blank message to:
> > >         mailto:[log in to unmask]
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> > >
> > > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> >
> >--
> >----------
> >Patrick Cordue
> >Director
> >Innovative Solutions Ltd
> >www.isl-solutions.co.nz
> >
> > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
> >
> >        To leave the Fish-Sci list, Send blank message to:
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> >
> > ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><> ><>
>
> Romuald N. Lipcius, Ph.D.
> 2006 Aldo Leopold Leadership Fellow
> &
> Professor of Marine Science
> Virginia Institute of Marine Science, The College of William and Mary
> 1208 Greate Road, Gloucester Point, VA 23062 USA
> 804-684-7330, [log in to unmask]
> http://www.vims.edu/fish/faculty/lipcius_rn.html
>
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-- 
----------
Patrick Cordue
Director
Innovative Solutions Ltd
www.isl-solutions.co.nz

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