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Subject: Re: depensatory exploitation
From: Rom Lipcius <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Sat, 29 Jul 2006 16:16:40 -0400
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Thanks for the comment and suggestion, Trevor.

As I'm sure you know, the normal assumption in 
most fisheries is that exploitation rate or 
fishing mortality rate is constant as a function 
of abundance. Hence, in fisheries management we 
attempt to find that rate (F or u) which produces 
a sustainably fished stock, for example by 
maximizing yield or protecting a fraction of the 
spawning stock using target or limit biological 
reference points. In this case the form of the 
function relating exploitation rate to abundance 
is linear (= constant), and is analogous to a 
predator's Type I functional response. Constant 
exploitation rates as a function of abundance may 
not be that uncommon, as exemplified by the following recent paper:

David B. Eggleston, Eric G. Johnson, G. Todd 
Kellison and David A. Nadeau. 2003. Intense 
removal and non-saturating functional responses 
by recreational divers on spiny lobster Panulirus 
argus. Mar. Ecol. Prog. Ser. 257: 197207.

When the response is non-linear, it is not 
necessarily depensatory, particularly at low to 
moderate population levels. The actual form 
depends on a combination of fisher behavior and 
the behavior and response of the species, similar 
to what we see in predator-prey dynamics. The two 
major non-linear forms of the response are a Type 
II functional response (= depensatory) in which 
exploitation rate or proportional mortality 
increases as abundance declines. The other 
function is a Type III functional response (= 
compensatory) whereby proportional 
mortality/exploitation rate decreases as 
abundance declines from moderate to low abundance levels.

A compensatory response is commonly observed when 
a prey or fishery species becomes more and more 
spread out as abundance declines, which makes it 
more difficult both logistically and 
economically, to catch a sufficient number of 
prey to make the fishing worthwhile. A 
depensatory response might be expected when the 
prey remain aggregated even at low abundance, 
such as in schooling fish, such that it is still 
worthwhile to fish the schools until every last 
one has been caught. Whether one would expect a 
compensatory, depensatory or constant 
exploitation or fishing mortality rate is thus 
not a straightforward problem with a simple 
answer. For many, if not most, vertebrate 
predators, the response is usually Type III 
(compensatory), so that we might expect the 
response of fishers to be compensatory if comparable mechanisms are in effect.

You are correct that there is likely to be a fair 
amount of information in stock assessments on 
fishing mortality rates and exploitation rates 
along with stock abundance/biomass, but I am not 
aware of many attempts to characterize the form 
of the function quantitatively (other than in 
theoretical treatments), particularly with direct 
population estimates of exploitation rate and 
abundance, as opposed to modeled estimates of 
exploitation or fishing mortality rates. I am now 
thinking that it would be useful to conduct a 
comprehensive review of this issue across various 
fisheries, and will be requesting suggestions as 
to where I can most readily acquire this information.

I should also note that Lobo Orensanz, Gordon 
Kruse and Tito de Morais kindly provided some 
initial leads on this issue, in particular the 
following publication that I am currently using 
to extract some of the published information:

Orensanz, J.M., Armstrong, J., Armstrong, D.A. 
and Hilborn, R. 1998. Crustacean resources are 
vulnerable to serial depletion- The multifaceted 
decline of shrimp and crab fisheries in the 
greater Gulf of Alaska. Reviews in Fish Biology and Fisheries 8: 117-176.

Regards,
Rom

At 06:36 PM 7/28/2006, Trevor J. Kenchington wrote:
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>
>Rom Lipcius wrote:
>
>>Yes, exploitation rate or fishing mortality rate increases at
>>decreased stock/population levels, for instance when fishers
>>attempt to catch the same number of animals annually, irrespective
>>of stock size.
>>
>>>>I am investigating depensatory exploitation of the Chesapeake Bay
>>>>blue crab population, and have been searching for published
>>>>examples of depensatory exploitation, particularly in marine
>>>>invertebrates. I have found some examples for fish, but none yet
>>>>for marine invertebrates. Does anyone know of any published
>>>>examples of depensatory exploitation in marine (or estuarine)
>>>>invertebrates?
>
>Thus defined, I would expect some degree of "depensatory
>exploitation" to be a normal feature of all fisheries (though, as
>ever, there will be specific exceptions), at least across some ranges
>of biomass and subject to masking by other  factors (e.g. a collapse
>in market prices as biomass declines).
>
>Most stock assessments that document time series of biomass and
>fishing mortality should provide examples.
>
>
>Trevor Kenchington


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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